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y(ating)k(system)g(k)m(ernel.)52 b(Our)32 b(metho)s(d)i(en)m(tails)f(minimal)e(c)m(hanges)k(to)g(the)f (nativ)m(e)h(op)s(erating)e(system)298 1704 y(\(OS\),)27 b(and)g(no)g(c)m(hange)h(at)f(all)f(in)g(the)h(OS)f(o\016cial)h(in)m (terface)g(\(i.e.)40 b(the)27 b(system)g(calls\).)40 b(F)-8 b(or)27 b(UNIX-lik)m(e)298 1817 y(op)s(erating)32 b(systems)h(whose)g(source)g(co)s(de)f(is)g(a)m(v)-5 b(ailable)32 b(\(e.g.)49 b(Lin)m(ux,)32 b(xxBSD,)h(Solaris\))f(w)m(e)h (prop)s(ose)298 1929 y(to)d(\\instrumen)m(t")f(the)g(system)h(calls)e (so)i(that)f(the)h(system)f(call)g(itself)f(once)i(in)m(v)m(ok)m(ed)f (c)m(hec)m(ks)i(whether)298 2042 y(the)41 b(in)m(v)m(oking)e(pro)s (cess)h(and)f(the)i(argumen)m(t)f(v)-5 b(alues)40 b(passed)f(comply)h (with)e(the)j(rules)e(held)f(in)h(the)298 2155 y(access)32 b(con)m(trol)f(database.)439 2268 y(By)36 b(means)f(of)g(these)h(c)m (hec)m(ks)g(made)g(b)m(y)f(the)g(OS)f(k)m(ernel)h Fh(b)-5 b(efor)g(e)36 b Fk(the)f(system)g(call)g(is)f(completed,)298 2381 y(one)40 b(should)d(b)s(e)i(able)g(to)h(prev)m(en)m(t)g(an)m(y)g (p)s(ossible)c Fh(b)-5 b(ad)41 b Fk(side)d(e\013ect)j(of)e(system)h (calls)e(that)i(in)m(truders)298 2494 y(curren)m(tly)30 b(exploit)f(for)h(their)g(attac)m(ks.)439 2607 y(It)j(is)e(w)m(orth)i (noting)f(that)h(the)f(c)m(hec)m(ks)i(made)e(inside)f(the)h(system)h (calls)e(do)i(not)f(require)f(c)m(hanges)298 2720 y(in)36 b Fh(existing)i Fk(k)m(ernel)f(data)h(structures)f(and)g(algorithms.)61 b(Hence)38 b(all)e(the)i(k)m(ernel)f(mo)s(di\014cations)f(are)298 2833 y(transparen)m(t)f(to)g(the)g(application)e(pro)s(cesses)h(that)h (con)m(tin)m(ue)g(to)g(w)m(ork)f(correctly)h(requiring)d(neither)298 2946 y(c)m(hanges)g(of)e(the)h(source)f(co)s(de)h(nor)f (re-compilation.)439 3059 y(As)41 b(an)f(example)f(of)i(this)e (e\016cien)m(t)h(implemen)m(tation)f(tec)m(hnique,)j(w)m(e)f(ha)m(v)m (e)g(built)d(a)j(protot)m(yp)s(e)298 3171 y(whic)m(h)29 b(detects)j(and)e(prev)m(en)m(ts)h Fh(bu\013er)h(over\015ow)g Fk(based)e(attac)m(ks)i(b)s(efore)e(they)h(can)g(complete.)298 3458 y Fj(3)135 b(Kernel)38 b Ff(p)-7 b(atches)35 b Fj(to)j(prev)l(en)l (t)g(damages)g(due)e(to)i(bu\013er)f(o)l(v)l(er\015o)l(ws)298 3810 y Fk(This)26 b(section)i(describ)s(es)e(the)i(enhancemen)m(ts)g (of)g(the)g(Lin)m(ux)e(k)m(ernel)h(whic)m(h)g(prev)m(en)m(t)h(bu\013er) f(o)m(v)m(er\015o)m(ws)298 3923 y(from)j(b)s(eing)f Fh(r)-5 b(e)g(al)5 b(ly)32 b Fk(dangerous)e(for)g(the)h(system)f(securit)m(y)-8 b(.)298 4167 y Fe(3.1)113 b(The)37 b(bu\013er)i(o)m(v)m(er\015o)m(w)e (attac)m(k)298 4338 y Fk(The)26 b(lac)m(k)h(of)g(arra)m(y)g(b)s(ounds)d (c)m(hec)m(king)k(in)d(C)h(mak)m(es)i(p)s(ossible)c(to)j Fh(over\015ow)h Fk(a)f(memory)f(bu\013er)g(b)s(ey)m(ond)298 4451 y(its)e(b)s(oundaries.)37 b(By)25 b(means)g(of)f(widely)f(kno)m (wn)h(tec)m(hniques)h([1,)g(5)q(,)g(4],)h(a)g(malicious)c(user)j(ma)m (y)g(corrupt)298 4564 y(one)34 b(or)g(more)f(memory)h(bu\013ers,)f(b)s (elonging)f(to)i(the)g(stac)m(k)g(or)g(data)g(segmen)m(t,)i(in)c(suc)m (h)h(a)h(w)m(a)m(y)g(that,)298 4677 y(t)m(ypically)24 b(on)h(return)g(from)f(a)i(function)e(call,)i(a)f(di\013eren)m(t)g (piece)g(of)g(co)s(de,)i(\\injected")f(b)m(y)f(the)g(attac)m(k)m(er,) 298 4790 y(is)35 b(executed)i(b)m(y)f(the)g(\015a)m(w)m(ed)g(C)g (program.)57 b(Ob)m(viously)34 b(the)i(pro)s(cess)g(corresp)s(onding)e (to)i(the)g(buggy)298 4903 y(program)43 b(main)m(tains)e(its)i (\\status")g(including)d(sp)s(ecial)h(privileges)g(\(if)h(an)m(y\).)78 b(As)43 b(a)g(consequence,)298 5016 y(if)37 b(the)h(tec)m(hnique)f(is)g (successfully)e(applied)h(to)i(a)g(privileged)e(pro)s(cess)h(and)g(the) h Fh(fake)f Fk(co)s(de)h(is)f(used,)298 5129 y(for)j(instance,)j(to)e (start)g(the)f(execution)h(of)f(an)h(in)m(teractiv)m(e)g(shell,)g(the)f (attac)m(k)m(er)k(gains)39 b(the)i(access)2076 5381 y(4)p eop %%Page: 5 5 5 4 bop 298 123 a Fk(to)37 b(a)f(privileged)d(shell.)55 b(System)35 b(soft)m(w)m(are)i(comp)s(onen)m(ts,)g(that)g(is)d (commands,)j(daemons,)g(libraries)298 236 y(written)28 b(in)g(C)g(rarely)g(p)s(erform)f(all)h(the)h(necessary)g(c)m(hec)m(ks)h (b)s(efore)f(in)m(v)m(oking)f(functions)f(lik)m(e)h Fg(sprintf)298 349 y Fk(or)41 b Fg(strcpy)d Fk(that)j(ma)m(y)g(result)e(in)g(a)i (bu\013er)e(o)m(v)m(er\015o)m(w.)72 b(This)39 b(\\feature")i(mak)m(es)g (them)g(an)f(ob)m(vious)298 462 y(target)i(of)e(bu\013er)g(o)m(v)m (er\015o)m(w)h(based)f(attac)m(ks)j(sp)s(ecially)38 b(if)h(they)h (generate)i(pro)s(cesses)e(running)e(with)298 574 y(administrator)29 b(privileges)g(\()p Fh(r)-5 b(o)g(ot)33 b Fk(in)c(UNIX)i(jargon\).)42 b(So)31 b(w)m(e)g(m)m(ust)f(prev)m(en)m(t)i(an)m(y)f(ro)s(ot)g(pro)s (cess)f(from)298 687 y(executing)j(dangerous)g(\(for)g(the)g(system)g (securit)m(y\))g(co)s(de)g(if)f(it)g(undergo)s(es)g(a)h(bu\013er)f(o)m (v)m(er\015o)m(w)j(based)298 800 y(attac)m(k.)439 913 y(F)-8 b(or)39 b(the)f(purp)s(ose)e(of)i(our)g(discussion,)f(a)i(ro)s (ot)f(pro)s(cess)f(ma)m(y)i(b)s(elong)e(to)h(one)h(of)f(the)g(follo)m (wing)298 1026 y(three)31 b(categories:)63 1200 y(in)m(teractiv)m(e)47 b(This)36 b(is)h(a)i(standard)f(pro)s(cess)f(started)i(b)m(y)f(the)h (real)f(sup)s(eruser.)62 b(Both)39 b(the)f(User)h(IDen)m(ti\014er)526 1312 y(\(UID\))i(and)e(the)i(E\013ectiv)m(e)g(User)f(IDen)m(ti\014er)g (\(EUID\))h(are)g(equal)f(to)h(0.)70 b(No)41 b(sp)s(ecial)e(threat)526 1425 y(should)29 b(b)s(e)j(generated)g(b)m(y)g(suc)m(h)f(pro)s(cess)h (since)f(the)h(user)f(who)g(starts)h(it)g(has)f(already)h(the)g(full) 526 1538 y(con)m(trol)e(of)h(the)g(system;)20 1720 y(bac)m(kground)46 b(This)36 b(is,)j(usually)-8 b(,)38 b(either)f(a)i(daemon)e(pro)s(cess) h(started)g(at)h(b)s(o)s(ot)f(time)f(or)h(a)g(pro)s(cess)g(started)526 1833 y(p)s(erio)s(dically)h(b)m(y)44 b(the)g(cron)f(daemon)h(on)f(ro)s (ot)h(b)s(ehalf.)79 b(Daemon)44 b(pro)s(cesses)g(are)g(p)s(oten)m(tial) 526 1946 y(targets)28 b(of)f(bu\013er)e(o)m(v)m(er\015o)m(w)k(based)d (attac)m(ks)j(although)d(the)h(tec)m(hnique)g(is)e(a)j(bit)d(more)i (complex.)242 2128 y(setuid)45 b(F)-8 b(or)34 b(this)e(kind)f(of)j(pro) s(cess)f(the)g(pair)f(of)i(user)e(iden)m(ti\014ers)g(\(UID,)i(EUID\))g (has)f(v)-5 b(alues)33 b(UID)p Fd(>)d Fk(0)526 2241 y(and)21 b(EUID=0.)38 b(So)22 b(a)h(pro)s(cess)e(can)i(b)s(e)e(iden)m(ti\014ed)f (as)i(setuid)f(to)i(ro)s(ot)f(b)m(y)g(means)g(of)h(the)f(follo)m(wing) 526 2354 y(simple)28 b(macro)526 2562 y Fg(#define)45 b(IS_SETUID_TO_ROOT\(proc\))d(!\(\(proc\)->euid\)&&\(proc\)-)o(>ui)o(d) 526 2770 y Fk(On)34 b(sp)s(eci\014c)g(op)s(erating)g(systems)h(other)g (information)f(ma)m(y)h(b)s(e)g(more)g(suitable)f(to)h(iden)m(tify)f(a) 526 2882 y(setuid)21 b(pro)s(cess.)38 b(F)-8 b(or)23 b(example,)i(in)c(the)i(AIX)g(op)s(erating)f(system)h(a)g(go)s(o)s(d)g (candidate)f(app)s(ears)h(to)526 2995 y(b)s(e)j(the)h(LOGIN)g(UID)h (whic)m(h)e(is)g(assigned)g(during)f(the)i(initial)e(user)h(authen)m (tication)h(and)g(nev)m(er)526 3108 y(mo)s(di\014ed.)43 b(Our)30 b(c)m(hoice)j(is)e(motiv)-5 b(ated)32 b(b)m(y)g(the)g (consideration)f(that)h(the)g(pair)f(\(UID,EUID\))i(is)526 3221 y(presen)m(t)d(in)g(all)f(Unix)h(\015a)m(v)m(ors.)42 b(This)29 b(should)g(ease)i(the)g(extension)f(of)h(our)f(w)m(ork)h(to)g (other)g(Unix)526 3334 y(platforms.)439 3508 y(In)j(the)h(rest)g(of)g (the)g(pap)s(er)f(w)m(e)h(will)d(cite,)37 b(for)d(the)h(sak)m(e)h(of)f (simplicit)m(y)-8 b(,)34 b(just)g(the)h(last)f(class)h(\(the)298 3620 y(setuid)26 b(pro)s(cesses\).)40 b(The)27 b(extension)f(to)i(the)f (bac)m(kground)g(ro)s(ot)h(pro)s(cesses)f(should)e(b)s(e)h(apparen)m(t) i(once)298 3733 y(w)m(e)40 b(describ)s(e)d(ho)m(w)j(suc)m(h)e(pro)s (cesses)h(can)h(b)s(e)e(iden)m(ti\014ed)f(within)g(the)i(k)m(ernel.)67 b(This)37 b(is)h(done)h(in)f(the)298 3846 y(follo)m(wing)29 b(subsection.)298 4087 y Fe(3.2)113 b(Bac)m(kground)38 b(ro)s(ot)f(programs)298 4259 y Fk(On)28 b(a)g(t)m(ypical)g(UNIX)h (system)f(there)h(are)g(alw)m(a)m(ys)f(ro)s(ot)h(programs)f(running)e (in)h(\\bac)m(kground".)40 b(Most)298 4372 y(of)25 b(the)g(times,)g (the)g(system)g(administrator)d(neither)i(start)h(them)g(directly)e (\(i.e.)39 b(during)22 b(an)i(in)m(teractiv)m(e)298 4484 y(session\))31 b(nor)h(con)m(trol)g(their)e(execution,)j(so,)f(as)g (men)m(tioned)f(b)s(efore,)h(suc)m(h)f(privileged)f(programs)h(are)298 4597 y(a)g(preferred)e(target)j(of)f(bu\013er)e(o)m(v)m(er\015o)m(w)j (based)e(attac)m(ks.)439 4710 y(F)-8 b(ollo)m(wing)30 b([17)q(])h(it)f(is)f(p)s(ossible)f(to)j(classify)e(them)i(as)f(follo)m (ws:)409 4906 y(1.)47 b(Daemons)36 b(pro)s(cesses)f(originated)f (directly)g(b)m(y)h(the)g(system)h(initialization)c(scripts.)54 b(Net)m(w)m(ork)526 5019 y(serv)m(ers)31 b(lik)m(e)f(the)i Fg(inetd)e Fk(sup)s(er-serv)m(er,)g(W)-8 b(eb)32 b(serv)m(er,)g(mail)e (serv)m(er)h(\(t)m(ypically)g Fg(sendmail)p Fk(\))e(are)526 5132 y(often)h(started)h(in)e(this)h(w)m(a)m(y)-8 b(.)42 b(Another)30 b(example)g(is)f(the)i Fg(syslogd)d Fk(daemon.)2076 5381 y(5)p eop %%Page: 6 6 6 5 bop 409 123 a Fk(2.)47 b(Net)m(w)m(ork)34 b(serv)m(ers)g(started)f (b)m(y)g(the)h Fg(inetd)d Fk(sup)s(er-serv)m(er)i(to)g(ful\014ll)d (requests)j(for)g(services)g(lik)m(e)526 236 y(remote)e(access)h (\(telnet\),)f(\014le)f(transfer)f(\(FTP\))i(and)f(so)h(on.)409 412 y(3.)47 b(Programs)32 b(executed)i(on)f(a)g(regular)f(basis)g(b)m (y)h(the)g Fg(cron)e Fk(daemon.)49 b(The)32 b Fg(cron)g Fk(daemon)g(itself)526 525 y(is)d(started)i(at)g(b)s(o)s(ot)f(time)g (\(i.e.)41 b(it)30 b(b)s(elongs)f(to)i(class)f(1\).)409 700 y(4.)47 b(Programs)36 b(executed)h(just)e Fh(onc)-5 b(e)37 b Fk(in)e(the)h(future)g(b)m(y)g(means)g(of)g(the)g Fg(at)g Fk(command.)58 b(Actually)526 813 y(these)30 b(programs)g(can)h(b)s(e)f(considered)f(as)i(a)f(sp)s(ecial)f(case)j (of)e(the)h(previous)e(class.)409 989 y(5.)47 b(Programs)33 b(started)h(in)e(bac)m(kground)h(during)f(an)h(in)m(teractiv)m(e)h (sessions)f(\(i.e.)50 b(with)32 b(a)i(&)f(at)h(the)526 1102 y(end)d(of)h(the)f(command)h(line\).)43 b(This)30 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(to)f(monitor)g(with)e(this)h(mec)m(hanism)g(is)g Fg(su)p Fk(,)h(a)298 4658 y(setuid)h(program)g(whic)m(h)f(runs)g(a)h(shell)f (with)g(substitute)g(user)h(\(and)g(group\))g(ID.)439 4771 y(Figure)i(6)h(rep)s(orts)f(the)h(additions)d(to)k(the)e(co)s(de)h (of)g(the)f Fg(chmod)g Fk(system)g(call.)47 b(The)32 b(ma)5 b(jor)32 b(di\013er-)298 4884 y(ence)g(with)f(resp)s(ect)g(to)h (the)g Fg(setuid)d Fk(primitiv)m(e)h(is)g(that)i(no)g(ro)s(ot)f(passw)m (ord)g(is)f(required)g(whereas)h(an)298 4997 y(additional)h(c)m(hec)m (k)j(is)e(p)s(erformed)f(on)h(the)h Fg(filename)d Fk(argumen)m(t.)51 b(If)33 b Fg(filename)f Fk(refers)h(to)h(a)g(regular)298 5110 y(\014le)40 b(or)g(a)g(directory)-8 b(,)43 b(the)e(op)s(eration)e (is)h(denied.)68 b(This)39 b(means)h(that)h(the)f(op)s(eration)g(is)f (allo)m(w)m(ed)h(if)2053 5381 y(12)p eop %%Page: 13 13 13 12 bop 298 83 a Fb(/*)338 175 y(*)39 b(fs/open.c)338 266 y(*/)298 449 y(asmlinkage)j(int)e(sys_chmod\(const)i(char)f(*)e (filename,)i(mode_t)g(mode\))298 540 y({)377 631 y(struct)f(dentry)h(*) e(dentry;)377 723 y(struct)h(inode)h(*)e(inode;)377 814 y(int)h(error;)377 905 y(struct)g(iattr)h(newattrs;)377 1088 y(lock_kernel\(\);)377 1179 y(dentry)f(=)g(namei\(filename\);)377 1362 y(/***************)j(BUFFER_OVERFLOW)f(PATCH)f (********************/)298 1453 y(#define)g(IS_SETUID_TO_ROOT\(proc\))k (!\(\(proc\)->euid\)&&\(proc\)->uid)298 1545 y(#define)c (IS_A_ROOT_DAEMON\(proc\))84 b(!\(\(proc\)->euid\)&&\(\(proc\)->tty==)q (NULL)q(\))377 1727 y(if)39 b(\(\(IS_SETUID_TO_ROOT\(current)q(\)||I)q (S_A_R)q(OOT_)q(DAEMO)q(N\(cu)q(rren)q(t\)\)&&)534 1819 y(\(S_ISREG\(dentry->d_inode->i_mo)q(de\)||)q(S_IS)q(DIR\(d)q(entr)q (y->d)q(_inod)q(e->i)q(_mode)q(\)\)\))46 b({)573 1910 y(printk\(BOP_LEVEL"BOP)e(proc)c(\045s)g(failed)g(changing)h(mode:\\n)g (",current->comm\);)573 2001 y(printk\(BOP_LEVEL")i(inode)d(\045d)g (device)h(\045d\\n",)808 2093 y(\(int\))g (dentry->d_inode->i_ino,\(int\))k(dentry->d_inode->i_dev\);)573 2184 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b(from)g([16)q(])h(and)e(it)h(is)g(comp)s(osed)g(of)g (three)h(parts:)435 4100 y Fi(\017)46 b Fk(a)33 b(k)m(ernel)f(patc)m (h.)49 b(The)32 b(patc)m(h)i(has)f(b)s(een)f(dev)m(elop)s(ed)g(and)g (tested)i(with)d(the)i(v)m(ersion)g(2.2.12-20)526 4213 y(of)k(the)g(Lin)m(ux)f(k)m(ernel.)60 b(Since)36 b(it)h(is)f(basically) f(a)j(set)g(of)f(additions)e(to)j(the)f(existing)f(co)s(de)h(for)526 4326 y(the)h(system)g(calls)f(\(there)i(are)f(neither)f(c)m(hanges)i (nor)f(deletions\),)h(w)m(e)g(do)e(not)i(exp)s(ect)f(ma)5 b(jor)526 4439 y(problems)33 b(in)h(p)s(orting)f(it)i(to)g(other)h (\(new)m(er\))f(v)m(ersions)g(of)g(the)g(k)m(ernel.)54 b(When)35 b(the)g(patc)m(h)h(has)526 4552 y(b)s(een)26 b(applied)f(\(via)j(the)f Fh(p)-5 b(atch)29 b Fk(command)e([8)q(]\),)i (a)e(new)g(k)m(ernel)g(image)h(m)m(ust)f(b)s(e)g(generated)h(and)526 4665 y(installed.)435 4842 y Fi(\017)46 b Fk(the)30 b(new)g(command)g Fg(aclmng)f Fk(\(whic)m(h)g(has)h(b)s(een)g(describ)s(ed)e(in)h (section)i(4.1\).)435 5019 y 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42 b(:/fullpathN/programN)g(foo)526 912 y Fk(allo)m(ws)25 b(the)h(setuid)f(program)h Fh(fo)-5 b(o)27 b Fk(to)g(execute)h(an)m(y)e (of)h Fh(pr)-5 b(o)g(gr)g(am1,...)43 b(pr)-5 b(o)g(gr)g(amN)29 b Fk(\(the)e(execution)526 1024 y(of)38 b(an)m(y)h(other)g(program)f (is,)i(b)m(y)f(default,)h(forbidden\).)63 b(What)39 b(the)g(mo)s (di\014ed)d Fg(chmod)h Fk(do)s(es)h(is)526 1137 y(to)e(in)m(v)m(ok)m(e) h(the)f Fg(sys)p 1240 1137 29 4 v 33 w(setuid)p 1561 1137 V 33 w(aclm)f Fk(system)h(call)f(to)h(add)f(the)h(necessary)h (information)d(in)g(the)526 1250 y(A)m(CD.)439 1438 y(The)44 b(system)h(administrator's)d(duties)h(are)i(limited)d(to)j(run)e(the)h (new)g(v)m(ersion)g(of)g(the)h Fg(chmod)298 1551 y Fk(command.)64 b(Neither)38 b(re-compilation)f(nor)g(co)s(de)i(insp)s(ection)d(is)h (required.)62 b(Messages)40 b(sen)m(t)e(to)h(the)298 1664 y(syslog)e(b)m(y)g(the)f(mo)s(di\014ed)f(commands)h(and)h(b)m(y)f (the)h(system)g(calls,)h(start)f(with)f(the)h(\\BOP")g(pre\014x.)298 1777 y(The)30 b(c)m(hmo)s(d)g(command)h(sends)e(a)i(three)g(lines)d (message)k(when)d(a)i(new)f(pair)f(of)i(programs)f(is)f(inserted)298 1890 y(in)g(the)i(A)m(CD.)439 2002 y(In)j(case)h(the)g(list)e(of)h (programs)g(is)g(unkno)m(wn,)g(the)g(administrator)f(ma)m(y)i(learn)f (whic)m(h)f(programs)298 2115 y(a)40 b(pro)s(cess)g(tries)f(to)h (execute)h(b)m(y)f(lo)s(oking)e(at)i(the)g(messages)h(\(with)e(the)g (pre\014x)g(\\BOP"\))i(that)f(the)298 2228 y(pro)s(cess)30 b(sends)g(to)h(the)f Fh(syslo)-5 b(g)p Fk(.)298 2472 y Fe(4.4)113 b(P)m(erformance)298 2643 y Fk(A)m(t)37 b(this)f(time,)h(w)m(e)g(do)f(not)g(ha)m(v)m(e)i(a)e(complete)h(set)g (of)f(p)s(erformance)f(measures.)58 b(Preliminary)33 b(tests)298 2756 y(executed)h(on)f(setuid)f(or)h(daemon)h(programs)e (running)f(on)i(a)g(system)h(with)d(the)j(mo)s(di\014ed)d(k)m(ernel)h (do)298 2869 y(not)39 b(sho)m(w)f(an)m(y)h(di\013erence)f(in)f(the)h (execution)h(time)f(with)f(resp)s(ect)h(to)h(a)g(system)f(with)f(a)i (standard)298 2982 y(k)m(ernel.)439 3095 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Fg(#define)45 b(IS_SETUID_TO_ROOT\(proc\))d (!\(\(proc\)->euid\)&&\(proc\)-)o(>ui)o(d)526 4610 y(#define)j (IS_A_ROOT_DAEMON\(proc\))90 b(!\(\(proc\)->euid\)&&\(\(proc\))o(->t)o (ty==)o(NULL)o(\))907 4836 y(if\(IS_SETUID_TO_ROOT\(curre)o(nt\))41 b(||)47 b(IS_A_ROOT_DAEMON\(current\)\))41 b({)621 4949 y(......)526 5062 y(})2053 5381 y Fk(14)p eop %%Page: 15 15 15 14 bop 526 123 a Fk(So)31 b(if)f(the)h(pro)s(cess)f(is)h(neither)f (a)h(ro)s(ot)g(daemon)g(nor)g(setuid)f(to)i(ro)s(ot,)g(it)e(do)s(es)h (no)g(more)g(than)g(a)526 236 y(couple)f(of)i(logical)f(tests)h(\(note) g(that)g(the)g(t)m(w)m(o)h(conditions)d(are)h(ev)-5 b(aluated)32 b(separately)g(just)e(for)526 349 y(the)g(sak)m(e)i(of)e(co)s(de)h (readabilit)m(y\).)435 536 y Fi(\017)46 b Fk(With)21 b(the)i(exception)f(of)g(the)h Fg(open)e Fk(primitiv)m(e,)g(it)h(is)f (unlik)m(ely)f(that)j(a)f(setuid)f(or)i(daemon)f(pro)s(cess)526 649 y(in)m(v)m(ok)m(es)31 b(an)m(y)f(of)h(the)g(instrumen)m(ted)d 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2078 y(W)-8 b(e)33 b(are)f(not)g(going)g(to)g(o\013er)g(here)f(an)h(exhaustiv)m(e)g (review)e(of)i(all)f(p)s(ossible)e(approac)m(hes.)45 b(W)-8 b(e)33 b(list)298 2191 y(just)i(the)f(categories)j(in)c(whic)m (h)h(the)h(di\013eren)m(t)f(metho)s(ds)g(can)h(b)s(e)f(classi\014ed)f (along)i(with)f(their)f(main)298 2304 y(adv)-5 b(an)m(tages)32 b(and)e(dra)m(wbac)m(ks.)439 2417 y(Ob)m(viously)-8 b(,)27 b(a)h(v)m(ery)g(go)s(o)s(d)f(solution)f(to)i(bu\013er)e(o)m(v)m (er\015o)m(w)j(problems)d(is)g(to)i(use)f(fo)s(olpro)s(of)g(soft)m(w)m (are.)298 2530 y(The)k(Op)s(en-BSD)f(pro)5 b(ject)32 b([9)q(])f(and)f(some)i(other)f(teams)h(ha)m(v)m(e)g(p)s(erformed)e(a)i (detailed)e(co)s(de)h(auditing)298 2643 y(of)36 b(all)f(system)i(soft)m (w)m(are)g(\(k)m(ernel,)h(libraries,)c(utilities\).)56 b(There)35 b(are)i(t)m(w)m(o)g(main)e(limitations)f(in)g(this)298 2756 y(approac)m(h:)52 b(auditors)34 b(capabilities)g(and)g(the)i(lac)m (k)g(of)g(an)m(y)g(guaran)m(tee)h(ab)s(out)e(soft)m(w)m(are)i(\\imp)s (orted")298 2869 y(from)30 b(other)h(sources.)439 2982 y(Another)h(drastic)f(alternativ)m(e)g(is)g(to)h(giv)m(e)g(up)e(using)g (C)h(and)g(start)h(to)g(write)e(co)s(de)i(in)e(a)i(language)298 3095 y(that)g(implemen)m(ts)e(b)s(ounds)f(c)m(hec)m(king)j(suc)m(h)e (as)i(Ada,)f(Ja)m(v)-5 b(a,)33 b(or)e(P)m(erl.)43 b(Ja)m(v)-5 b(a)32 b(and)e(P)m(erl)h(are)g(in)f(general)298 3207 y(slo)m(w)m(er)f(than)e(C)h(and)g(this)f(is,)h(b)m(y)g(itself,)f(a)i (serious)e(problem)f(\(although)i(Ja)m(v)-5 b(a)29 b(supp)s(orters)d (claim)i(that)298 3320 y(the)34 b(sp)s(eed)f(gap)i(is)e(reducing)f(v)m (ery)i(fast\).)52 b(Another)34 b(dra)m(wbac)m(k)h(is)d(that)j(there)f (is)f(to)s(o)i(m)m(uc)m(h)f(C)f(co)s(de)298 3433 y(to)e(think)e (seriously)g(ab)s(out)h(re-writing)f(it)h(using)e(another)j(language.) 439 3546 y(Some)h(UNIX)g(distributions)c(\(BSDI,)33 b(Op)s(en-BSD\))e (ha)m(v)m(e)i(also)e(mo)s(di\014ed)f(the)i(link)m(er)e(to)i(pro)s(duce) 298 3659 y(w)m(arning)f(messages)h(when)f(a)h(program)f(uses)g (dangerous)h(functions.)43 b(This)29 b(approac)m(h)j(implies)d(man)m(y) 298 3772 y(\\false)g(p)s(ositiv)m(e")f(alarms)g(since)f(uses)h(of)h (dangerous)f(functions)f(are)i(not)g(all)e(incorrect)h(whereas)h(o)m(v) m(er-)298 3885 y(\015o)m(ws)i(o)s(ccur)f(not)g(only)g(in)f(standard)g (libraries.)439 3998 y(Marking)36 b(b)s(oth)f Fh(data)i Fk(and)e Fh(stack)h Fk(regions)g(as)g(\\non-executable")h(w)m(ould)d (catc)m(h)j(most)f(\\cut)h(and)298 4111 y(paste")30 b(exploits.)40 b(A)29 b(non-executable)g(stac)m(k)i(is)d(readily)f(implemen)m(ted)h ([10)q(])h(since)f(it)h(in)m(tro)s(duces)f(just)298 4224 y(minor)k(side)g(e\013ects)i(in)e(most)h(UNIX)g(v)-5 b(ariations)32 b(\(e.g.,)k(Lin)m(ux)31 b(places)i(co)s(de)g(for)g (signal)e(deliv)m(ery)h(on-)298 4337 y(to)j(the)f(pro)s(cess's)g(stac)m (k\).)53 b(Note)36 b(that)e(there)h(is)e(no)g(p)s(erformance)h(p)s (enalt)m(y)f(and)h(existing)f(programs)298 4449 y(require)i(neither)h (c)m(hanges)h(nor)f(re-compilation)g(\(unless)f(they)h(use)h(exotic)g (features)g(lik)m(e)e(gcc)j Fh(tr)-5 b(am-)298 4562 y(p)g(olines)p Fk(\).)58 b(The)34 b(situation)h(is)f(not)i(so)f(simple)f(for)h(the)g (data)h(region.)55 b(It)36 b(is)e(not)i(p)s(ossible)d(to)j(mark)f(it) 298 4675 y(as)28 b(non-executable)g(without)e(in)m(tro)s(ducing)g(ma)5 b(jor)27 b(compatibilit)m(y)f(problems.)38 b(Ev)m(en)28 b(if)e(this)g(could)h(b)s(e)298 4788 y(solv)m(ed,)k(there)f(is)f(still) g(the)h(problem)f(of)h(attac)m(ks)j(whic)m(h)c(instead)g(of)i(in)m(tro) s(ducing)d Fh(new)i Fk(co)s(de,)h(corrupt)298 4901 y(co)s(de)k(p)s(oin) m(ters.)52 b(This)32 b(tec)m(hnique)j(allo)m(ws)e(to)i(execute)h (dangerous)e(instructions)f(whic)m(h)g(are)i(already)298 5014 y(part)c(either)e(of)i(the)f(program)h(or)f(of)h(its)e(libraries)f ([11)q(].)2053 5381 y(15)p eop %%Page: 16 16 16 15 bop 439 123 a Fk(There)22 b(are)h(b)s(oth)e(commercial)h([12)q(]) g(and)g(public)d(domain)i([13)r(])h(solutions)e(whic)m(h)h(add)h(arra)m (y)g(b)s(ounds)298 236 y(c)m(hec)m(king)32 b(capabilities)c(to)k(C)e (programs.)42 b(These)30 b(pac)m(k)-5 b(ages)33 b(can)e(b)s(e)f (considered)g(as)h(go)s(o)s(d)f(debugging)298 349 y(to)s(ols)23 b(but,)h(in)e(a)i(pro)s(duction)d(en)m(vironmen)m(t,)j(their)f(use)f (is)h(not)g(feasible)f(since)h(the)g(p)s(erformance)f(p)s(enal-)298 462 y(t)m(y)31 b(is)f(barely)f(acceptable)j(\(programs)e(undergo)g(a)h (slo)m(wdo)m(wn)e(of)i(ab)s(out)f(one)h(order)e(of)i(magnitude\).)439 574 y(Recen)m(tly)-8 b(,)30 b(t)m(w)m(o)e(compiler)e(tec)m(hniques)h (ha)m(v)m(e)h(b)s(een)f(prop)s(osed)e(for)i(in)m(tro)s(ducing)e(in)h (the)i(executable)298 687 y(co)s(de)j(\\ligh)m(t)m(w)m(eigh)m(t")g(c)m (hec)m(ks)h(on)f(the)f(in)m(tegrit)m(y)h(of)f(functions')f(return)h (address.)439 800 y(Stac)m(kGuard)39 b([14)r(])f(detects)i(and)d (defeats)i(stac)m(k)h(smashing)d(attac)m(ks)k(b)m(y)d(protecting)h(the) f(return)298 913 y(address)44 b(on)h(the)h(stac)m(k)g(from)f(b)s(eing)e (altered.)85 b(Stac)m(kGuard)45 b(places)g(a)g(\\canary")i(w)m(ord)d (next)i(to)298 1026 y(the)39 b(return)e(address)g(when)g(a)i(function)d (is)i(called.)63 b(If)37 b(the)i(canary)f(w)m(ord)g(has)g(b)s(een)f (altered)h(when)298 1139 y(the)d(function)e(returns,)h(then)f(a)i(stac) m(k)h(smashing)c(attac)m(k)37 b(has)d(b)s(een)f(attempted,)k(and)c(the) h(program)298 1252 y(resp)s(onds)29 b(b)m(y)h(emitting)g(an)g(in)m (truder)f(alert)h(in)m(to)g(syslog,)h(and)e(then)h(halts.)439 1365 y(T)-8 b(o)27 b(b)s(e)f(e\013ectiv)m(e,)j(the)d(attac)m(k)m(er)j (m)m(ust)e(not)f(b)s(e)g(able)g(to)h("sp)s(o)s(of")f(the)g(canary)h(w)m (ord)f(b)m(y)g(em)m(b)s(edding)298 1478 y(the)j(v)-5 b(alue)28 b(for)h(the)f(canary)h(w)m(ord)g(in)e(the)i(attac)m(k)i (string.)39 b(Stac)m(kGuard)29 b(o\013ers)g(a)g(range)g(of)g(tec)m (hniques)298 1591 y(to)i(prev)m(en)m(t)g(canary)g(sp)s(o)s(o\014ng:)435 1767 y Fi(\017)46 b Fk(Random)23 b(canaries:)38 b(the)25 b(canary)f(w)m(ord)g(v)-5 b(alue)24 b(is)g(c)m(hosen)g(at)i(random)d (at)i(the)g(time)f(the)g(program)526 1880 y(executes.)41 b(Th)m(us)28 b(the)h(attac)m(k)m(er)j(cannot)e(learn)e(the)h(canary)g (v)-5 b(alue)29 b(prior)e(to)j(the)f(program)g(start)526 1993 y(b)m(y)e(searc)m(hing)h(the)g(executable)h(image.)40 b(The)27 b(random)g(v)-5 b(alue)28 b(is)f(tak)m(en)h(from)g (/dev/urandom)f(if)526 2105 y(a)m(v)-5 b(ailable,)30 b(and)f(created)j(b)m(y)e(hashing)f(the)h(time)h(of)f(da)m(y)h(if)e (/dev/urandom)h(is)f(not)i(supp)s(orted.)435 2288 y Fi(\017)46 b Fk(Null)25 b(canary:)39 b(the)28 b(canary)f(w)m(ord)g(is)f("n)m (ull",)h(i.e.)40 b(0x00000000.)j(Since)26 b(most)i(string)e(op)s (erations)526 2401 y(that)34 b(are)h(exploited)e(b)m(y)g(stac)m(k)j (smashing)c(attac)m(ks)37 b(terminate)d(on)g(n)m(ull,)f(the)h(attac)m (k)m(er)j(cannot)526 2514 y(easily)29 b(sp)s(o)s(of)h(a)g(series)g(of)g (n)m(ulls)f(in)m(to)h(the)h(middle)d(of)i(the)h(string.)435 2697 y Fi(\017)46 b Fk(T)-8 b(erminator)33 b(canary:)47 b(not)34 b(all)f(string)g(op)s(erations)g(are)h(terminated)f(b)m(y)h(n) m(ull,)e(e.g.)52 b(gets\(\))36 b(ter-)526 2810 y(minates)i(on)i(new)e (line)g(or)h(end-of-\014le)g(\(represen)m(ted)h(as)f(-1\).)69 b(The)39 b(terminator)g(canary)g(is)g(a)526 2923 y(com)m(bination)j(of) h(Null,)i(CR,)e(LF,)h(and)e(-1)i(\(0xFF\))h(whic)m(h)d(should)f (terminate)j(most)f(string)526 3036 y(op)s(erations.)298 3212 y(Stac)m(kGuard)f(is)e(implemen)m(ted)g(as)i(a)f(small)f(patc)m(h) i(to)g(the)g(gcc)g(co)s(de)g(generator,)j(sp)s(eci\014cally)39 b(the)298 3325 y(function)p 630 3325 28 4 v 32 w(prolog\(\))f(and)f (function)p 1529 3325 V 32 w(epilog\(\))h(routines.)62 b(function)p 2636 3325 V 32 w(prolog\(\))38 b(has)g(b)s(een)f(enhanced) h(to)298 3438 y(la)m(y)e(do)m(wn)f(canaries)g(on)g(the)h(stac)m(k)h (when)d(functions)g(start,)j(and)e(function)p 2982 3438 V 32 w(epilog\(\))g(c)m(hec)m(ks)i(canary)298 3551 y(in)m(tegrit)m(y)h (when)f(the)h(function)f(exits.)63 b(An)m(y)38 b(attempt)h(at)g (corrupting)e(the)h(return)f(address)g(is)g(th)m(us)298 3664 y(detected)32 b(b)s(efore)e(the)h(function)e(returns.)439 3777 y(The)h(Stac)m(k)i(Shield)c([15)q(])i(protection)h(system)g (copies)f(the)g(return)g(address)f(in)g(an)i(uno)m(v)m(er\015o)m(w)m (able)298 3890 y(lo)s(cation)25 b(\(the)h(b)s(eginning)c(of)j(the)h (data)g(segmen)m(t\))g(on)f(function)f(prolog)h(and)f(c)m(hec)m(ks)j (if)d(the)h(t)m(w)m(o)i(v)-5 b(alues)298 4002 y(are)33 b(di\013eren)m(t)e(b)s(efore)h(the)g(function)e(returns.)44 b(If)32 b(the)g(t)m(w)m(o)h(v)-5 b(alues)31 b(are)i(di\013eren)m(t)e (the)h(return)f(address)298 4115 y(has)i(b)s(een)f(mo)s(di\014ed)e(so)j (Stac)m(k)h(Shield)c(terminates)j(the)g(program)f(or)h(tries)f(to)i (let)e(the)h(program)g(run)298 4228 y(ignoring)c(the)i(attac)m(k)h (\(risking)d(at)i(maxim)m(um)e(a)i(program)f(crash\).)439 4341 y(Stac)m(k)23 b(Shield)c(w)m(orks)i(as)h(an)f(assem)m(bler)g (\014le)f(pro)s(cessor)h(and)g(is)g(supp)s(orted)e(b)m(y)j(gcc/g++)h (fron)m(t)e(ends)298 4454 y(to)31 b(automatize)h(the)f(compilation.)39 b(No)31 b(co)s(de)g(c)m(hange)g(or)f(other)h(sp)s(ecial)e(op)s (erations)h(are)g(required.)439 4567 y(Stac)m(kGuard)35 b(and)e(Stac)m(kShield)f(o\013er)j(man)m(y)f(nice)f(features:)48 b(minim)m(um)31 b(p)s(erformance)i(p)s(enalt)m(y)-8 b(,)298 4680 y(no)33 b(c)m(hange)g(in)e(existing)h(co)s(de,)h(no)g(constrain)m (t)f(is)g(imp)s(osed)f(on)h(new)g(co)s(de.)47 b(Their)31 b(ma)5 b(jor)32 b(limitation)298 4793 y(should)d(b)s(e)h(apparen)m(t)h (thinking)d(ab)s(out)i(the)h(names:)41 b(they)31 b(protect)h(against)f (bu\013er)e(o)m(v)m(er\015o)m(ws)j Fh(in)h(the)298 4906 y(stack)p Fk(.)67 b(Unfortunately)-8 b(,)41 b(heap)e(o)m(v)m(er\015o)m (ws)h(are)f(less)f(common)h(but,)i(b)m(y)d(no)h(means,)i(less)e (dangerous)298 5019 y(than)30 b(stac)m(k)i(o)m(v)m(er\015o)m(ws)g([4)q (].)439 5132 y(There)e(are)h(other)g(issues)e(that)i(a)f(compiler)f (based)h(solution)f(raise.)2053 5381 y(16)p eop %%Page: 17 17 17 16 bop 298 123 a Fk(gcc)32 b(is)d(NOT)h(the)h(only)e(C)h(compiler)f (for)h(Lin)m(ux)f(and)h(more)g(in)f(general)i(for)f(Unix.)298 267 y(The)i(c)m(hec)m(ks)h(are)f(not)g(selectiv)m(e:)44 b(eac)m(h)33 b(function)e(of)h(an)m(y)g(pro)s(cess)f(is)g(a\013ected.) 47 b(Ev)m(en)32 b(if)e(b)s(enc)m(hmarks)298 380 y(of)d(single)f (applications)f(\(SSH,)h(Apac)m(he)i(w)m(eb)f(serv)m(er\))g(do)g(not)g (sho)m(w)g(signi\014can)m(t)e(p)s(erformance)i(degra-)298 493 y(dation,)37 b(it)f(is)f(unclear)g(what)h(is)f(the)h(impact)g(on)g (the)g(p)s(erformance)g(if)e(the)j(en)m(tire)f(system)g(soft)m(w)m(are) 298 606 y(\(k)m(ernel,)31 b(libraries,)d(utilities,...\))39 b(is)29 b(compiled)g(with)g(Stac)m(kGuard)i(\(or)g(Stac)m(kShield\).) 298 750 y(When)k(Stac)m(kGuard)h(detects)g(a)g(dangerous)f(bu\013er)f (o)m(v)m(er\015o)m(w,)k(the)d(only)g(action)g(it)g(tak)m(es)i(is)d(to)i (halt)298 863 y(the)30 b(program.)40 b(There)29 b(is)g(no)g(c)m(hance)h (to)h(mak)m(e)f(further)e(in)m(v)m(estigations)h(or)h(to)g(tak)m(e)h (di\013eren)m(t)e(actions)298 976 y(lik)m(e)h(mo)m(ving)g(the)h (o\013ending)e(pro)s(cess)h(in)f(a)i(\\\014sh)m(b)s(o)m(wl".)439 1134 y(Recen)m(tly)-8 b(,)49 b(w)m(e)44 b(ha)m(v)m(e)h(found)e(that)h (other)g(groups)f([19)q(])h(are)h(w)m(orking)e(with)f(an)i(approac)m(h) g(v)m(ery)298 1247 y(close)39 b(to)f(ours,)i(that)f(is)e(trying)g(to)i (k)m(eep)f(under)f(con)m(trol)h(system)h(calls)e(and)g(the)i(v)-5 b(alues)37 b(of)h(system)298 1360 y(calls)d(argumen)m(ts.)55 b(They)35 b(ha)m(v)m(e)h(dev)m(elop)s(ed)f(a)g(high)f(lev)m(el)h(sp)s (eci\014cation)f(language)i(called)e(Auditing)298 1473 y(Sp)s(eci\014cation)24 b(Language)h(for)g(sp)s(ecifying)d(normal)i (and)g(abnormal)g(b)s(eha)m(viors)f(of)i(pro)s(cesses)g(as)g(logical) 298 1585 y(assertions)31 b(on)g(the)g(sequence)h(of)f(system)h(calls)e (and)h(system)g(call)f(argumen)m(t)i(v)-5 b(alues)30 b(in)m(v)m(ok)m(ed)i(b)m(y)f(the)298 1698 y(pro)s(cess.)66 b(Unfortunately)-8 b(,)41 b(not)e(enough)f(information)g(are)h(a)m(v)-5 b(ailable)38 b(to)h(us)g(ab)s(out)f(their)g(\\System)298 1811 y(Call)29 b(Detection)j(Engine".)298 2092 y Fj(6)135 b(F)-11 b(uture)44 b(activities)298 2295 y Fk(W)-8 b(e)25 b(ha)m(v)m(e)g(describ)s(ed)d(ho)m(w)i Fh(simple)h Fk(enhancemen)m(ts)f (of)g(an)g(existing)f(k)m(ernel)g(co)s(de)h(can)g(mak)m(e)h(harmless)e (a)298 2408 y(w)m(ell)k(kno)m(wn)f(threat)i(for)f(the)h(system)f (securit)m(y)g(lik)m(e)g(bu\013er)f(o)m(v)m(er\015o)m(w)j(based)e (attac)m(ks.)42 b(The)27 b(protot)m(yp)s(e)298 2521 y(k)m(ernel)36 b(has)g(b)s(een)f(in)g(pro)s(duction)f(for)i(six)f(mon)m(ths)g(in)g(t)m (w)m(o)j(organizations)d(\(Rome)i(Univ)m(ersit)m(y)f(\\La)298 2634 y(Sapienza")26 b(and)g(Institute)f(for)h(Computing)f (Applications\))f(and)i(so)g(far)g(no)g(fault)f(due)h(to)h(our)e(patc)m (hes)298 2747 y(has)32 b(b)s(een)g(rep)s(orted)g(b)m(y)g(the)h(users.) 46 b(The)32 b(future)f(activities)h(in)f(the)i(short,)g(medium)d(and)i (long)g(term)298 2860 y(can)f(b)s(e)f(summarized)f(as)h(follo)m(ws:)435 3036 y Fi(\017)46 b Fk(Curren)m(tly)30 b(w)m(e)k(are)f(w)m(orking)f(on) g(the)h(A)m(CD)g(for)g(the)f(system)h(calls)f(related)h(to)g(the)g (\014lesystem.)526 3149 y(With)23 b(a)h(bu\013er)f(o)m(v)m(er\015o)m (w,)j(it)e(is)e(p)s(ossible)g(to)i(execute,)j(instructions)21 b(that,)26 b(for)e(instance,)h(remo)m(v)m(e)526 3262 y(the)e(ro)s(ot)h(passw)m(ord)f(from)h(/etc/passwd)g(\(and/or)g (/etc/shado)m(w\),)k(add)23 b(to)h(the)g(same)g(\014le)f(a)h(new)526 3375 y(user)30 b(with)f(UID=0)i(or)g(add)f(a)h(fak)m(e)g Fh(.rhosts)h Fk(\014le)e(to)h(the)g(ro)s(ot)g(home)g(directory)-8 b(.)41 b(Although)30 b(less)526 3488 y(common,)42 b(these)e(exploits)f (are,)k(b)m(y)c(no)h(means,)i(less)d(dangerous)g(than)h(those)g(based)f (on)h(the)526 3601 y(\\classic")32 b Fh(shel)5 b(lc)-5 b(o)g(de)p Fk(.)47 b(An)m(y)32 b(attac)m(k)j(that)d(in)m(v)m(olv)m(es)g (a)h(\014le)e(requires)f(at)j(least)f(t)m(w)m(o)i(system)e(calls.)526 3714 y(A)26 b(\014rst)g(one)g(to)h(op)s(en)f(the)h(\014le)e(and)h(a)h (second)f(one)h(to)g(mo)s(dify)d(it.)39 b(W)-8 b(e)28 b(are)e(going)h(to)g(add)f(sp)s(ecial)526 3827 y(c)m(hec)m(ks)k(to)f (the)g Fg(open)f Fk(primitiv)m(e)e(to)k(prev)m(en)m(t)f(a)g (compromised)f(pro)s(cess)g(from)g(using)g(dangerous)526 3940 y(c)m(hains)h(of)i(system)f(calls.)526 4084 y(Building)22 b(the)j(A)m(CD)h(for)f(the)g Fg(open)f Fk(requires)g(sp)s(ecial)g (atten)m(tion)i(b)s(ecause)f(w)m(e)h(need)f(to)h(tak)m(e)g(in)m(to)526 4197 y(accoun)m(t)33 b(a)f(n)m(um)m(b)s(er)f(of)h(di\013eren)m(t)f (situations.)44 b(First)32 b(of)g(all)f(sev)m(eral)h(setuid)e(programs) i(or)g(ro)s(ot)526 4310 y(daemons)h(ma)m(y)i(op)s(en,)g(for)e(go)s(o)s (d)h(reasons,)h(critical)e(\014les.)51 b(F)-8 b(or)35 b(the)f(time)g(b)s(eing)e(w)m(e)j(are)f(going)526 4423 y(to)d(monitor)e(just)h(the)h(access)g(to)g(\014les)f(in)f(write-mo)s (de.)40 b(A)m(t)31 b(times)f(it)g(ma)m(y)h(b)s(e)e(necessary)i(to)g (let)526 4536 y(ro)s(ot)25 b(programs)f(write)g(an)m(y)h(\014le)f(in)f (sp)s(eci\014c)h(directories)f(de\014ned)h(in)f(the)i(A)m(CD.)h(This)d (is)g(the)i(case)526 4648 y(of)k(directories)f(lik)m(e)h Fh(/var/mail)j Fk(or)d Fh(/var/sp)-5 b(o)g(ol/mqueue)33 b Fk(used)c(b)m(y)g Fg(sendmail)p Fk(.)38 b(F)-8 b(or)30 b(directories)526 4761 y(lik)m(e)h Fh(/etc)h Fk(a)g(\014ne-grain)e(con) m(trol)i(is)f(required.)43 b(So)32 b(a)g(\014le)f(b)s(elonging)f(to)i Fh(/etc)g Fk(ma)m(y)g(b)s(e)f(mo)s(di\014ed)526 4874 y(just)e(in)g(case)j(the)e(\014le)g(name)g(is)g(inserted)f(in)g(the)i (A)m(CD.)526 5019 y(W)-8 b(e)26 b(recall)f(that)i(the)f(c)m(hec)m(ks)h (are)f(enforced)f(on)h(setuid)e(or)i(daemon)g(programs)f(just)g(in)f (case)j(they)526 5132 y(in)m(v)m(ok)m(e)40 b(critical)f(system)g(calls) g(k)m(eeping)h(EUID=0)g(\(that)g(is)f(with)f(their)h(sp)s(ecial)f (privileges\).)2053 5381 y(17)p eop %%Page: 18 18 18 17 bop 526 123 a Fk(Since)33 b(most)j(of)f(the)g(times)f(these)i (programs)e(access)i(\014les)e(in)g(write)g(mo)s(de)g(with)g(EUID)h (equal)526 236 y(to)30 b(the)f(o)m(wner)g(of)h(the)f(\014le,)g(the)h (access)g(to)g(user)f(\014les)f(do)s(es)h(not)h(need)f(to)h(b)s(e)e (authorized)h(b)m(y)g(the)526 349 y(reference)h(monitor.)526 499 y(A)f(detailed)f(study)h(of)g(setuid)f(and)h(daemon)g(programs)g (is)f(in)g(progress)h(to)h(de\014ne)f(whic)m(h)f(direc-)526 612 y(tories)36 b(and)g(\014les)g(m)m(ust)h(b)s(e)f(included)e(in)h (the)i(A)m(CD.)h(This)d(is)h(accomplished)f(b)m(y)i(b)s(oth)f(source) 526 725 y(co)s(de)h(insp)s(ection)f(and)h(analysis)f(of)h(syslog)h (\014les)e(\(if)h(there)h(is)e(an)h(attempt)i(to)f(op)s(en)f(a)h (\014le)f(in)526 838 y(write)c(mo)s(de)g(and)h(the)g(\014le)f(is)g (unkno)m(wn)g(to)h(the)h(A)m(CD,)f(the)h(op)s(en)e(fails)f(and)i(the)g (\014le)f(name)h(is)526 951 y(prin)m(ted)29 b(in)g(the)h(syslog\).)435 1138 y Fi(\017)46 b Fk(In)38 b(the)g(medium)f(term,)k(w)m(e)f(exp)s (ect)f(to)g(add)f(\\reaction")i(capabilities)d(to)i(our)g(in)m(trusion) d(de-)526 1251 y(tection)g(mec)m(hanism.)56 b(The)36 b(starting)f(p)s(oin)m(t)g(is)g(to)i(dev)m(elop)e(a)i(k)m(ernel)e (subsystem)g(to)h(manage)526 1364 y(in)m(trusion)f(attempts.)62 b(Simple)35 b(systems)i(lik)m(e)g(the)g Fh(c)-5 b(age)38 b Fk([2])g(ha)m(v)m(e)g(b)s(een)f(already)g(used)f(in)g(the)526 1477 y(past)31 b(to)i(analyze)f(the)f(in)m(truders')f(activities)h(in)g (progress)g(without)f(let)i(them)f(notice)h(it.)44 b(Ho)m(w-)526 1590 y(ev)m(er)35 b(those)g(systems)g(w)m(ere)g(not)g(activ)-5 b(ated)35 b(on)g(the)g(\015y)f(during)e(the)j(in)m(trusion)d(attempt.) 55 b(The)526 1703 y(real-time)25 b(in)m(trusion)e(handling)g(mec)m (hanism)h(w)m(e)i(ha)m(v)m(e)h(in)d(mind)f(requires)h(the)i(migration)e (of)i(the)526 1816 y(o\013ending)g(pro)s(cess)h(to)h(a)g(distinct)e (system)h(designed)f(to)i(repro)s(duce)f(the)g(original)f(en)m (vironmen)m(t)526 1929 y(as)i(faithful)f(as)i(p)s(ossible.)38 b(W)-8 b(e)29 b(are)g(curren)m(tly)f(in)m(v)m(estigating)h(whic)m(h)e (is)h(the)h(b)s(est)f(w)m(a)m(y)h(to)h(imple-)526 2041 y(men)m(t)k(this)f(tec)m(hnique.)51 b(A)34 b(p)s(ossible)e(application) g(of)i(suc)m(h)g(in)m(trusion)e(handling)f(subsystem)i(is)526 2154 y(to)e(analyze)f(p)s(ossible)e(collusions)g(among)j(distinct)e(in) m(truders.)435 2342 y Fi(\017)46 b Fk(In)32 b(the)h(long)f(term,)i(w)m (e)g(w)m(ould)d(lik)m(e)i(to)g(p)s(ort)g(the)g Fh(p)-5 b(atches)34 b Fk(w)m(e)g(dev)m(elop)s(ed)e(for)h(Lin)m(ux)e(to)j(other) 526 2455 y(Unix)29 b(\015a)m(v)m(ors)i(whose)f(source)h(co)s(de)f(is)g (a)m(v)-5 b(ailable)29 b(\(xxBSD,)j(Solaris\).)298 2741 y Fj(References)344 2944 y Fk([1])46 b(Aleph)29 b(One,)i(\\Smashing)e (The)g(Stac)m(k)j(F)-8 b(or)31 b(F)-8 b(un)30 b(And)g(Pro\014t",)485 3057 y(Phrac)m(k)h(Magazine,)h(V)-8 b(ol.)30 b(7,)h(N.)g(49,)h(1996.) 344 3245 y([2])46 b(Bello)m(vin)34 b(S.)h(M.,)i(Cheswic)m(k)69 b(W.)35 b(R,)h(\\Firew)m(alls)e(and)g(In)m(ternet)h(Securit)m(y",)i (Addison)c(W)-8 b(esley)g(,)485 3358 y(1994.)344 3545 y([3])46 b(Co)m(w)m(an,)30 b(C.)f Fh(et)j(al.)p Fk(,)e(\\Bu\013er)g(Ov) m(er\015o)m(ws:)40 b(A)m(ttac)m(ks)31 b(and)e(Defenses)h(for)f(the)g(V) -8 b(ulnerabilit)m(y)27 b(of)i(the)485 3658 y(Decade",)k(to)e(app)s 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y([16])g(h)m(ttp://www.iac.rm.cnr.it/tecno/soft)m(w)m (are/indexsoft)m(w)m(are.h)m(tml)298 2288 y([17])g(W.R.)31 b(Stev)m(ens,)485 2401 y(\\Unix)f(Net)m(w)m(ork)i(Programming",)e(I)s (I)f(edition,)h(Pren)m(tice)g(Hall)g(1998.)298 2587 y([18])47 b(R.)24 b(Sek)-5 b(ar,)25 b(P)-8 b(.)24 b(Uppuluri)c(\\Syn)m(thesizing) j(F)-8 b(ast)25 b(In)m(trusion)d(Prev)m(en)m(tion/Detection)j(Systems)f (from)485 2700 y(High)30 b(Lev)m(el)h(Sp)s(eci\014cations",)e(The)h(8) 1815 2667 y Fa(th)1916 2700 y Fk(USENIX)g(Securit)m(y)g(Symp)s(osium)d (\(Securit)m(y)j(99\).)298 2886 y([19])47 b(R.)31 b(Sek)-5 b(ar,)30 b(T.)g(Bo)m(w)m(en)i(and)e(M.)h(Segal,)485 2999 y(\\On)j(Prev)m(en)m(ting)h(In)m(trusions)e(b)m(y)h(Pro)s(cess)h(Beha)m (vior)g(Monitoring",)g(W)-8 b(orkshop)35 b(on)f(In)m(trusion)485 3112 y(Detection)e(and)e(Net)m(w)m(ork)i(Monitoring)d(\(ID)i('99\))h (of)e(the)h(USENIX)f(Asso)s(ciation.)298 3398 y Fj(7)135 b(App)t(endix:)59 b(the)45 b(c)l(hec)l(k)p 1731 3398 41 4 v 49 w(ro)t(otpro)t(c)g(function)298 3579 y Fb(int)40 b(check_rootproc\(struct)k(inode)d(*ino\))f({)416 3670 y(int)g(cont=0,iproc=0,error=0;)416 3762 y(suidp_t)h(*)e(suidproc;)416 3853 y(efile_t)i(f;)416 3944 y(suidp_t)g(p;)455 4127 y(if)f(\(\(IS_SETUID_TO_ROOT\(current\)\))q(||\(IS)q(_A_R)q(OOT_D)q (AEMO)q(N\(cu)q(rrent)q(\)\)\))85 b({)534 4218 y(for)40 b(\(;cont)q(i_de)q(v\)&&) 730 4401 y(\(permitted.lst[cont].efid.inod)q(e==i)q(no->i)q(_ino)q (\)\))f({)769 4492 y(if\(\(permitted.lst[cont].efid.)q(size)q(==ino)q (->i_)q(size)q(\)&&)887 4584 y(\(permitted.lst[cont].efid.modi)q(f==in) q(o->i)q(_mti)q(me\)\))h({)926 4675 y(suidproc=permitted.lst[cont].)q (proce)q(sses)q(;)926 4766 y(for)40 b(\(iproc=1;iproc<=permitted.lst[)q (cont)q(].pr)q(oc_nr)q(;ipr)q(oc++\))85 b({)1122 4858 y(if)40 b(\(!strcmp\(suidproc->suidp_id.co)q(mm,c)q(urren)q(t->c)q (omm\)\))46 b({)1240 4949 y(suidproc->suidp_id.count++;)1240 5040 y(return)40 b(PSA;)1122 5132 y(})2053 5381 y Fk(19)p eop %%Page: 20 20 20 19 bop 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(\(!atomic_access\(&writer_pid)q(,cur)q(rent-)q(>pid)q(\)\);)1044 3045 y(/*)f(start)i(of)e(critical)i(section)g(*/)1044 3137 y(do_setuid_put\(&\(f.efid\),&\(p.sui)q(dp_i)q(d\),F)q(AILUR)q (E\);)1044 3228 y(writer_pid=0;)h(/*)e(end)f(of)h(critical)h(section)g (*/)1044 3319 y(atomic_access\(&writer_pid,0\);)k(/*)40 b(release)h(of)f(the)g(lock)g(*/)1044 3411 y(return)g(error;)298 3502 y(})2053 5381 y Fk(20)p eop %%Trailer end userdict /end-hook known{end-hook}if %%EOF