Date: Tue, 22 Mar 2011 07:52:02 -0400 From: Dan Rosenberg <dan.j.rosenberg@...il.com> To: Tomas Hoger <thoger@...hat.com> Cc: oss-security@...ts.openwall.com, Ludwig Nussel <ludwig.nussel@...e.de>, Petr Baudis <pasky@...e.cz> Subject: Re: Suid mount helpers fail to anticipate RLIMIT_FSIZE On Tue, Mar 22, 2011 at 5:48 AM, Tomas Hoger <thoger@...hat.com> wrote: > On Mon, 14 Mar 2011 12:31:18 -0400 Dan Rosenberg wrote: > >> I've done some further investigation, and have found one of the >> underlying problems. addmntent() will return 0 (success) even if the >> write was truncated: >> >> return (fprintf (stream, "%s %s %s %s %d %d\n", >> mntcopy.mnt_fsname, >> mntcopy.mnt_dir, >> mntcopy.mnt_type, >> mntcopy.mnt_opts, >> mntcopy.mnt_freq, >> mntcopy.mnt_passno) >> < 0 ? 1 : 0); > > I must admit that I fail to see an obvious issue here. This should do > the right thing assuming fprintf returns what you expect (which does > not seem to happen due to stdio buffering). > You're right, I neglected to consider stdio buffering, so the fprintf() return code will always be the full number of characters written, even if they weren't actually written yet. >> Of course, this only matters if the process is catching the SIGXFSZ >> that gets thrown if the resource limit is exceeded, but nearly all >> suid mount helpers block or ignore signals (if they don't, that's an >> additional problem, because the process could be terminated mid-write, >> corrupting /etc/mtab or leaving a stale lockfile, for example). >> >> So, I think the first step is to patch glibc to return success in >> these functions if and only if the *full* contents have been written. >> Then, it will be possible to have proper error handling in these >> helper utilities. Currently, there's really no way for these programs >> to know whether or not their calls to addmntent() actually succeeded >> besides installing a special signal handler for SIGXFSZ (ugly). > > Do you have any specific idea for the fix? It seems following approach > may work: > > if (fprintf (stream, "%s %s %s %s %d %d\n", ...) < 0) > return 1; > > return (fflush(stream) == 0 ? 0 : 1); > This may work. I'll do some testing later today. > Detecting this error in endmntent() seems more problematic API-wise, > given that endmntent() currently "always returns 1". > > Do you plan to open bug in glibc bugzilla for this issue? > Sure, I'll open one today. Thanks, Dan > -- > Tomas Hoger / Red Hat Security Response Team >
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