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

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>

diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c
index a28df5206d95c24d6f3b4116753747f1fb2a67e3..11649615c50504cb5976d58c8bf012d563bf2bbc 100644 (file)
--- a/lib/strnlen_user.c
+++ b/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))))