From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:58139) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1R9fLK-0008B3-TA for qemu-devel@nongnu.org; Fri, 30 Sep 2011 11:49:44 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1R9fLJ-0005Ac-5P for qemu-devel@nongnu.org; Fri, 30 Sep 2011 11:49:42 -0400 Received: from mx1.redhat.com ([209.132.183.28]:28878) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1R9fLI-0005AW-SF for qemu-devel@nongnu.org; Fri, 30 Sep 2011 11:49:41 -0400 Date: Fri, 30 Sep 2011 21:19:30 +0530 From: Amit Shah Message-ID: <20110930154930.GJ21901@amit-x200.redhat.com> References: <1317379151-11557-1-git-send-email-stefanha@linux.vnet.ibm.com> <20110930112759.GI21901@amit-x200.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH] qed: fix use-after-free during l2 cache commit List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: Anthony Liguori , Stefan Hajnoczi , qemu-devel@nongnu.org On (Fri) 30 Sep 2011 [16:23:30], Stefan Hajnoczi wrote: > On Fri, Sep 30, 2011 at 12:27 PM, Amit Shah wrot= e: > > On (Fri) 30 Sep 2011 [11:39:11], Stefan Hajnoczi wrote: > >> QED's metadata caching strategy allows two parallel requests to race= for > >> metadata lookup. =A0The first one to complete will populate the meta= data > >> cache and the second one will drop the data it just read in favor of= the > >> cached data. > >> > >> There is a use-after-free in qed_read_l2_table_cb() and > >> qed_commit_l2_update() where l2_table->offset was used after the > >> l2_table may have been freed due to a metadata lookup race. =A0Fix t= his by > >> keeping the l2_offset in a local variable and not reaching into the > >> possibly freed l2_table. > >> > >> Reported-by: Amit Shah > >> Signed-off-by: Stefan Hajnoczi > >> --- > >> Hi Amit, > >> Thanks for reporting the assertion failure you saw at http://fpaste.= org/CDuv/. > >> Does this patch fix the problem? > > > > Yes, this fixes it. >=20 > Were you able to reliably reproduce the assertion failure before? Absolutely. I even reverted the patch and tried the same image; same segfault again. > I wonder because this only happens when two metadata lookups race > (which is rare enough on my setup that I've never seen this failure). > It might be worth trying a few times. Get the F16 beta-rc LXE live iso, install guest. It doesn't cleanly reboot, you have to kill the VM. Next start of the VM produces this segfault. https://alt.fedoraproject.org/pub/alt/stage/16-Beta.RC2/Live/x86_64/Fedor= a-16-Beta-x86_64-Live-LXDE.iso Amit