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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 13/10/2020 16:58, Jan Beulich wrote:
> On 09.10.2020 17:09, Andrew Cooper wrote:
>> At the time of XSA-170, the x86 instruction emulator really was broken, and
>> would allow arbitrary non-canonical values to be loaded into %rip.  This was
>> fixed after the embargo by c/s 81d3a0b26c1 "x86emul: limit-check branch
>> targets".
>>
>> However, in a demonstration that off-by-one errors really are one of the
>> hardest programming issues we face, everyone involved with XSA-170, myself
>> included, mistook the statement in the SDM which says:
>>
>>   If the processor supports N < 64 linear-address bits, bits 63:N must be 
>> identical
>>
>> to mean "must be canonical".  A real canonical check is bits 63:N-1.
>>
>> VMEntries really do tolerate a not-quite-canonical %rip, specifically to 
>> cater
>> to the boundary condition at 0x0000800000000000.
>>
>> Now that the emulator has been fixed, revert the XSA-170 change to fix
>> architectural behaviour at the boundary case.  The XTF test case for XSA-170
>> exercises this corner case, and still passes.
>>
>> Fixes: ffbbfda377 ("x86/VMX: sanitize rIP before re-entering guest")
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> But why revert the change rather than fix ...
>
>> @@ -4280,38 +4280,6 @@ void vmx_vmexit_handler(struct cpu_user_regs *regs)
>>  out:
>>      if ( nestedhvm_vcpu_in_guestmode(v) )
>>          nvmx_idtv_handling();
>> -
>> -    /*
>> -     * VM entry will fail (causing the guest to get crashed) if rIP (and
>> -     * rFLAGS, but we don't have an issue there) doesn't meet certain
>> -     * criteria. As we must not allow less than fully privileged mode to 
>> have
>> -     * such an effect on the domain, we correct rIP in that case (accepting
>> -     * this not being architecturally correct behavior, as the injected #GP
>> -     * fault will then not see the correct [invalid] return address).
>> -     * And since we know the guest will crash, we crash it right away if it
>> -     * already is in most privileged mode.
>> -     */
>> -    mode = vmx_guest_x86_mode(v);
>> -    if ( mode == 8 ? !is_canonical_address(regs->rip)
> ... the wrong use of is_canonical_address() here? By reverting
> you open up avenues for XSAs in case we get things wrong elsewhere,
> including ...
>
>> -                   : regs->rip != regs->eip )
> ... for 32-bit guests.

Because the only appropriate alternative would be ASSERT_UNREACHABLE()
and domain crash.

This logic corrupts guest state.

Running with corrupt state is every bit an XSA as hitting a VMEntry
failure if it can be triggered by userspace, but the latter safer and
much more obvious.

It was the appropriate security fix (give or take the functional bug in
it) at the time, given the complexity of retrofitting zero length
instruction fetches to the emulator.

However, it is one of a very long list of guest-state-induced VMEntry
failures, with non-trivial logic which we assert will pass, on a
fastpath, where hardware also performs the same checks and we already
have a runtime safe way of dealing with errors.  (Hence not actually
using ASSERT_UNREACHABLE() here.)

It isn't appropriate for this check to exist on its own (i.e. without
other guest state checks), and it isn't appropriate to live in a
fastpath.  In principle, some logic like this could live on the vmentry
failure path to try a second time, but then you're still creating an XSA
situation which is less obvious, and therefore isn't a clever move IMO.

~Andrew