linux-kernel - [PATCH nf] netfilter: nf_tables: unconditionally flush pending work before notifier

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Date: Tue,  2 Jul 2024 16:08:14 +0200
From: Florian Westphal <fw@...len.de>
To: <netfilter-devel@...r.kernel.org>
Cc: linux-kernel@...r.kernel.org,
	<netdev@...r.kernel.org>,
	syzkaller-bugs@...glegroups.com,
	Florian Westphal <fw@...len.de>,
	syzbot+4fd66a69358fc15ae2ad@...kaller.appspotmail.com
Subject: [PATCH nf] netfilter: nf_tables: unconditionally flush pending work before notifier

syzbot reports:

KASAN: slab-uaf in nft_ctx_update include/net/netfilter/nf_tables.h:1831
KASAN: slab-uaf in nft_commit_release net/netfilter/nf_tables_api.c:9530
KASAN: slab-uaf int nf_tables_trans_destroy_work+0x152b/0x1750 net/netfilter/nf_tables_api.c:9597
Read of size 2 at addr ffff88802b0051c4 by task kworker/1:1/45
[..]
Workqueue: events nf_tables_trans_destroy_work
Call Trace:
 nft_ctx_update include/net/netfilter/nf_tables.h:1831 [inline]
 nft_commit_release net/netfilter/nf_tables_api.c:9530 [inline]
 nf_tables_trans_destroy_work+0x152b/0x1750 net/netfilter/nf_tables_api.c:9597

Problem is that the notifier does a conditional flush, but its possible
that the table-to-be-removed is still referenced by transactions being
processed by the worker, so we need to flush unconditionally.

We could make the flush_work depend on whether we found a table to delete
in nf-next to avoid the flush for most cases.

AFAICS this problem is only exposed in nf-next, with
commit e169285f8c56 ("netfilter: nf_tables: do not store nft_ctx in transaction objects"),
with this commit applied there is an unconditional fetch of
table->family which is whats triggering the above splat.

Fixes: 2c9f0293280e ("netfilter: nf_tables: flush pending destroy work before netlink notifier")
Reported-and-tested-by: syzbot+4fd66a69358fc15ae2ad@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=4fd66a69358fc15ae2ad
Signed-off-by: Florian Westphal <fw@...len.de>
---
 net/netfilter/nf_tables_api.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/net/netfilter/nf_tables_api.c b/net/netfilter/nf_tables_api.c
index 02d75aefaa8e..683f6a4518ee 100644
--- a/net/netfilter/nf_tables_api.c
+++ b/net/netfilter/nf_tables_api.c
@@ -11552,8 +11552,7 @@ static int nft_rcv_nl_event(struct notifier_block *this, unsigned long event,
 
 	gc_seq = nft_gc_seq_begin(nft_net);
 
-	if (!list_empty(&nf_tables_destroy_list))
-		nf_tables_trans_destroy_flush_work();
+	nf_tables_trans_destroy_flush_work();
 again:
 	list_for_each_entry(table, &nft_net->tables, list) {
 		if (nft_table_has_owner(table) &&
-- 
2.44.2