Date: Thu, 25 Aug 2011 17:54:23 +0200 From: Yves-Alexis Perez <corsac@...ian.org> To: 639151@...s.debian.org Cc: Moritz Muehlenhoff <jmm@...ian.org>, robert.ancell@...onical.com, Sebastian Krahmer <krahmer@...e.de>, oss-security@...ts.openwall.com Subject: Re: [Pkg-xfce-devel] Bug#639151: Bug#639151: Bug#639151: Local privilege escalation On mer., 2011-08-24 at 20:55 +0200, Yves-Alexis Perez wrote: > And, out of curiosity, how would you achieve privilege escalation? You > should be able to erase/rewrite arbitrary files, including /etc/shadow, > but you don't really have control on what's written there. In gdm (CVE-2011-0727 I guess) the issue was that a g_file_copy() was run as root from files under user control (.dmrc and the avatar), to a cache dir with write permissions (afaict). So it was easy to put whatever stuff you need in the original file and make a symlink to /etc/shadow in the destination folder so the g_file_copy() would erase that: res = g_file_copy (src_file, dst_file, G_FILE_COPY_OVERWRITE | G_FILE_COPY_NOFOLLOW_SYMLINKS, NULL, NULL, NULL, &error); I'm not too sure what G_FILE_COPY_OVERWRITE means, if it truncate()s and write over of if it unlink()s and start fresh (digging in glib to find out). Apparenlty in the fallback case (not sure if it's the case here) it ends up doing a g_file_replace()). In any case, in lightdm case, for .Xauthority file it uses g_file_replace() which creates a temporary file and then rename over the new file, so in the worst case you overwrite a system file with xauthority data. Same thing for .dmrc, you can overwrite system files but with dmrc data which look like [Desktop] Session=xfce Lang=fr_FR.UTF-8 so it doesn't look easy to gain root access with that. LightDM maintains a cache for dmrc files in /var/cache/lightdm but the folder is created 0700 so it doesn't look like one can put symlinks there and have it use a user-controled .dmrc. All in all, I'm not too sure there's a privilege escalation for Xauthority/.dmrc files (but if one exists, I'm interested in how to do it, by curiosity). But you still damage pretty much any arbitrary file, which is still an easy DoS. Regards, -- Yves-Alexis Download attachment "signature.asc" of type "application/pgp-signature" (837 bytes)
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