[PATCH] x86/entry/32: Add an ASM_CLAC to entry_SYSENTER_32

[Andy Lutomirski <luto@kernel.org>]

Both before and after 5f310f739b4c ("x86/entry/32: Re-implement
SYSENTER using the new C path"), we relied on a uaccess very early
in the SYSENTER path to clear AC.  After that change, though, we can
potentially make it all the way into C code with AC set, which
enlarges the attack surface for SMAP bypass by doing SYSENTER with
AC set.

Strengthen the SMAP protection by addding the missing ASM_CLAC right
at the beginning.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---

This is probably an x86/urgent candidate.  It fixes a minor
hardening regression in 4.4.

It's lightly tested.  It's hard to test well right now because the
4.5 series is completely broken for 32-bit SMAP sytems.

arch/x86/entry/entry_32.S | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index f3facd40fd2d..9d6165c171eb 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -294,6 +294,7 @@ sysenter_past_esp:
 	pushl	$__USER_DS		/* pt_regs->ss */
 	pushl	%ebp			/* pt_regs->sp (stashed in bp) */
 	pushfl				/* pt_regs->flags (except IF = 0) */
+	ASM_CLAC			/* Clear AC after saving FLAGS */
 	orl	$X86_EFLAGS_IF, (%esp)	/* Fix IF */
 	pushl	$__USER_CS		/* pt_regs->cs */
 	pushl	$0			/* pt_regs->ip = 0 (placeholder) */
-- 
2.5.0