netfilter: x_tables: check for bogus target offset

We're currently asserting that targetoff + targetsize <= nextoff.

Extend it to also check that targetoff is >= sizeof(xt_entry).
Since this is generic code, add an argument pointing to the start of the
match/target, we can then derive the base structure size from the delta.

We also need the e->elems pointer in a followup change to validate matches.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/linux/netfilter/x_tables.h

@@ -242,7 +242,7 @@ void xt_unregister_match(struct xt_match *target);
 int xt_register_matches(struct xt_match *match, unsigned int n);
 void xt_unregister_matches(struct xt_match *match, unsigned int n);
 
-int xt_check_entry_offsets(const void *base,
+int xt_check_entry_offsets(const void *base, const char *elems,
 			   unsigned int target_offset,
 			   unsigned int next_offset);
 
@@ -494,7 +494,7 @@ void xt_compat_target_from_user(struct xt_entry_target *t, void **dstptr,
 				unsigned int *size);
 int xt_compat_target_to_user(const struct xt_entry_target *t,
 			     void __user **dstptr, unsigned int *size);
-int xt_compat_check_entry_offsets(const void *base,
+int xt_compat_check_entry_offsets(const void *base, const char *elems,
 				  unsigned int target_offset,
 				  unsigned int next_offset);
 

net/ipv4/netfilter/arp_tables.c

@@ -592,7 +592,8 @@ static inline int check_entry_size_and_hooks(struct arpt_entry *e,
 	if (!arp_checkentry(&e->arp))
 		return -EINVAL;
 
-	err = xt_check_entry_offsets(e, e->target_offset, e->next_offset);
+	err = xt_check_entry_offsets(e, e->elems, e->target_offset,
+				     e->next_offset);
 	if (err)
 		return err;
 
@@ -1254,7 +1255,7 @@ check_compat_entry_size_and_hooks(struct compat_arpt_entry *e,
 	if (!arp_checkentry(&e->arp))
 		return -EINVAL;
 
-	ret = xt_compat_check_entry_offsets(e, e->target_offset,
+	ret = xt_compat_check_entry_offsets(e, e->elems, e->target_offset,
 					    e->next_offset);
 	if (ret)
 		return ret;

net/ipv4/netfilter/ip_tables.c

@@ -754,7 +754,8 @@ check_entry_size_and_hooks(struct ipt_entry *e,
 	if (!ip_checkentry(&e->ip))
 		return -EINVAL;
 
-	err = xt_check_entry_offsets(e, e->target_offset, e->next_offset);
+	err = xt_check_entry_offsets(e, e->elems, e->target_offset,
+				     e->next_offset);
 	if (err)
 		return err;
 
@@ -1513,7 +1514,7 @@ check_compat_entry_size_and_hooks(struct compat_ipt_entry *e,
 	if (!ip_checkentry(&e->ip))
 		return -EINVAL;
 
-	ret = xt_compat_check_entry_offsets(e,
+	ret = xt_compat_check_entry_offsets(e, e->elems,
 					    e->target_offset, e->next_offset);
 	if (ret)
 		return ret;

net/ipv6/netfilter/ip6_tables.c

@@ -766,7 +766,8 @@ check_entry_size_and_hooks(struct ip6t_entry *e,
 	if (!ip6_checkentry(&e->ipv6))
 		return -EINVAL;
 
-	err = xt_check_entry_offsets(e, e->target_offset, e->next_offset);
+	err = xt_check_entry_offsets(e, e->elems, e->target_offset,
+				     e->next_offset);
 	if (err)
 		return err;
 
@@ -1525,7 +1526,7 @@ check_compat_entry_size_and_hooks(struct compat_ip6t_entry *e,
 	if (!ip6_checkentry(&e->ipv6))
 		return -EINVAL;
 
-	ret = xt_compat_check_entry_offsets(e,
+	ret = xt_compat_check_entry_offsets(e, e->elems,
 					    e->target_offset, e->next_offset);
 	if (ret)
 		return ret;

net/netfilter/x_tables.c

@@ -546,14 +546,17 @@ struct compat_xt_standard_target {
 	compat_uint_t verdict;
 };
 
-/* see xt_check_entry_offsets */
-int xt_compat_check_entry_offsets(const void *base,
+int xt_compat_check_entry_offsets(const void *base, const char *elems,
 				  unsigned int target_offset,
 				  unsigned int next_offset)
 {
+	long size_of_base_struct = elems - (const char *)base;
 	const struct compat_xt_entry_target *t;
 	const char *e = base;
 
+	if (target_offset < size_of_base_struct)
+		return -EINVAL;
+
 	if (target_offset + sizeof(*t) > next_offset)
 		return -EINVAL;
 
@@ -577,12 +580,16 @@ EXPORT_SYMBOL(xt_compat_check_entry_offsets);
  * xt_check_entry_offsets - validate arp/ip/ip6t_entry
  *
  * @base: pointer to arp/ip/ip6t_entry
+ * @elems: pointer to first xt_entry_match, i.e. ip(6)t_entry->elems
  * @target_offset: the arp/ip/ip6_t->target_offset
  * @next_offset: the arp/ip/ip6_t->next_offset
  *
  * validates that target_offset and next_offset are sane.
  * Also see xt_compat_check_entry_offsets for CONFIG_COMPAT version.
  *
+ * This function does not validate the targets or matches themselves, it
+ * only tests that all the offsets and sizes are correct.
+ *
  * The arp/ip/ip6t_entry structure @base must have passed following tests:
  * - it must point to a valid memory location
  * - base to base + next_offset must be accessible, i.e. not exceed allocated
@@ -591,12 +598,18 @@ EXPORT_SYMBOL(xt_compat_check_entry_offsets);
  * Return: 0 on success, negative errno on failure.
  */
 int xt_check_entry_offsets(const void *base,
+			   const char *elems,
 			   unsigned int target_offset,
 			   unsigned int next_offset)
 {
+	long size_of_base_struct = elems - (const char *)base;
 	const struct xt_entry_target *t;
 	const char *e = base;
 
+	/* target start is within the ip/ip6/arpt_entry struct */
+	if (target_offset < size_of_base_struct)
+		return -EINVAL;
+
 	if (target_offset + sizeof(*t) > next_offset)
 		return -EINVAL;