[tip: x86/cpu] x86/shstk: Return the correct error value when user shadow stacks are disabled

["tip-bot2 for Bill Roberts" <tip-bot2@linutronix.de>]

The following commit has been merged into the x86/cpu branch of tip:

Commit-ID:     9025f3bf73f0565eaa271ecc15f468b1d1e4cbaa
Gitweb:        https://git.kernel.org/tip/9025f3bf73f0565eaa271ecc15f468b1d1e4cbaa
Author:        Bill Roberts <bill.roberts@arm.com>
AuthorDate:    Tue, 07 Jul 2026 13:45:42 -05:00
Committer:     Borislav Petkov (AMD) <bp@alien8.de>
CommitterDate: Mon, 14 Sep 2026 18:57:04 -07:00

x86/shstk: Return the correct error value when user shadow stacks are disabled

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".

  [ bp: Fixup commit title. ]

Signed-off-by: Bill Roberts <bill.roberts@arm.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Link: https://patch.msgid.link/20260707184542.1721085-1-bill.roberts@arm.com
---
 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 0ca6490..eb690ba 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)