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.
Thanks, Roger.
