Re: [Xen-devel] backport of XSA-274 patch to 4.9.x kernel (could use a review)
> On Aug 7, 2018, at 11:49 AM, Boris Ostrovsky
> wrote:
>
>> On 08/07/2018 01:20 PM, George Dunlap wrote:
>>> On Mon, Aug 6, 2018 at 8:10 PM, Chris Brannon 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
>>> Cc: Borislav Petkov
>>> Cc: Dominik Brodowski
>>> Cc: Ingo Molnar
>>> Cc: "H. Peter Anvin"
>>> Cc: Thomas Gleixner
>>> Cc: Boris Ostrovsky
>>> Cc: Juergen Gross
>>> Cc: xen-devel@lists.xenproject.org
>>> Cc: x...@kernel.org
>>> Cc: sta...@vger.kernel.org
>>> Cc: Andy Lutomirski
>>> Fixes: 3ac6d8c787b8 ("x86/entry/64: Clear registers for
>>> exceptions/interrupts, to reduce speculation attack surface")
>>> Reported-and-tested-by: "M. Vefa Bicakci"
>>> Signed-off-by: Sarah Newman
>> 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. ]
>
On further review, I don’t like that suggestion. What if the callback came from
user code. It’s not supposed to happen on modern kernels, but I’m not sure I
trust that.
>
>
>
> -boris
>
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Re: [Xen-devel] backport of XSA-274 patch to 4.9.x kernel (could use a review)
On 08/07/2018 01:20 PM, George Dunlap wrote:
> On Mon, Aug 6, 2018 at 8:10 PM, Chris Brannon 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
>> Cc: Borislav Petkov
>> Cc: Dominik Brodowski
>> Cc: Ingo Molnar
>> Cc: "H. Peter Anvin"
>> Cc: Thomas Gleixner
>> Cc: Boris Ostrovsky
>> Cc: Juergen Gross
>> Cc: xen-devel@lists.xenproject.org
>> Cc: x...@kernel.org
>> Cc: sta...@vger.kernel.org
>> Cc: Andy Lutomirski
>> Fixes: 3ac6d8c787b8 ("x86/entry/64: Clear registers for
>> exceptions/interrupts, to reduce speculation attack surface")
>> Reported-and-tested-by: "M. Vefa Bicakci"
>> Signed-off-by: Sarah Newman
> 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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Re: [Xen-devel] backport of XSA-274 patch to 4.9.x kernel (could use a review)
On Mon, Aug 6, 2018 at 8:10 PM, Chris Brannon 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
> Cc: Borislav Petkov
> Cc: Dominik Brodowski
> Cc: Ingo Molnar
> Cc: "H. Peter Anvin"
> Cc: Thomas Gleixner
> Cc: Boris Ostrovsky
> Cc: Juergen Gross
> Cc: xen-devel@lists.xenproject.org
> Cc: x...@kernel.org
> Cc: sta...@vger.kernel.org
> Cc: Andy Lutomirski
> Fixes: 3ac6d8c787b8 ("x86/entry/64: Clear registers for
> exceptions/interrupts, to reduce speculation attack surface")
> Reported-and-tested-by: "M. Vefa Bicakci"
> Signed-off-by: Sarah Newman
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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