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

amluto · suryasaimadhu · commit 2a418cf3f5f1 · 2019-02-25T20:17:05.000+01:00
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: 11f1a4b ("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
diff --git 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();					\
