x86/uaccess: Fix 32-bit __get_user_asm_u64() when CC_HAS_ASM_GOTO_OUTPUT=y

Commit 865c50e1 ("x86/uaccess: utilize CONFIG_CC_HAS_ASM_GOTO_OUTPUT")
added an optimised version of __get_user_asm() for x86 using 'asm goto'.

Like the non-optimised code, the 32-bit implementation of 64-bit
get_user() expands to a pair of 32-bit accesses.  Unlike the
non-optimised code, the _original_ pointer is incremented to copy the
high word instead of loading through a new pointer explicitly
constructed to point at a 32-bit type.  Consequently, if the pointer
points at a 64-bit type then we end up loading the wrong data for the
upper 32-bits.

This was observed as a mount() failure in Android targeting i686 after
b0cfcdd9 ("d_path: make 'prepend()' fill up the buffer exactly on
overflow") because the call to copy_from_kernel_nofault() from
prepend_copy() ends up in __get_kernel_nofault() and casts the source
pointer to a 'u64 __user *'.  An attempt to mount at "/debug_ramdisk"
therefore ends up failing trying to mount "/debumdismdisk".

Use the existing '__gu_ptr' source pointer to unsigned int for 32-bit
__get_user_asm_u64() instead of the original pointer.

Cc: Bill Wendling <morbo@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Reported-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Fixes: 865c50e1 ("x86/uaccess: utilize CONFIG_CC_HAS_ASM_GOTO_OUTPUT")
Signed-off-by: Will Deacon <will@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/include/asm/uaccess.h

@@ -301,8 +301,8 @@ do {									\
 	unsigned int __gu_low, __gu_high;				\
 	const unsigned int __user *__gu_ptr;				\
 	__gu_ptr = (const void __user *)(ptr);				\
-	__get_user_asm(__gu_low, ptr, "l", "=r", label);		\
-	__get_user_asm(__gu_high, ptr+1, "l", "=r", label);		\
+	__get_user_asm(__gu_low, __gu_ptr, "l", "=r", label);		\
+	__get_user_asm(__gu_high, __gu_ptr+1, "l", "=r", label);	\
 	(x) = ((unsigned long long)__gu_high << 32) | __gu_low;		\
 } while (0)
 #else