[PATCH nf-next] netfilter: ebtables: bound num_counters like nentries in do_replace()

[Jiayuan Chen <jiayuan.chen@linux.dev>]

do_replace_finish() allocates the counter buffer before it is validated:

   counterstmp = vmalloc_array(repl->num_counters, sizeof(*counterstmp));

do_replace() only checks num_counters against INT_MAX / sizeof(struct
ebt_counter), so vmalloc_array() can be asked for up to 134217726 * 16 =
2147483616 bytes (~2 GiB).

num_counters must in fact equal nentries: do_replace_finish() later
rejects the request when repl->num_counters != t->private->nentries.
get_counters() folds the per-CPU counters back into one entry per rule,
so what userspace gets is bounded by nentries, never by nentries *
nr_cpus. Apply the same upper bound used for nentries (MAX_EBT_ENTRIES)
to the incoming num_counters so the over-sized allocation can no longer
be requested.

The allocation is still kept outside the ebt_mutex, since vmalloc() may
sleep and trigger reclaim; only the bound is tightened.

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 net/bridge/netfilter/ebtables.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/net/bridge/netfilter/ebtables.c b/net/bridge/netfilter/ebtables.c
index f20c039e44c8..042d31278713 100644
--- a/net/bridge/netfilter/ebtables.c
+++ b/net/bridge/netfilter/ebtables.c
@@ -39,6 +39,8 @@
 #define COUNTER_OFFSET(n) (SMP_ALIGN(n * sizeof(struct ebt_counter)))
 #define COUNTER_BASE(c, n, cpu) ((struct ebt_counter *)(((char *)c) + \
 				 COUNTER_OFFSET(n) * cpu))
+#define MAX_EBT_ENTRIES (((INT_MAX - sizeof(struct ebt_table_info)) / \
+			 NR_CPUS - SMP_CACHE_BYTES) / sizeof(struct ebt_counter))
 
 struct ebt_pernet {
 	struct list_head tables;
@@ -1124,10 +1126,9 @@ static int do_replace(struct net *net, sockptr_t arg, unsigned int len)
 		return -EINVAL;
 
 	/* overflow check */
-	if (tmp.nentries >= ((INT_MAX - sizeof(struct ebt_table_info)) /
-			NR_CPUS - SMP_CACHE_BYTES) / sizeof(struct ebt_counter))
+	if (tmp.nentries >= MAX_EBT_ENTRIES)
 		return -ENOMEM;
-	if (tmp.num_counters >= INT_MAX / sizeof(struct ebt_counter))
+	if (tmp.num_counters >= MAX_EBT_ENTRIES)
 		return -ENOMEM;
 
 	tmp.name[sizeof(tmp.name) - 1] = 0;
@@ -2265,10 +2266,9 @@ static int compat_copy_ebt_replace_from_user(struct ebt_replace *repl,
 	if (tmp.entries_size == 0)
 		return -EINVAL;
 
-	if (tmp.nentries >= ((INT_MAX - sizeof(struct ebt_table_info)) /
-			NR_CPUS - SMP_CACHE_BYTES) / sizeof(struct ebt_counter))
+	if (tmp.nentries >= MAX_EBT_ENTRIES)
 		return -ENOMEM;
-	if (tmp.num_counters >= INT_MAX / sizeof(struct ebt_counter))
+	if (tmp.num_counters >= MAX_EBT_ENTRIES)
 		return -ENOMEM;
 
 	memcpy(repl, &tmp, offsetof(struct ebt_replace, hook_entry));
-- 
2.43.0