Re: [Qemu-devel] [PATCH] qed: fix use-after-free during l2 cache commit

[Amit Shah]

On (Fri) 30 Sep 2011 [16:23:30], Stefan Hajnoczi wrote:
> On Fri, Sep 30, 2011 at 12:27 PM, Amit Shah <address@hidden> wrote:
> > 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.  The first one to complete will populate the metadata
> >> 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.  Fix this by
> >> keeping the l2_offset in a local variable and not reaching into the
> >> possibly freed l2_table.
> >>
> >> Reported-by: Amit Shah <address@hidden>
> >> Signed-off-by: Stefan Hajnoczi <address@hidden>
> >> ---
> >> 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.
> 
> 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/Fedora-16-Beta-x86_64-Live-LXDE.iso


                Amit