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

[Bill Roberts]

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>
---
 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)
-- 
2.54.0