audit: fix out-of-bounds read in audit_compare_dname_path()

When a watch on dir=/ is combined with an fsnotify event for a
single-character name directly under / (e.g., creating /a), an
out-of-bounds read can occur in audit_compare_dname_path().

The helper parent_len() returns 1 for "/". In audit_compare_dname_path(),
when parentlen equals the full path length (1), the code sets p = path + 1
and pathlen = 1 - 1 = 0. The subsequent loop then dereferences
p[pathlen - 1] (i.e., p[-1]), causing an out-of-bounds read.

Fix this by adding a pathlen > 0 check to the while loop condition
to prevent the out-of-bounds access.

Cc: [email protected]
Fixes: e92eebb ("audit: fix suffixed '/' filename matching")
Reported-by: Stanislav Fort <[email protected]>
Suggested-by: Linus Torvalds <[email protected]>
Signed-off-by: Stanislav Fort <[email protected]>
[PM: subject tweak, sign-off email fixes]
Signed-off-by: Paul Moore <[email protected]>

Author: stanislavfortaisle

kernel/auditfilter.c

@@ -1326,7 +1326,7 @@ int audit_compare_dname_path(const struct qstr *dname, const char *path, int par
 
 	/* handle trailing slashes */
 	pathlen -= parentlen;
-	while (p[pathlen - 1] == '/')
+	while (pathlen > 0 && p[pathlen - 1] == '/')
 		pathlen--;
 
 	if (pathlen != dlen)