Re: [PATCH v2] x86/shstk: shadow stack enabling write return code change

[Edgecombe, Rick P]

On Tue, 2026-07-07 at 13:45 -0500, Bill Roberts wrote:
> The WRSS instruction (the special instruction that writes to shadow stacks)
> cannot be used in userspace unless IA32_U_CET.SH_STK_EN=1 (user shadow
> stack is enabled). So the kernel *should* return -EINVAL to userspace if
> it tries to enable it when shadow stack is disabled. However, currently,
> it will return -EPERM. But, that error code doesn't fit the condition as
> the failure is due to an invalid state change request not a permission
> issue.
> 
> Investigating userspace call sites, like glibc and criu (checkpoint code),
> they do not rely on this specific error message, nor could a userspace
> effectively utilize this specific return error code to indicate a
> difference in "I cannot enable write because of invalid permissions"
> versus "I cannot enable write because the shadow stack is disabled".
> 
> Signed-off-by: Bill Roberts <bill.roberts@xxxxxxx>

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>

> ---
>  arch/x86/kernel/shstk.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
> index 0ca64900192f..eb690ba90180 100644
> --- a/arch/x86/kernel/shstk.c
> +++ b/arch/x86/kernel/shstk.c
> @@ -490,7 +490,7 @@ static int wrss_control(bool enable)
>  	 * when disabling.
>  	 */
>  	if (!features_enabled(ARCH_SHSTK_SHSTK))
> -		return -EPERM;
> +		return -EINVAL;
>  
>  	/* Already enabled/disabled? */
>  	if (features_enabled(ARCH_SHSTK_WRSS) == enable)