Date: Tue, 16 Jan 2007 21:43:58 +1100 (EST) From: Bruce Evans <bde@zeta.org.au> To: Pawel Jakub Dawidek <pjd@FreeBSD.org> 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: <20070116211016.T6114@delplex.bde.org> In-Reply-To: <20070116084243.GA1117@garage.freebsd.pl> 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> <20070116084243.GA1117@garage.freebsd.pl>
next in thread | previous in thread | raw e-mail | index | archive | help
On Tue, 16 Jan 2007, Pawel Jakub Dawidek wrote: > On Tue, Jan 16, 2007 at 02:42:17PM +1100, Bruce Evans wrote: >> install -S ... >> ... 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. I did forget it. I just assumed that doing the same thing as mkstemp() is as secure as possible, and it is. Old versions of mkstemp() couldn't use O_NOFOLLOW since O_NOFOLLOW has only existed since Y2K. New versions don't use it because it is unnecessary. Exclusive access isn't enough for security since if open() followed a dangling link it would create a security hole with (O_CREAT | O_EXCL). But there is no problem since O_EXCL implies not following symlinks even if O_NOFOLLOW is not supported. This is documented in open(2) and better documented in POSIX. Bruce
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20070116211016.T6114>