[tip:core/urgent] x86/uaccess: Don't leak AC flag into fentry from mcsafe_handle_tail()

[tip-bot for Josh Poimboeuf]

Commit-ID:  5e307a6bc7b600999742675dd182bcb8a6fe308e
Gitweb:     https://git.kernel.org/tip/5e307a6bc7b600999742675dd182bcb8a6fe308e
Author:     Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
AuthorDate: Wed, 17 Jul 2019 20:36:43 -0500
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Thu, 18 Jul 2019 21:01:05 +0200

x86/uaccess: Don't leak AC flag into fentry from mcsafe_handle_tail()

After adding mcsafe_handle_tail() to the objtool uaccess safe list,
objtool reports:

  arch/x86/lib/usercopy_64.o: warning: objtool: mcsafe_handle_tail()+0x0: call to __fentry__() with UACCESS enabled

With SMAP, this function is called with AC=1, so it needs to be careful
about which functions it calls.  Disable the ftrace entry hook, which
can potentially pull in a lot of extra code.

Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/8e13d6f0da1c8a3f7603903da6cbf6d582bbfe10.1563413318.git.jpoimboe@xxxxxxxxxx

---
 arch/x86/lib/usercopy_64.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/lib/usercopy_64.c b/arch/x86/lib/usercopy_64.c
index e0e006f1624e..fff28c6f73a2 100644
--- a/arch/x86/lib/usercopy_64.c
+++ b/arch/x86/lib/usercopy_64.c
@@ -60,7 +60,7 @@ EXPORT_SYMBOL(clear_user);
  * but reuse __memcpy_mcsafe in case a new read error is encountered.
  * clac() is handled in _copy_to_iter_mcsafe().
  */
-__visible unsigned long
+__visible notrace unsigned long
 mcsafe_handle_tail(char *to, char *from, unsigned len)
 {
 	for (; len; --len, to++, from++) {