Re: [PATCH 0/3] x86: Initial pieces for guest CET support

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

On 27/04/2021 07:46, Jan Beulich wrote:
> On 26.04.2021 19:54, Andrew Cooper wrote:
>> Some initial pieces for guest support.  Everything will currently malfunction
>> for VMs which explicitly opt in to CET_SS/IBT.
>>
>> Still TODO as a minimum:
>>  * Teach the pagewalk logic about shadow stack accesses and errors.
>>  * Emulator support for the new instructions.  WRUSS is an irritating corner
>>    case, requiring a change to how we express pagewalk inputs, as
>>    user/supervisor is no longer dependent on CPL.
> I can put this on my todo list, considering that I'm the one to play
> with the emulator the most. Just let me know if you would prefer to
> do it yourself. (Otherwise my next item there after AMX is now
> complete would have been KeyLocker insns.)

My plan was actually to start with WRSS and do the pagewalk side of
things, including refreshing my comprehensive pagewalk XTF test.  This
will block work on all the other shadow stack instructions.

There are 3 emulator complexities for shadow stack instructions.  SSP
itself as a register, WRUSS no longer being CPL-based for
user/supervisor, and the fact that RSTORSSP in particular uses an atomic
block which microcode can express, but can't be encoded at an ISA
level.  I've got no idea what to do about this last problem, because we
can't map the two guest frames and re-issue the instruction - the
aliasing check on the tokens forces us to map the two frames in their
correct linear addresses.


IBT is quite different.  There are only two new instructions,
ENDBR{32,64} but tweaks to lots of other instructions (all indirect
control transfers), as well as 0x3e for the notrack prefix, and the
legacy code page bitmap.  The tracker bits themselves are somewhat
irritating to access in the {U,S}_CET MSRs.

Furthermore, I don't have access to hardware with CET-IBT (the SDP seems
to have let out some blue smoke), so any support here would be speculative.


Other bits I'd forgotten from the first set of bullet points.
* {RD,WR}MSR support for all the MSRs, including finally breaking into
the non-trivial work of context-dependent state.
* Changes to Task Switch.

>>  * Context switching of U/S_CET state.  Recommended way is with XSAVES, except
>>    the S_CET has broken sematics - it ends up as a mix of host and guest
>>    state, and isn't safe to XRSTOR without editing what the CPU wrote out.
> Hmm, I wasn't aware of quirks here - would you mind going into more
> detail?

I'll double check my notes.

>
>> The above ought to suffice for getting some XTF testing in place.  For general
>> guest support:
>>  * In-guest XSAVES support.  Windows is the only OS to support CET at the time
>>    of writing, and it cross-checks for XSAVES.  Linux expected to perform the
>>    same cross-check in due course.
> What specifically do you mean here? The XSAVES CPU feature is marked
> 'S', so ought to be visible to Windows. Hence I guess you mean support
> for the respective XSS bits?

We really shouldn't have exposed XSAVES without XSS bits, but yes - what
matters is that the {U,S}_CET XSTATE components are usable in the guest.

~Andrew