UPSTREAM: usercopy: avoid potentially undefined behavior in pointer math

UPSTREAM: 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>
Change-Id: I73b13be651cf35c03482f2014bf2c3dd291518ab
(cherry picked from commit 7329a655875a2f4bd6984fe8a7e00a6981e802f3)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>