From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	Xiang Mei <xmei5@asu.edu>,
	Weiming Shi <bestswngs@gmail.com>,
	Ido Schimmel <idosch@nvidia.com>,
	Paolo Abeni <pabeni@redhat.com>,
	Sasha Levin <sashal@kernel.org>
Subject: [PATCH 6.1 058/609] ipv4: fib: free fib_alias with kfree_rcu() on insert error path
Date: Mon, 17 Aug 2026 15:25:54 +0200
Message-ID: <20260817132545.395755984@linuxfoundation.org>
In-Reply-To: <20260817132543.039278408@linuxfoundation.org>
References: <20260817132543.039278408@linuxfoundation.org>
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6.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Weiming Shi <bestswngs@gmail.com>

[ Upstream commit f2f152e94a67bc746afaf05a1b2702c195553112 ]

fib_table_insert() publishes new_fa into the leaf's fa_list with
fib_insert_alias() before calling the fib entry notifiers. When a
notifier fails, the error path removes new_fa with fib_remove_alias()
(hlist_del_rcu) and frees it right away with kmem_cache_free().

fib_table_lookup() walks that list under rcu_read_lock() only, so a
concurrent lookup that already reached new_fa keeps reading it after the
free:

 BUG: KASAN: slab-use-after-free in fib_table_lookup (net/ipv4/fib_trie.c:1601)
 Read of size 1 at addr ffff88810676d4eb by task exploit/297
 Call Trace:
  fib_table_lookup (net/ipv4/fib_trie.c:1601)
  ip_route_output_key_hash_rcu (net/ipv4/route.c:2814)
  ip_route_output_key_hash (net/ipv4/route.c:2705)
  __ip4_datagram_connect (net/ipv4/datagram.c:49)
  udp_connect (net/ipv4/udp.c:2144)
  __sys_connect (net/socket.c:2167)
  __x64_sys_connect (net/socket.c:2173)
  do_syscall_64
  entry_SYSCALL_64_after_hwframe
 which belongs to the cache ip_fib_alias of size 56

Triggering the error path needs CAP_NET_ADMIN and a registered fib
notifier that can reject a route; a netdevsim device whose IPv4 FIB
resource is exhausted is enough.

Free new_fa with alias_free_mem_rcu(), as fib_table_delete() already
does for a fib_alias removed from the trie.

Fixes: a6c76c17df02 ("ipv4: Notify route after insertion to the routing table")
Reported-by: Xiang Mei <xmei5@asu.edu>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260704171421.1786806-1-bestswngs@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/ipv4/fib_trie.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
index c9e1526e749b2b..53e7664eeb0a2a 100644
--- a/net/ipv4/fib_trie.c
+++ b/net/ipv4/fib_trie.c
@@ -1390,7 +1390,7 @@ int fib_table_insert(struct net *net, struct fib_table *tb,
 out_remove_new_fa:
 	fib_remove_alias(t, tp, l, new_fa);
 out_free_new_fa:
-	kmem_cache_free(fn_alias_kmem, new_fa);
+	alias_free_mem_rcu(new_fa);
 out:
 	fib_release_info(fi);
 err:
-- 
2.53.0