Re: [Xen-devel] backport of XSA-274 patch to 4.9.x kernel (could use a review)

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 08/07/2018 01:20 PM, George Dunlap wrote:
> On Mon, Aug 6, 2018 at 8:10 PM, Chris Brannon <cmb@xxxxxxxxx> 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 <brgerst@xxxxxxxxx>
>> Cc: Borislav Petkov <bp@xxxxxxxxx>
>> Cc: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
>> Cc: Juergen Gross <jgross@xxxxxxxx>
>> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
>> Cc: x86@xxxxxxxxxx
>> Cc: stable@xxxxxxxxxxxxxxx
>> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
>> Fixes: 3ac6d8c787b8 ("x86/entry/64: Clear registers for 
>> exceptions/interrupts, to reduce speculation attack surface")
>> Reported-and-tested-by: "M. Vefa Bicakci" <m.v.b@xxxxxxxxxx>
>> Signed-off-by: Sarah Newman <srn@xxxxxxxxx>
> 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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