riscv: evaluate put_user() arg before enabling user access

The <asm/uaccess.h> header has a problem with put_user(a, ptr) if
the 'a' is not a simple variable, such as a function. This can lead
to the compiler producing code as so:

1:	enable_user_access()
2:	evaluate 'a' into register 'r'
3:	put 'r' to 'ptr'
4:	disable_user_acess()

The issue is that 'a' is now being evaluated with the user memory
protections disabled. So we try and force the evaulation by assigning
'x' to __val at the start, and hoping the compiler barriers in
 enable_user_access() do the job of ordering step 2 before step 1.

This has shown up in a bug where 'a' sleeps and thus schedules out
and loses the SR_SUM flag. This isn't sufficient to fully fix, but
should reduce the window of opportunity. The first instance of this
we found is in scheudle_tail() where the code does:

$ less -N kernel/sched/core.c

4263  if (current->set_child_tid)
4264         put_user(task_pid_vnr(current), current->set_child_tid);

Here, the task_pid_vnr(current) is called within the block that has
enabled the user memory access. This can be made worse with KASAN
which makes task_pid_vnr() a rather large call with plenty of
opportunity to sleep.
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Reported-by: syzbot+e74b94fe601ab9552d69@syzkaller.appspotmail.com
Suggested-by: Arnd Bergman <arnd@arndb.de>

--
Changes since v1:
- fixed formatting and updated the patch description with more info

Changes since v2:
- fixed commenting on __put_user() (schwab@linux-m68k.org)

Change since v3:
- fixed RFC in patch title. Should be ready to merge.
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/include/asm/uaccess.h

@@ -306,7 +306,9 @@ do {								\
  * data types like structures or arrays.
  *
  * @ptr must have pointer-to-simple-variable type, and @x must be assignable
- * to the result of dereferencing @ptr.
+ * to the result of dereferencing @ptr. The value of @x is copied to avoid
+ * re-ordering where @x is evaluated inside the block that enables user-space
+ * access (thus bypassing user space protection if @x is a function).
  *
  * Caller must check the pointer with access_ok() before calling this
  * function.
@@ -316,12 +318,13 @@ do {								\
 #define __put_user(x, ptr)					\
 ({								\
 	__typeof__(*(ptr)) __user *__gu_ptr = (ptr);		\
+	__typeof__(*__gu_ptr) __val = (x);			\
 	long __pu_err = 0;					\
 								\
 	__chk_user_ptr(__gu_ptr);				\
 								\
 	__enable_user_access();					\
-	__put_user_nocheck(x, __gu_ptr, __pu_err);		\
+	__put_user_nocheck(__val, __gu_ptr, __pu_err);		\
 	__disable_user_access();				\
 								\
 	__pu_err;						\