x86/entry/32: Add an ASM_CLAC to entry_SYSENTER_32

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>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/3e36be110724896e32a4a1fe73bacb349d3cba94.1456262295.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

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) */