linux-kernel - Re: [PATCH v2 06/39] x86/fpu: Add helper for modifying xstate

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Message-ID: <202210032104.A1029D23@keescook>
Date:   Mon, 3 Oct 2022 21:05:04 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH v2 06/39] x86/fpu: Add helper for modifying xstate

On Mon, Oct 03, 2022 at 08:05:13PM +0000, Edgecombe, Rick P wrote:
> On Mon, 2022-10-03 at 10:48 -0700, Kees Cook wrote:
> > > The easiest way to modify supervisor xfeature data is to force
> > > restore
> > > the registers and write directly to the MSRs. Often times this is
> > > just fine
> > > anyway as the registers need to be restored before returning to
> > > userspace.
> > > Do this for now, leaving buffer writing optimizations for the
> > > future.
> > 
> > Just for my own clarity, does this mean lock/load _needs_ to happen
> > before MSR access, or is it just a convenient place to do it? From
> > later
> > patches it seems it's a requirement during MSR access, which might be
> > a
> > good idea to detail here. It answers the question "when is this
> > function
> > needed?"
> 
> The CET state is xsaves managed. It gets lazily restored before
> returning to userspace with the rest of the fpu stuff. This function
> will force restore all the fpu state to the registers early and lock
> them from being automatically saved/restored. Then the tasks CET state
> can be modified in the MSRs, before unlocking the fpregs. Last time I
> tried to modify the state directly in the xsave buffer when it was
> efficient, but it had issues and Thomas suggested this.

Okay, gotcha. Thanks!

-- 
Kees Cook