netfilter: ebtables: reject blobs that don't provide all entry points

Harshit Mogalapalli says:
 In ebt_do_table() function dereferencing 'private->hook_entry[hook]'
 can lead to NULL pointer dereference. [..] Kernel panic:

general protection fault, probably for non-canonical address 0xdffffc0000000005: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000028-0x000000000000002f]
[..]
RIP: 0010:ebt_do_table+0x1dc/0x1ce0
Code: 89 fa 48 c1 ea 03 80 3c 02 00 0f 85 5c 16 00 00 48 b8 00 00 00 00 00 fc ff df 49 8b 6c df 08 48 8d 7d 2c 48 89 fa 48 c1 ea 03 <0f> b6 14 02 48 89 f8 83 e0 07 83 c0 03 38 d0 7c 08 84 d2 0f 85 88
[..]
Call Trace:
 nf_hook_slow+0xb1/0x170
 __br_forward+0x289/0x730
 maybe_deliver+0x24b/0x380
 br_flood+0xc6/0x390
 br_dev_xmit+0xa2e/0x12c0

For some reason ebtables rejects blobs that provide entry points that are
not supported by the table, but what it should instead reject is the
opposite: blobs that DO NOT provide an entry point supported by the table.

t->valid_hooks is the bitmask of hooks (input, forward ...) that will see
packets.  Providing an entry point that is not support is harmless
(never called/used), but the inverse isn't: it results in a crash
because the ebtables traverser doesn't expect a NULL blob for a location
its receiving packets for.

Instead of fixing all the individual checks, do what iptables is doing and
reject all blobs that differ from the expected hooks.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Reported-by: Harshit Mogalapalli <[email protected]>
Reported-by: syzkaller <[email protected]>
Signed-off-by: Florian Westphal <[email protected]>

Author: Florian Westphal

include/linux/netfilter_bridge/ebtables.h

@@ -94,10 +94,6 @@ struct ebt_table {
 	struct ebt_replace_kernel *table;
 	unsigned int valid_hooks;
 	rwlock_t lock;
-	/* e.g. could be the table explicitly only allows certain
-	 * matches, targets, ... 0 == let it in */
-	int (*check)(const struct ebt_table_info *info,
-	   unsigned int valid_hooks);
 	/* the data used by the kernel */
 	struct ebt_table_info *private;
 	struct nf_hook_ops *ops;

net/bridge/netfilter/ebtable_broute.c

@@ -36,18 +36,10 @@ static struct ebt_replace_kernel initial_table = {
 	.entries	= (char *)&initial_chain,
 };
 
-static int check(const struct ebt_table_info *info, unsigned int valid_hooks)
-{
-	if (valid_hooks & ~(1 << NF_BR_BROUTING))
-		return -EINVAL;
-	return 0;
-}
-
 static const struct ebt_table broute_table = {
 	.name		= "broute",
 	.table		= &initial_table,
 	.valid_hooks	= 1 << NF_BR_BROUTING,
-	.check		= check,
 	.me		= THIS_MODULE,
 };
 

net/bridge/netfilter/ebtable_filter.c

@@ -43,18 +43,10 @@ static struct ebt_replace_kernel initial_table = {
 	.entries	= (char *)initial_chains,
 };
 
-static int check(const struct ebt_table_info *info, unsigned int valid_hooks)
-{
-	if (valid_hooks & ~FILTER_VALID_HOOKS)
-		return -EINVAL;
-	return 0;
-}
-
 static const struct ebt_table frame_filter = {
 	.name		= "filter",
 	.table		= &initial_table,
 	.valid_hooks	= FILTER_VALID_HOOKS,
-	.check		= check,
 	.me		= THIS_MODULE,
 };
 

net/bridge/netfilter/ebtable_nat.c

@@ -43,18 +43,10 @@ static struct ebt_replace_kernel initial_table = {
 	.entries	= (char *)initial_chains,
 };
 
-static int check(const struct ebt_table_info *info, unsigned int valid_hooks)
-{
-	if (valid_hooks & ~NAT_VALID_HOOKS)
-		return -EINVAL;
-	return 0;
-}
-
 static const struct ebt_table frame_nat = {
 	.name		= "nat",
 	.table		= &initial_table,
 	.valid_hooks	= NAT_VALID_HOOKS,
-	.check		= check,
 	.me		= THIS_MODULE,
 };
 

net/bridge/netfilter/ebtables.c

@@ -1040,8 +1040,7 @@ static int do_replace_finish(struct net *net, struct ebt_replace *repl,
 		goto free_iterate;
 	}
 
-	/* the table doesn't like it */
-	if (t->check && (ret = t->check(newinfo, repl->valid_hooks)))
+	if (repl->valid_hooks != t->valid_hooks)
 		goto free_unlock;
 
 	if (repl->num_counters && repl->num_counters != t->private->nentries) {
@@ -1231,11 +1230,6 @@ int ebt_register_table(struct net *net, const struct ebt_table *input_table,
 	if (ret != 0)
 		goto free_chainstack;
 
-	if (table->check && table->check(newinfo, table->valid_hooks)) {
-		ret = -EINVAL;
-		goto free_chainstack;
-	}
-
 	table->private = newinfo;
 	rwlock_init(&table->lock);
 	mutex_lock(&ebt_mutex);