[PATCH v4.9] x86/uaccess: Don't leak the AC flag into __put_user() value evaluation

[Maxim Zhukov <mussitantesmortem@gmail.com>]

From: Andy Lutomirski <luto@kernel.org>

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@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/20190225125231.845656645@infradead.org
---
 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 a8d85a687cf4..39bbc225558c 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -292,7 +292,7 @@ do {									\
 		__put_user_asm(x, ptr, retval, "l", "k", "ir", errret);	\
 		break;							\
 	case 8:								\
-		__put_user_asm_u64((__typeof__(*ptr))(x), ptr, retval,	\
+		__put_user_asm_u64(x, ptr, retval,	\
 				   errret);				\
 		break;							\
 	default:							\
@@ -427,8 +427,10 @@ do {									\
 #define __put_user_nocheck(x, ptr, size)			\
 ({								\
 	int __pu_err;						\
+	__typeof__(*(ptr)) __pu_val;				\
 	__uaccess_begin();					\
-	__put_user_size((x), (ptr), (size), __pu_err, -EFAULT);	\
+	__pu_val = x;						\
+	__put_user_size(__pu_val, (ptr), (size), __pu_err, -EFAULT);	\
 	__uaccess_end();					\
 	__builtin_expect(__pu_err, 0);				\
 })
-- 
2.21.0