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[95.169.12.199]) by smtp.gmail.com with ESMTPSA id 5a478bee46e88-34141a4a00dsm31259622eec.1.2026.09.28.09.25.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 28 Sep 2026 09:25:42 -0700 (PDT) From: Chengfeng Ye To: David Ahern , Ido Schimmel , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Martin KaFai Lau , Wei Wang Cc: Hangbin Liu , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Chengfeng Ye , stable@vger.kernel.org Subject: [PATCH net v2] ipv4: Fix device use-after-free in ip_mc_output() Date: Tue, 29 Sep 2026 00:25:34 +0800 Message-ID: <20260928162534.2122207-1-nicoyip.dev@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit ip_mc_output() reads rt->dst.dev and stores it in skb->dev without RCU protection. dst_dev_put() can concurrently replace the destination device and release the old device after an RCU grace period, leaving the multicast output path with a stale skb->dev. The stale device can be dereferenced by the post-routing path. In particular, a socket using IP_PMTUDISC_PROBE reaches ip_skb_dst_mtu() from ip_finish_output() and reads the freed device's MTU. KASAN reported: BUG: KASAN: slab-use-after-free in ip_skb_dst_mtu+0x634/0x740 Read of size 4 at addr ffff88810921c038 by task poc/98 Call Trace: ip_skb_dst_mtu+0x634/0x740 __ip_finish_output.part.0+0x22/0x2c0 ip_mc_output+0x287/0x930 ip_send_skb+0x11d/0x150 udp_send_skb+0x63e/0xdf0 udp_sendmsg+0x1235/0x1da0 __sys_sendto+0x32c/0x3a0 Freed by task 99: kfree+0x131/0x3c0 device_release+0xc8/0x240 kobject_put+0x14d/0x280 netdev_run_todo+0x4cb/0xc70 rtnl_dellink+0x362/0xa90 Protect the whole multicast output path with an RCU read-side critical section, as ip_output() already does. Load the destination device through skb_dst_dev_rcu() and keep it protected through the post-routing hooks and ip_finish_output(). Fixes: 4a6ce2b6f2ec ("net: introduce a new function dst_dev_put()") Cc: stable@vger.kernel.org Signed-off-by: Chengfeng Ye --- Changes in v2: - Protect the whole ip_mc_output() path instead of only the MTU read. - Load the destination device with skb_dst_dev_rcu(), matching ip_output(). - Use the commit that made dst device replacement RCU-protected as Fixes. Link: https://lore.kernel.org/r/20260927071051.3693368-1-nicoyip.dev@gmail.com/ [v1] net/ipv4/ip_output.c | 19 +++++++++++++------ 1 file changed, 13 insertions(+), 6 deletions(-) diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c index a24cc8ee11d3..cf44597896cd 100644 --- a/net/ipv4/ip_output.c +++ b/net/ipv4/ip_output.c @@ -367,8 +367,11 @@ static int ip_mc_finish_output(struct net *net, struct sock *sk, int ip_mc_output(struct net *net, struct sock *sk, struct sk_buff *skb) { struct rtable *rt = skb_rtable(skb); - struct net_device *dev = rt->dst.dev; + struct net_device *dev; + int ret; + rcu_read_lock(); + dev = skb_dst_dev_rcu(skb); /* * If the indicated interface is up and running, send the packet. */ @@ -406,7 +409,8 @@ int ip_mc_output(struct net *net, struct sock *sk, struct sk_buff *skb) if (ip_hdr(skb)->ttl == 0) { kfree_skb(skb); - return 0; + ret = 0; + goto out; } } @@ -418,10 +422,13 @@ int ip_mc_output(struct net *net, struct sock *sk, struct sk_buff *skb) ip_mc_finish_output); } - return NF_HOOK_COND(NFPROTO_IPV4, NF_INET_POST_ROUTING, - net, sk, skb, NULL, skb->dev, - ip_finish_output, - !(IPCB(skb)->flags & IPSKB_REROUTED)); + ret = NF_HOOK_COND(NFPROTO_IPV4, NF_INET_POST_ROUTING, + net, sk, skb, NULL, skb->dev, + ip_finish_output, + !(IPCB(skb)->flags & IPSKB_REROUTED)); +out: + rcu_read_unlock(); + return ret; } int ip_output(struct net *net, struct sock *sk, struct sk_buff *skb) -- 2.43.0