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

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

[Chengfeng Ye]

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>
---
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

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

[Antoine Tenart]

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

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

[Jiayuan Chen]

On 9/29/26 12:25 AM, Chengfeng Ye wrote:
> ip_mc_output() reads rt->dst.dev and stores it in skb->dev without RCU


Could you take a look at this thread ?

https://lore.kernel.org/netdev/ahveCu-zzFlpeVut@v4bel/


It seems there are still a lot of similar paths to be considered.


> 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>
> ---
> 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)

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

[Ido Schimmel]

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

It can also be rt_flush_dev(), if the dst entry is uncached.

> and release the old device after an RCU grace period, leaving the multicast

The reference release is immediate, only the free happens after a GP.

> 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.

ip_skb_dst_mtu() reads the device from the dst entry, not from skb->dev.

> 
> 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>
> ---
> 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(-)

The diff itself looks fine.

FTR, I'm aware of the discussion that wants an RCU critical section in
ip_local_out(), but:

1. Adding one in ip_mc_output() is consistent with other output
functions: ip_output(), ip6_output(), ip_mr_output(), ip6_mr_output(),
etc.

2. This patch doesn't affect the fast path, unlike an RCU critical
section in ip_local_out() [1].

3. Patching ip_local_out() doesn't cover all the process context callers
of dst_output() either.

And yes, I'm aware that more places suffer from this "problem". I opened
an internal ticket after seeing [2]. I will get to it eventually and
will target it at net-next since these UAFs are not very realistic as
evident from [2].

[1] https://lore.kernel.org/netdev/85c6016d-fb15-4c61-aab9-61448911f934@redhat.com/
[2] https://lore.kernel.org/netdev/20260819164528.3761320-1-jjy600901@snu.ac.kr/