Re: [PATCH] x86/fpu: Fix the os panic issue caused by the XGETBV instruction

[Chang S. Bae]

On 1/1/2025 11:54 PM, Tony W Wang-oc wrote:
> From: Lyle Li <LyleLi@xxxxxxxxxxx>
>
> The callers of the xfeatures_in_use function must ensure that the
> current processor has the X86_FEATURE_XGETBV1 feature. However, in some
> places where xfeatures_in_use is called, there is no check to see if the
> processor supports this feature, leading to the execution of the XGETBV
> XCR1 instruction on processors that do not support this feature,
> triggering a #GP exception, and ultimately causing an OS panic.

I doubt this is a real issue. An XFD implementation without XGETBV1 is 
considerably broken; every AMX system includes XGETBV1. Similarly, as 
far as I can see, PKU implementations also include XGETBV1. QEMU's CPU 
feature list [1] seems consistent with this.

Maybe a wild clearcpuid use may clear off the XGETBV1 flag. Adding this 
dependency to the table would make the relationship explicit:

static const struct cpuid_dep cpuid_deps[] = {
...
+       { X86_FEATURE_PKU,                      X86_FEATURE_XGETBV1   },
        {}
 };

Note that XFD is already listed as dependent on XGETBV1.

But I doubt the kernel needs to be resilient to deliberately 
misconfigured or crazy virtual machine setups.

Thanks,
Chang

[1] https://github.com/qemu/qemu/blob/master/target/i386/cpu.c