On 08/07/2018 01:20 PM, George Dunlap wrote:
> 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?
LGTM.
Note also that Andy's patch had slightly longer commit message
(including some of the tags that you are missing), with this suggestion:
[ Note to stable maintainers: this should probably get applied to all
kernels. If you're nervous about that, a more conservative fix to
add xorl %ebx,%ebx; incl %ebx before the jump to error_exit should
also fix the problem. ]
-boris
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