netfilter: x_tables: check standard target size too

We have targets and standard targets -- the latter carries a verdict.

The ip/ip6tables validation functions will access t->verdict for the
standard targets to fetch the jump offset or verdict for chainloop
detection, but this happens before the targets get checked/validated.

Thus we also need to check for verdict presence here, else t->verdict
can point right after a blob.

Spotted with UBSAN while testing malformed blobs.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
(cherry picked from commit 7ed2abdd)
BugLink: https://bugs.launchpad.net/bugs/1595350Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Brad Figg <brad.figg@canonical.com>

net/netfilter/x_tables.c

@@ -540,6 +540,13 @@ int xt_compat_match_to_user(const struct xt_entry_match *m,
 }
 EXPORT_SYMBOL_GPL(xt_compat_match_to_user);
 
+/* non-compat version may have padding after verdict */
+struct compat_xt_standard_target {
+	struct compat_xt_entry_target t;
+	compat_uint_t verdict;
+};
+
+/* see xt_check_entry_offsets */
 int xt_compat_check_entry_offsets(const void *base,
 				  unsigned int target_offset,
 				  unsigned int next_offset)
@@ -557,6 +564,10 @@ int xt_compat_check_entry_offsets(const void *base,
 	if (target_offset + t->u.target_size > next_offset)
 		return -EINVAL;
 
+	if (strcmp(t->u.user.name, XT_STANDARD_TARGET) == 0 &&
+	    target_offset + sizeof(struct compat_xt_standard_target) != next_offset)
+		return -EINVAL;
+
 	return 0;
 }
 EXPORT_SYMBOL(xt_compat_check_entry_offsets);
@@ -596,6 +607,10 @@ int xt_check_entry_offsets(const void *base,
 	if (target_offset + t->u.target_size > next_offset)
 		return -EINVAL;
 
+	if (strcmp(t->u.user.name, XT_STANDARD_TARGET) == 0 &&
+	    target_offset + sizeof(struct xt_standard_target) != next_offset)
+		return -EINVAL;
+
 	return 0;
 }
 EXPORT_SYMBOL(xt_check_entry_offsets);