linux-kernel - [tip:x86/urgent] x86/entry/32: Add an ASM_CLAC to entry_SYSENTER

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Date:	Wed, 24 Feb 2016 21:53:53 -0800
From:	tip-bot for Andy Lutomirski <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	dvlasenk@...hat.com, bp@...en8.de, luto@...capital.net,
	hpa@...or.com, luto@...nel.org, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, tglx@...utronix.de,
	brgerst@...il.com, mingo@...nel.org, peterz@...radead.org
Subject: [tip:x86/urgent] x86/entry/32: Add an ASM_CLAC to entry_SYSENTER_32

Commit-ID:  04d1d281dcfe683a53cddfab8371fc8bb302b069
Gitweb:     http://git.kernel.org/tip/04d1d281dcfe683a53cddfab8371fc8bb302b069
Author:     Andy Lutomirski <luto@...nel.org>
AuthorDate: Tue, 23 Feb 2016 13:19:29 -0800
Committer:  Ingo Molnar <mingo@...nel.org>
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@...nel.org>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/3e36be110724896e32a4a1fe73bacb349d3cba94.1456262295.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 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) */