Re: [PATCH] x86/cpufeatures: Force disable X86_FEATURE_ENQCMD and remove update_pasid()

[Thomas Gleixner]

On Mon, May 31 2021 at 10:43, Borislav Petkov wrote:
> On Sat, May 29, 2021 at 11:17:30AM +0200, Thomas Gleixner wrote:
>> #2 is broken beyond repair. The comment in the code claims that it is safe
>>    to invoke this in an IPI, but that's just wishful thinking.
>> 
>>    FPU state of a running task is protected by fregs_lock() which is
>>    nothing else than a local_bh_disable(). As BH disabled regions run
>>    usually with interrupts enabled the IPI can hit a code section which
>>    modifies FPU state and there is absolutely no guarantee that any of the
>>    assumptions which are made for the IPI case is true.
>
> ... so on a PASID system, your trivial reproducer would theoretically
> fire the same way and corrupt FPU state just as well.

This is worse and you can't selftest it because the IPI can just hit in
the middle of _any_ FPU state operation and corrupt state.

Thanks,

        tglx