[tip:x86/urgent] x86/uaccess: Don't leak the AC flag into __put_user() value evaluation

[tip-bot for Andy Lutomirski]

Commit-ID:  2a418cf3f5f1caf911af288e978d61c9844b0695
Gitweb:     https://git.kernel.org/tip/2a418cf3f5f1caf911af288e978d61c9844b0695
Author:     Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate: Fri, 22 Feb 2019 17:17:04 -0800
Committer:  Borislav Petkov <bp@xxxxxxx>
CommitDate: Mon, 25 Feb 2019 20:17:05 +0100

x86/uaccess: Don't leak the AC flag into __put_user() value evaluation

When calling __put_user(foo(), ptr), the __put_user() macro would call
foo() in between __uaccess_begin() and __uaccess_end().  If that code
were buggy, then those bugs would be run without SMAP protection.

Fortunately, there seem to be few instances of the problem in the
kernel. Nevertheless, __put_user() should be fixed to avoid doing this.
Therefore, evaluate __put_user()'s argument before setting AC.

This issue was noticed when an objtool hack by Peter Zijlstra complained
about genregs_get() and I compared the assembly output to the C source.

 [ bp: Massage commit message and fixed up whitespace. ]

Fixes: 11f1a4b9755f ("x86: reorganize SMAP handling in user space accesses")
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Acked-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/20190225125231.845656645@xxxxxxxxxxxxx
---
 arch/x86/include/asm/uaccess.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index a77445d1b034..28376aa2d053 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -284,7 +284,7 @@ do {									\
 		__put_user_goto(x, ptr, "l", "k", "ir", label);		\
 		break;							\
 	case 8:								\
-		__put_user_goto_u64((__typeof__(*ptr))(x), ptr, label);	\
+		__put_user_goto_u64(x, ptr, label);			\
 		break;							\
 	default:							\
 		__put_user_bad();					\
@@ -431,8 +431,10 @@ do {									\
 ({								\
 	__label__ __pu_label;					\
 	int __pu_err = -EFAULT;					\
+	__typeof__(*(ptr)) __pu_val;				\
+	__pu_val = x;						\
 	__uaccess_begin();					\
-	__put_user_size((x), (ptr), (size), __pu_label);	\
+	__put_user_size(__pu_val, (ptr), (size), __pu_label);	\
 	__pu_err = 0;						\
 __pu_label:							\
 	__uaccess_end();					\