Date: Tue, 16 Jan 2007 09:42:43 +0100 From: Pawel Jakub Dawidek <pjd@FreeBSD.org> To: Bruce Evans <bde@zeta.org.au> Cc: freebsd-security@freebsd.org, Dirk Engling <erdgeist@erdgeist.org>, Colin Percival <cperciva@freebsd.org> Subject: Re: HEADS UP: Re: FreeBSD Security Advisory FreeBSD-SA-07:01.jail Message-ID: <20070116084243.GA1117@garage.freebsd.pl> In-Reply-To: <20070116133259.N5056@delplex.bde.org> References: <45A6DB76.40800@freebsd.org> <20070113112937.GI90718@garage.freebsd.pl> <45ABDC7C.6060407@erdgeist.org> <20070115210826.GA2839@garage.freebsd.pl> <45ABEEEE.4030609@erdgeist.org> <20070115220039.GB2839@garage.freebsd.pl> <45AC29EA.70009@erdgeist.org> <45AC2E9F.20901@freebsd.org> <45AC35A6.7090103@erdgeist.org> <20070116133259.N5056@delplex.bde.org>
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--mP3DRpeJDSE+ciuQ Content-Type: text/plain; charset=iso-8859-2 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Jan 16, 2007 at 02:42:17PM +1100, Bruce Evans wrote: > On Tue, 16 Jan 2007, Dirk Engling wrote: >=20 > >Colin Percival wrote: > > > >>No. `cp -f` unlinks the existing file and creates a new file, but will > >>still follow a symlink if one is created between the "unlink" syscall a= nd > >>the "open" syscall. > >... > >You are right. Atomically in binary is not atomical enough. > > > >mv in its rename()-form will do the job, so we need to create a file in > >. by mktemp and mv it to the real name when filled. >=20 > install -S already implements this, but not robustly enough to be secure. > It only creates the temporary file if the target doesn't already exists, > so it is subject to the usual races otherwise. 'S' stands for "safe" > (no-clobber), not secure, so this is reasonable. However, it can easily > be made both safer (actually no-clobber) and securer by opening the file > with O_EXCL and exiting if the file exists at the time of the open. > Perhaps cp -f should do the same. (Both have paths where they do a > forced unlink() followed by an open(). This open() can easily use O_EXCL= ). Interesting. I was sure it won't work as you described, because the target file can be a symlink and open(2) by default follows symlinks. I thought that you just forget about O_NOFOLLOW flag, but it seems, that with O_EXCL open(2) doesn't follow symlinks so it will work. > mv(1) can never be trusted to use its rename() form since it uses > copying to move across file systems and there is no way to control this. > mv(1)'s rewriting of "mv file dir" to "rename file dir/file" is also > a problem (I keep rename(1) handy to avoid it). I haven't followed > most of this thread so I don't know what the attacker can do here. > Changing the target to a symlink to a directory on a different file > system would exploit both of the problems in mv. That's true. Dirk's proposal is to create a file with mktemp(1) in the same directory where we're going to rename(2) the file, but I don't think mktemp(1) will be safe here: good-guy attacker-within-a-jail cd /jail/var/log mktemp foo.XXX rm -f foo.XXX ln -s /etc/spwd.db foo.XXX copy /path/to/jail_console.log foo.XXX mv -f foo.XXX console.log --=20 Pawel Jakub Dawidek http://www.wheel.pl pjd@FreeBSD.org http://www.FreeBSD.org FreeBSD committer Am I Evil? Yes, I Am! --mP3DRpeJDSE+ciuQ Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (FreeBSD) iD8DBQFFrJADForvXbEpPzQRAoJJAJ4phBMyAMl1ZhWnj/3r3cNeWDnChQCgoWGx TxRmmvgdjgWsVF+VOosFaXU= =vPbG -----END PGP SIGNATURE----- --mP3DRpeJDSE+ciuQ--
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