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.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: f5509cc ("mm: Hardened usercopy")
Signed-off-by: Prasad Sodagudi <[email protected]>
Signed-off-by: Isaac J. Manjarres <[email protected]>
Co-developed-by: Prasad Sodagudi <[email protected]>
Reviewed-by: William Kucharski <[email protected]>
Acked-by: Kees Cook <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Trilok Soni <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/usercopy.c

@@ -147,7 +147,7 @@ static inline void check_bogus_address(const unsigned long ptr, unsigned long n,
 				       bool to_user)
 {
 	/* Reject if object wraps past end of memory. */
-	if (ptr + n < ptr)
+	if (ptr + (n - 1) < ptr)
 		usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n);
 
 	/* Reject if NULL or ZERO-allocation. */