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 <l...@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
