mm/usercopy: Use memory range to be accessed for wraparound check

Currently, when checking to see if accessing n bytes starting at
address "ptr" will cause a wraparound in the memory addresses,
the check in check_bogus_address() adds an extra byte, which is
incorrect, as the range of addresses that will be accessed is
[ptr, ptr + (n - 1)].

This can lead to incorrectly detecting a wraparound in the
memory address, when trying to read 4 KB from memory that is
mapped to the the last possible page in the virtual address
space, when in fact, accessing that range of memory would not
cause a wraparound to occur.

Use the memory range that will actually be accessed when
considering if accessing a certain amount of bytes will cause
the memory address to wrap around.

Change-Id: Ie71a5a5cb9f6dc15d82a77202b4aaa775eb549f7
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>

mm/usercopy.c

@@ -121,7 +121,7 @@ static inline const char *check_kernel_text_object(const void *ptr,
 static inline const char *check_bogus_address(const void *ptr, unsigned long n)
 {
 	/* Reject if object wraps past end of memory. */
-	if ((unsigned long)ptr + n < (unsigned long)ptr)
+	if ((unsigned long)ptr + (n - 1) < (unsigned long)ptr)
 		return "<wrapped address>";
 
 	/* Reject if NULL or ZERO-allocation. */