x86/uaccess: fix code generation in put_user()

Quoting https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html:

  You can define a local register variable and associate it with a
  specified register...

  The only supported use for this feature is to specify registers for
  input and output operands when calling Extended asm (see Extended
  Asm). This may be necessary if the constraints for a particular
  machine don't provide sufficient control to select the desired
  register.

On 32-bit x86, this is used to ensure that gcc will put an 8-byte value
into the %edx:%eax pair, while all other cases will just use the single
register %eax (%rax on x86-64).  While the _ASM_AX actually just expands
to "%eax", note this comment next to get_user() which does something
very similar:

 * The use of _ASM_DX as the register specifier is a bit of a
 * simplification, as gcc only cares about it as the starting point
 * and not size: for a 64-bit value it will use %ecx:%edx on 32 bits
 * (%ecx being the next register in gcc's x86 register sequence), and
 * %rdx on 64 bits.

However, getting this to work requires that there is no code between the
assignment to the local register variable and its use as an input to the
asm() which can possibly clobber any of the registers involved -
including evaluation of the expressions making up other inputs.

In the current code, the ptr expression used directly as an input may
cause such code to be emitted.  For example, Sean Christopherson
observed that with KASAN enabled and ptr being current->set_child_tid
(from chedule_tail()), the load of current->set_child_tid causes a call
to __asan_load8() to be emitted immediately prior to the __put_user_4
call, and Naresh Kamboju reports that various mmstress tests fail on
KASAN-enabled builds.

It's also possible to synthesize a broken case without KASAN if one uses
"foo()" as the ptr argument, with foo being some "extern u64 __user
*foo(void);" (though I don't know if that appears in real code).

Fix it by making sure ptr gets evaluated before the assignment to
__val_pu, and add a comment that __val_pu must be the last thing
computed before the asm() is entered.

Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Fixes: d55564cf ("x86: Make __put_user() generate an out-of-line call")
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/include/asm/uaccess.h

@@ -208,16 +208,24 @@ extern void __put_user_nocheck_2(void);
 extern void __put_user_nocheck_4(void);
 extern void __put_user_nocheck_8(void);
 
+/*
+ * ptr must be evaluated and assigned to the temporary __ptr_pu before
+ * the assignment of x to __val_pu, to avoid any function calls
+ * involved in the ptr expression (possibly implicitly generated due
+ * to KASAN) from clobbering %ax.
+ */
 #define do_put_user_call(fn,x,ptr)					\
 ({									\
 	int __ret_pu;							\
+	void __user *__ptr_pu;						\
 	register __typeof__(*(ptr)) __val_pu asm("%"_ASM_AX);		\
 	__chk_user_ptr(ptr);						\
+	__ptr_pu = (ptr);						\
 	__val_pu = (x);							\
 	asm volatile("call __" #fn "_%P[size]"				\
 		     : "=c" (__ret_pu),					\
 			ASM_CALL_CONSTRAINT				\
-		     : "0" (ptr),					\
+		     : "0" (__ptr_pu),					\
 		       "r" (__val_pu),					\
 		       [size] "i" (sizeof(*(ptr)))			\
 		     :"ebx");						\