Re: [PATCH] x86/fpu: Disable shstk if no CET_USER state

[Sean Christopherson <seanjc@google.com>]

On Fri, Apr 03, 2026, David Kaplan wrote:
> Some hypervisors (including QEMU 10.1.5) may report CET_SS support in
> CPUID Fn7 but fail to report that CET_USER state is supported in
> supervisor xstate.  Linux relies on XSAVES/XRSTORS to swap CET state
> during context switch and assumes it is supported when CET_SS is
> present.
> 
> As a result, if a user process is run with shadow stacks enabled and
> then is switched away from, the system may crash because the new process
> may be incorrectly run with shadow stacks enabled.
> 
> Detect this broken configuration and disable user shadow stacks unless
> CET_USER is supported in xstate.

It's not actually broken though, is it?  Just "odd".  AFAICT, neither the SDM
nor the APM _requires_ CET_{U,S} to be supported in XSS if shadow stacks are
suppported.

> Signed-off-by: David Kaplan <david.kaplan@amd.com>
> ---
>  arch/x86/kernel/fpu/xstate.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index 76153dfb58c9..188323442b4d 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -855,6 +855,17 @@ void __init fpu__init_system_xstate(unsigned int legacy_size)
>  		goto out_disable;
>  	}
>  
> +	if (boot_cpu_has(X86_FEATURE_USER_SHSTK) &&
> +	    !(fpu_kernel_cfg.max_features & XFEATURE_MASK_CET_USER)) {
> +		/*
> +		 * The kernel relies on XSAVES/XRSTORS to context switch shadow
> +		 * stack state.  If this isn't present, disable user shadow
> +		 * stacks.
> +		 */
> +		pr_err("x86/fpu: CET_USER not supported in xstate when CET is supported.  Disabling shadow stacks.\n");
> +		setup_clear_cpu_cap(X86_FEATURE_USER_SHSTK);

Doesn't this apply to IBT as well?  This code is also misplaced, as it needs to
live after at least this code:

	if (!cpu_feature_enabled(X86_FEATURE_XSAVES))
		fpu_kernel_cfg.max_features &= XFEATURE_MASK_USER_SUPPORTED;
	else
		fpu_kernel_cfg.max_features &= XFEATURE_MASK_USER_SUPPORTED |
					XFEATURE_MASK_SUPERVISOR_SUPPORTED;

and should probably play nice with the "out_disable" path too.

All in all, setup_cet() seems like a much better fit, but unfortunately that
runs before fpu__init_system() :-(

> +	}
> +
>  	fpu_kernel_cfg.independent_features = fpu_kernel_cfg.max_features &
>  					      XFEATURE_MASK_INDEPENDENT;
>  
> 
> base-commit: d998c62f267213aeb815cf654908608eb7c00db2
> -- 
> 2.53.0
>