Re: [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for shadow stack

Borislav Petkov
2018-11-08T23:52:44Z

On Thu, Nov 08, 2018 at 12:40:02PM -0800, Yu-cheng Yu wrote:
> In fpu_init_system_xstate(), we test and clear features that are not enabled.
> There we depend on the order of these elements. This is the tenth "unknown
> xstate feature".

Aha, those are *reserved* bits - not unused, in XCR0.

Do an s/unused/reserved/g pls.

Now let's see, you have 0 for the 10th bit which happens to be

#define X86_FEATURE_FPU ( 0*32+ 0) /* Onboard FPU */

too. And if we look at the code:

for (i = 0; i < ARRAY_SIZE(xsave_cpuid_features); i++) {
if (!boot_cpu_has(xsave_cpuid_features[i]))
xfeatures_mask_all &= ~BIT_ULL(i);

guess what happens if i == 10.

I know, the subsequent & SUPPORTED_XFEATURES_MASK saves you from the
#GP but that's still not good enough. The loop should not even call
boot_cpu_has() for reserved feature bits.

Thx.

--
Regards/Gruss,
Boris.

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