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

From: "Roger Pau Monné" <roger.pau@citrix.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Xen-devel <xen-devel@lists.xenproject.org>,
	Jan Beulich <JBeulich@suse.com>, Wei Liu <wl@xen.org>,
	Jun Nakajima <jun.nakajima@intel.com>,
	Kevin Tian <kevin.tian@intel.com>
Subject: Re: [PATCH] x86/vmx: Revert "x86/VMX: sanitize rIP before re-entering guest"
Date: Wed, 23 Aug 2023 15:31:21 +0200	[thread overview]
Message-ID: <ZOYKKS60iWq6Zx51@MacBook-Air-de-Roger.local> (raw)
In-Reply-To: <327109c1-f8ad-f8c4-01b9-ea19dc5df247@citrix.com>

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.