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

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

On 23/08/2023 2:31 pm, Roger Pau Monné wrote:
> On Wed, Aug 23, 2023 at 12:56:48PM +0100, Andrew Cooper wrote:
>> 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.
> Right, but taking this patch without also having 81d3a0b26c1 will lead
> to a vulnerable system, hence why I think the dependency would better
> be on 81d3a0b26c1.
>
> Anyway, I don't think it's worth arguing over, so if you want to leave
> it as-is I won't object.

We don't really have a depends-on tag, or only-safe-with or whatever,
and a change like that is stretching the definition of Fixes IMO.

81d3a0b26c1 is more than 7 years old now, so there's not going to be a
practical problem backporting.  For everything else, I expect people to
read the commit message and apply common sense.

~Andrew