Re: [PATCH net v2] ipv4: Fix device use-after-free in ip_mc_output()

[Antoine Tenart <atenart@kernel.org>]

On Tue, Sep 29, 2026 at 12:25:34AM +0800, Chengfeng Ye wrote:
> 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 <nicoyip.dev@gmail.com>

Reviewed-by: Antoine Tenart <atenart@kernel.org>

> ---
> 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