On Mon, Aug 6, 2018 at 8:10 PM, Chris Brannon <c...@prgmr.com> wrote:
> I just got the following patch from a colleague. It's a backport of
> the XSA 274 kernel patch to 4.9.x kernels. The kernel patch given in
> the XSA would not apply cleanly. Would someone mind reviewing it? It
> would be much appreciated.
>
> commit b3681dd548d06deb2e1573890829dff4b15abf46 upstream.
>
> This version applies to v4.9.
>
> error_entry and error_exit communicate the user vs kernel status of
> the frame using %ebx. This is unnecessary -- the information is in
> regs->cs. Just use regs->cs.
>
> This makes error_entry simpler and makes error_exit more robust.
>
> It also fixes a nasty bug. Before all the Spectre nonsense, The
> xen_failsafe_callback entry point returned like this:
>
> ALLOC_PT_GPREGS_ON_STACK
> SAVE_C_REGS
> SAVE_EXTRA_REGS
> ENCODE_FRAME_POINTER
> jmp error_exit
>
> And it did not go through error_entry. This was bogus: RBX
> contained garbage, and error_exit expected a flag in RBX.
> Fortunately, it generally contained *nonzero* garbage, so the
> correct code path was used. As part of the Spectre fixes, code was
> added to clear RBX to mitigate certain speculation attacks. Now,
> depending on kernel configuration, RBX got zeroed and, when running
> some Wine workloads, the kernel crashes. This was introduced by:
>
> commit 3ac6d8c787b8 ("x86/entry/64: Clear registers for
> exceptions/interrupts, to reduce speculation attack surface")
>
> With this patch applied, RBX is no longer needed as a flag, and the
> problem goes away.
>
> Cc: Brian Gerst <brge...@gmail.com>
> Cc: Borislav Petkov <b...@alien8.de>
> Cc: Dominik Brodowski <li...@dominikbrodowski.net>
> Cc: Ingo Molnar <mi...@redhat.com>
> Cc: "H. Peter Anvin" <h...@zytor.com>
> Cc: Thomas Gleixner <t...@linutronix.de>
> Cc: Boris Ostrovsky <boris.ostrov...@oracle.com>
> Cc: Juergen Gross <jgr...@suse.com>
> Cc: xen-devel@lists.xenproject.org
> Cc: x...@kernel.org
> Cc: sta...@vger.kernel.org
> Cc: Andy Lutomirski <l...@kernel.org>
> Fixes: 3ac6d8c787b8 ("x86/entry/64: Clear registers for
> exceptions/interrupts, to reduce speculation attack surface")
> Reported-and-tested-by: "M. Vefa Bicakci" <m....@runbox.com>
> Signed-off-by: Sarah Newman <s...@prgmr.com>
I think you need to retain Andy's SoB, and add your own underneath.
This looks plausible to me -- Andy / Boris, any opinions?
-George
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