Re: [PATCH v1 1/1] x86/fred: Clear the WFE bit in missing-ENDBRANCH #CP

[Xin Li <xin@zytor.com>]

On 9/11/2024 4:35 PM, Andrew Cooper wrote:
> On 12/09/2024 12:19 am, Xin Li (Intel) wrote:
>> The WFE, i.e., WAIT_FOR_ENDBRANCH, bit in the augmented CS of FRED
>> stack frame is set to 1 in missing-ENDBRANCH #CP exceptions.
>>
>> The CPU will generate another missing-ENDBRANCH #CP if the WFE bit
>> is left as 1, because the indirect branch tracker will be set in
>> the WAIT_FOR_ENDBRANCH state upon completion of the following ERETS
>> instruction and the CPU will restart from the IP that just caused a
>> previous missing-ENDBRANCH #CP.
>>
>> Clear the WFE bit to avoid dead looping in missing-ENDBRANCH #CP.
>>
>> Signed-off-by: Xin Li (Intel) <xin@zytor.com>
> 
> Ah - good.  Finally some evidence that this hole in CET has been plugged
> by FRED.
> 
> However, I'd suggest describing it differently.

Yep, it's a much better story including historical issues with IDT.

I will use it as the comment above ibt_clear_fred_wfe() in the next
iteration, but keep the change log.  Is it okay?

> 
> 
> By definition, all missing-ENDBRANCH #CPs are a result of WFE && !ENDBR.
> 
> But, in original CET under IDT delivery, any transfer for
> interrupt/exception/etc that does not change privilege will clobber the
> WFE state because MSR_{U,S}_CET.WFE is intentionally set by microcode so
> as to expect to find an ENDBR at the interrupt/exception/syscall entrypoint.
> 
> In practice, this means interrupts and exceptions hitting the kernel, or
> user interrupts, loose the WFE state of the interrupted context.  And
> yes, this means that a well timed interrupt (to the precise instruction
> boundary) will let an attacker sneak a bad function pointer past the
> CET-IBT enforcement.
> 
> In FRED, the WFE state of the interrupted context (even if it is the
> same privilege) is preserved and restored, in order to close this hole.
> 
> Therefore, the intentional #CP selfchecks need to clear WFE when they
> are deemed to have succeeded, now that FRED is causing the state not to
> get lost.
> 
> ~Andrew