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Subject: [PATCH] netfilter: nf_tables: fix use-after-free in nft_set_elem lookup
Author: kartikey406@gmail.com

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git main

Add RCU read-side critical section in nft_get_set_elem() to protect
against concurrent element deletion by garbage collection.

The lookup path in nft_rbtree_get() dereferences set elements through
interval->from and interval->to pointers via nft_array_get_cmp().
These elements can be freed by nft_trans_gc_trans_free() running in
an RCU callback context while the lookup is in progress, leading to
a use-after-free.

The crash occurs in nft_array_get_cmp() when it calls nft_set_ext_key()
on a freed element:

  BUG: KASAN: slab-use-after-free in nft_array_get_cmp+0x1f6/0x2a0

Protect the element lookup with rcu_read_lock()/rcu_read_unlock() to
ensure the RCU grace period extends until after we're done accessing
the element data. This prevents the GC callback from freeing elements
while they're being accessed.

Reported-by: syzbot+d417922a3e7935517ef6@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=d417922a3e7935517ef6
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
 net/netfilter/nf_tables_api.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/netfilter/nf_tables_api.c b/net/netfilter/nf_tables_api.c
index be4924aeaf0e..57affe22e0bd 100644
--- a/net/netfilter/nf_tables_api.c
+++ b/net/netfilter/nf_tables_api.c
@@ -6540,7 +6540,10 @@ static int nft_get_set_elem(struct nft_ctx *ctx, const struct nft_set *set,
 			return err;
 	}
 
+	rcu_read_lock();
 	err = nft_setelem_get(ctx, set, &elem, flags);
+	rcu_read_unlock();
+
 	if (err < 0)
 		return err;
 
-- 
2.43.0