lib: Fix strnlen_user() to not touch memory after specified maximum

[ Upstream commit f18c34e4 ]

If the specified maximum length of the string is a multiple of unsigned
long, we would load one long behind the specified maximum.  If that
happens to be in a next page, we can hit a page fault although we were
not expected to.

Fix the off-by-one bug in the test whether we are at the end of the
specified range.
Signed-off-by: Jan Kara <jack@suse.cz>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>

lib/strnlen_user.c

@@ -57,7 +57,8 @@ static inline long do_strnlen_user(const char __user *src, unsigned long count,
 			return res + find_zero(data) + 1 - align;
 		}
 		res += sizeof(unsigned long);
-		if (unlikely(max < sizeof(unsigned long)))
+		/* We already handled 'unsigned long' bytes. Did we do it all ? */
+		if (unlikely(max <= sizeof(unsigned long)))
 			break;
 		max -= sizeof(unsigned long);
 		if (unlikely(__get_user(c,(unsigned long __user *)(src+res))))