[PATCH nf] netfilter: nft_set_rbtree: skip transaction elements during GC

[bestswngs@gmail.com]

From: Weiming Shi <bestswngs@gmail.com>

Since nft_set_commit_update() runs set commit callbacks before processing
NEWSETELEM transactions, nft_rbtree_gc_scan() can observe elements added by
the transaction being committed.

The scan records an interval end in rbe_end without checking the element's
transaction state. A later, unrelated expired start then moves both
elements to the expired list. The synchronous GC queue can free the new end
element before the transaction subsequently activates it, causing a
use-after-free.

Only consider elements that are fully active in both generations. This
keeps transaction-state elements out of the GC scan and preserves interval
pairing across skipped elements.

KASAN reports:

  BUG: KASAN: slab-use-after-free in nft_setelem_activate
  nft_setelem_activate net/netfilter/nf_tables_api.c:7047
  nf_tables_commit net/netfilter/nf_tables_api.c:11137

  Allocated by task 130:
  nft_set_elem_init net/netfilter/nf_tables_api.c:6794
  nft_add_set_elem net/netfilter/nf_tables_api.c:7523

  Freed by task 130:
  nft_trans_gc_trans_free net/netfilter/nf_tables_api.c:10506
  rcu_core kernel/rcu/tree.c:2919

Fixes: 1e3b9e1c77fe ("netfilter: nf_tables: call set ops .commit when building new ruleset blob")
Reported-by: <co+ee5e50ef2670e5f4@bugs.sh>
Assisted-by: LLM
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
 net/netfilter/nft_set_rbtree.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/netfilter/nft_set_rbtree.c b/net/netfilter/nft_set_rbtree.c
index 9894832281c4b..12431b55752fb 100644
--- a/net/netfilter/nft_set_rbtree.c
+++ b/net/netfilter/nft_set_rbtree.c
@@ -900,6 +900,8 @@ static void nft_rbtree_gc_scan(struct nft_set *set)
 		next = rb_next(node);
 
 		rbe = rb_entry(node, struct nft_rbtree_elem, node);
+		if (!nft_set_elem_active(&rbe->ext, NFT_GENMASK_ANY))
+			continue;
 
 		/* elements are reversed in the rbtree for historical reasons,
 		 * from highest to lowest value, that is why end element is
-- 
2.55.0