Re: [PATCH] x86/vmx: Revert "x86/VMX: sanitize rIP before re-entering guest"

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 23/08/2023 12:15 pm, Roger Pau Monné wrote:
> On Wed, Apr 05, 2023 at 10:52:45PM +0100, Andrew Cooper wrote:
>> At the time of XSA-170, the x86 instruction emulator was genuinely broken.  It
>> would load arbitrary values into %rip and putting a check here probably was
>> the best stopgap security fix.  It should have been reverted following c/s
>> 81d3a0b26c1 "x86emul: limit-check branch targets" which corrected the emulator
>> behaviour.
>>
>> However, everyone involved in XSA-170, myself included, failed to read the SDM
>> correctly.  On the subject of %rip consistency checks, the SDM stated:
>>
>>   If the processor supports N < 64 linear-address bits, bits 63:N must be
>>   identical
>>
>> A non-canonical %rip (and SSP more recently) is an explicitly legal state in
>> x86, and the VMEntry consistency checks are intentionally off-by-one from a
>> regular canonical check.
>>
>> The consequence of this bug is that Xen will currently take a legal x86 state
>> which would successfully VMEnter, and corrupt it into having non-architectural
>> behaviour.
>>
>> Furthermore, in the time this bugfix has been pending in public, I
>> successfully persuaded Intel to clarify the SDM, adding the following
>> clarification:
>>
>>   The guest RIP value is not required to be canonical; the value of bit N-1
>>   may differ from that of bit N.
>>
>> Fixes: ffbbfda377 ("x86/VMX: sanitize rIP before re-entering guest")
> I think the fixes tag should likely be "x86emul: limit-check branch
> targets", since it's that commit that missed the revert done here?

Well, not really.  ffbbfda377 really does have a bug, irrespective of
the changes in the emulator.

The presence of 81d3a0b26c1 is why this bugfix is a full revert of
ffbbfda377, and not just an off-by-1 adjustment.

>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> Acked-by: Roger Pau Monné <roger.pau@citrix.com>

Thanks.

~Andrew