Re: [PATCH v4 06/39] x86/fpu: Add helper for modifying xstate

Kees Cook
2022-12-03T02:25:30Z

On Fri, Dec 02, 2022 at 04:35:33PM -0800, 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.
>
> 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.
>
> Tested-by: Pengfei Xu <pengfei.xu@intel.com>
> Tested-by: John Allen <john.allen@amd.com>
> Suggested-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

--
Kees Cook