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

From: tip-bot for Andy Lutomirski
Date: Thu Feb 25 2016 - 00:54:58 EST

Commit-ID: 04d1d281dcfe683a53cddfab8371fc8bb302b069
Gitweb: http://git.kernel.org/tip/04d1d281dcfe683a53cddfab8371fc8bb302b069
Author: Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate: Tue, 23 Feb 2016 13:19:29 -0800
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 24 Feb 2016 08:43:04 +0100

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@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/3e36be110724896e32a4a1fe73bacb349d3cba94.1456262295.git.luto@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
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 77d8c51..bb3e376 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) */