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

[Kees Cook]

On Thu, Sep 29, 2022 at 03:29:03PM -0700, Rick Edgecombe wrote:
> Just like user xfeatures, supervisor xfeatures can be active in the
> registers or present in the task FPU buffer. If the registers are
> active, the registers can be modified directly. If the registers are
> not active, the modification must be performed on the task FPU buffer.
> 
> When the state is not active, the kernel could perform modifications
> directly to the buffer. But in order for it to do that, it needs
> to know where in the buffer the specific state it wants to modify is
> located. Doing this is not robust against optimizations that compact
> the FPU buffer, as each access would require computing where in the
> buffer it is.
> 
> 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?"

> 
> Add a new function fpregs_lock_and_load() that can simultaneously call
> fpregs_lock() and do this restore. Also perform some extra sanity
> checks in this function since this will be used in non-fpu focused code.

Nit: this is called "fpu_lock_and_load" in the patch itself.

> 
> Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-- 
Kees Cook