Re: [PATCH v2 4/4] x86/fpu: Correct the legacy state offset and size information

[Sean Christopherson <seanjc@google.com>]

On Thu, Sep 22, 2022, Chang S. Bae wrote:
> MXCSR is architecturally part of the SSE state. But, the kernel code
> presumes it as part of the FP component. Adjust the offset and size for
> these legacy states.
> 
> Notably, each legacy component area is not contiguous, unlike extended
> components. Add a warning message when these offset and size are
> referenced.
> 
> Fixes: ac73b27aea4e ("x86/fpu/xstate: Fix xstate_offsets, xstate_sizes for non-extended xstates")
> Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
> Cc: x86@kernel.org
> Cc: linux-kernel@vger.kernel.org
> ---
>  arch/x86/kernel/fpu/xstate.c | 21 +++++++++++++++------
>  1 file changed, 15 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index a3f7045d1f8e..ac2ec5d6e7e4 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -143,8 +143,13 @@ static unsigned int xfeature_get_offset(u64 xcomp_bv, int xfeature)
>  	 * offsets.
>  	 */
>  	if (!cpu_feature_enabled(X86_FEATURE_XCOMPACTED) ||
> -	    xfeature <= XFEATURE_SSE)
> +	    xfeature <= XFEATURE_SSE) {
> +		if (xfeature <= XFEATURE_SSE)
> +			pr_warn("The legacy state (%d) is discontiguously located.\n",
> +				xfeature);

pr_warn() here isn't warranted.  copy_uabi_to_xstate() calls this with non-extended
features, which is perfectly fine since it manually handles MXCSR.  And that helper
is directly reachable by userspace, i.e. userspace can spam the pr_warn().