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@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

1 files 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();                                        \