Fix potential infoleak in older kernels

Not upstream as it is not needed there.

So a patch something like this might be a safe way to fix the
potential infoleak in older kernels.

THIS IS UNTESTED. It's a very obvious patch, though, so if it compiles
it probably works. It just initializes the output variable with 0 in
the inline asm description, instead of doing it in the exception
handler.

It will generate slightly worse code (a few unnecessary ALU
operations), but it doesn't have any interactions with the exception
handler implementation.
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

arch/x86/include/asm/uaccess.h

@@ -347,7 +347,7 @@ do {									\
 #define __get_user_asm_u64(x, ptr, retval, errret) \
 	 __get_user_asm(x, ptr, retval, "q", "", "=r", errret)
 #define __get_user_asm_ex_u64(x, ptr) \
-	 __get_user_asm_ex(x, ptr, "q", "", "=r")
+	 __get_user_asm_ex(x, ptr, "q", "", "=&r")
 #endif
 
 #define __get_user_size(x, ptr, size, retval, errret)			\
@@ -389,13 +389,13 @@ do {									\
 	__chk_user_ptr(ptr);						\
 	switch (size) {							\
 	case 1:								\
-		__get_user_asm_ex(x, ptr, "b", "b", "=q");		\
+		__get_user_asm_ex(x, ptr, "b", "b", "=&q");		\
 		break;							\
 	case 2:								\
-		__get_user_asm_ex(x, ptr, "w", "w", "=r");		\
+		__get_user_asm_ex(x, ptr, "w", "w", "=&r");		\
 		break;							\
 	case 4:								\
-		__get_user_asm_ex(x, ptr, "l", "k", "=r");		\
+		__get_user_asm_ex(x, ptr, "l", "k", "=&r");		\
 		break;							\
 	case 8:								\
 		__get_user_asm_ex_u64(x, ptr);				\
@@ -409,7 +409,7 @@ do {									\
 	asm volatile("1:	mov"itype" %1,%"rtype"0\n"		\
 		     "2:\n"						\
 		     _ASM_EXTABLE(1b, 2b - 1b)				\
-		     : ltype(x) : "m" (__m(addr)))
+		     : ltype(x) : "m" (__m(addr)), "0" (0))
 
 #define __put_user_nocheck(x, ptr, size)			\
 ({								\