Re: [PATCH 4/8] lib/crypto: x86: Stop using cpu_has_xfeatures()

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Removing `cpu_has_xfeatures()` causes a kernel #UD panic on systems advertising AVX without XSAVE.
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commit c4725d088a9c170c27e417f42ccdfa48f567b845
Author: Eric Biggers <[email protected]>

lib/crypto: x86: Stop using cpu_has_xfeatures()

This commit removes calls to cpu_has_xfeatures() across the x86 crypto 
library, relying entirely on boot_cpu_has() to check for features. The 
assumption is that x86 explicitly clears AVX and AVX-512 flags if their 
xfeatures are missing.

> diff --git a/lib/crypto/x86/poly1305.h b/lib/crypto/x86/poly1305.h
> index ee92e3740a787..b061b9926fa5d 100644
> --- a/lib/crypto/x86/poly1305.h
> +++ b/lib/crypto/x86/poly1305.h
> @@ -143,15 +143,12 @@ static void poly1305_emit(const struct poly1305_state *ctx,
>  #define poly1305_mod_init_arch poly1305_mod_init_arch
>  static void poly1305_mod_init_arch(void)
>  {
> -	if (boot_cpu_has(X86_FEATURE_AVX) &&
> -	    cpu_has_xfeatures(XFEATURE_MASK_SSE | XFEATURE_MASK_YMM, NULL))
> +	if (boot_cpu_has(X86_FEATURE_AVX))
>  		static_branch_enable(&poly1305_use_avx);

[Severity: High]
Will this cause a invalid opcode panic on systems advertising AVX without XSAVE
such as virtualized environments using a missing xsave cpu flag?

If XSAVE is missing natively, fpu__init_system_xstate() in
arch/x86/kernel/fpu/xstate.c returns early without clearing features:

arch/x86/kernel/fpu/xstate.c:fpu__init_system_xstate() {
        ...
        if (!boot_cpu_has(X86_FEATURE_XSAVE)) {
                pr_info("x86/fpu: x87 FPU will use %s\n",
                        boot_cpu_has(X86_FEATURE_FXSR) ? "FXSAVE" : "FSAVE");
                return;
        }
        ...
}

This early return bypasses clear_cpu_caps_with_missing_xfeatures(). Since
the CPUID dependency enforcement is only triggered when a feature is explicitly
cleared, X86_FEATURE_AVX is never cleared in this specific situation.

Without the cpu_has_xfeatures() check here, the crypto subsystems will see
boot_cpu_has(X86_FEATURE_AVX) as true and enable AVX instructions. Because
XSAVE is disabled, the OS never sets CR4.OSXSAVE=1, which can lead to a panic
when AVX instructions subsequently execute.

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Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=4