Re: [PATCH 1/8] x86/fpu: Check for missing AVX and AVX-512 xstate bits

On Fri, Jun 26, 2026 at 05:00:37AM +0000, [email protected] wrote:
> [Severity: Critical]
> This is a pre-existing issue, but Memory Protection Keys (PKU) isolation
> can be bypassed because we don't verify X86_FEATURE_PKU against the
> presence of XFEATURE_PKRU in xstate here.
> 
> If a hypervisor reports X86_FEATURE_PKU in CPUID but masks out the
> XFEATURE_PKRU xstate component, the kernel's setup_pku() enables CR4.PKE
> and forces X86_FEATURE_OSPKE.  Because X86_FEATURE_OSPKE is artificially
> forced, the feature is not cleared.
> 
> CR4.PKE=1 permits userspace to use WRPKRU, but because XCR0.PKRU=0,
> xsaves/xrstors silently ignore the PKRU register during task context
> switches, leading to state leakage across tasks.
> 
> Should we also validate PKU against missing xstate components in this
> function to prevent isolation bypass?

I *think* Sashiko means here our preexisting fpu__init_system_xstate() here
but I'm not sure...

> [Severity: High]
> Does this logic get bypassed if XSAVE or FPU support is missing?
> 
> Earlier in fpu__init_system_xstate(), there are early returns if
> X86_FEATURE_FPU or X86_FEATURE_XSAVE are missing:
> 
> 	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;
> 	}
> 
> If a hypervisor natively enumerates AVX=1 but XSAVE=0 (such as QEMU
> with -cpu host,-xsave), check_cpufeature_deps() only warns about unmet

Yah, we don't care. We won't support non-sensical guest configs.

-- 
Regards/Gruss,
    Boris.

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