usercopy: avoid potentially undefined behavior in pointer math

check_bogus_address() checked for pointer overflow using this expression,
where 'ptr' has type 'const void *':

	ptr + n < ptr

Since pointer wraparound is undefined behavior, gcc at -O2 by default
treats it like the following, which would not behave as intended:

	(long)n < 0

Fortunately, this doesn't currently happen for kernel code because kernel
code is compiled with -fno-strict-overflow.  But the expression should be
fixed anyway to use well-defined integer arithmetic, since it could be
treated differently by different compilers in the future or could be
reported by tools checking for undefined behavior.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

diff --git a/mm/usercopy.c b/mm/usercopy.c
index 8ebae91..82f81df 100644 (file)
--- a/mm/usercopy.c
+++ b/mm/usercopy.c
@@ -124,7 +124,7 @@ 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 (ptr + n < ptr)
+       if ((unsigned long)ptr + n < (unsigned long)ptr)
                return "<wrapped address>";
 
        /* Reject if NULL or ZERO-allocation. */