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

[Hangbin Liu]

On Sun, Sep 27, 2026 at 03:10:51PM +0800, Chengfeng Ye wrote:
> When a socket ignores the path MTU, ip_skb_dst_mtu() reads the device
> MTU without RCU protection. The multicast and broadcast output path
> through ip_mc_output() can reach this helper without an RCU read lock.
> Netfilter releases its internal read lock before calling
> ip_finish_output().
> 
> A sender can load dst->dev and then be preempted before reading dev->mtu.
> Concurrent device unregistration replaces dst->dev with blackhole_netdev
> and drops the old device reference. After the grace period and remaining
> references drain, the device can be freed before the sender resumes and
> reads its MTU, causing a use-after-free.
> 
> 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
> 
>   Allocated by task 97:
>    __kvmalloc_node_noprof+0x1d4/0x620
>    alloc_netdev_mqs+0x81/0x12c0
>    rtnl_create_link+0xaa3/0xe30
>    rtnl_newlink+0xabc/0x1f70
> 
>   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 device lookup and MTU read with RCU and use dst_dev_rcu() to
> access the device pointer. Keep the MTU limit and headroom calculation
> unchanged.
> 
> Fixes: 628a5c561890 ("[INET]: Add IP(V6)_PMTUDISC_RPOBE")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Chengfeng Ye <nicoyip.dev@xxxxxxxxx>
> ---
>  include/net/ip.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/include/net/ip.h b/include/net/ip.h
> index 6f602df72ee6..14d6f77f3bb8 100644
> --- a/include/net/ip.h
> +++ b/include/net/ip.h
> @@ -543,7 +543,9 @@ static inline unsigned int ip_skb_dst_mtu(struct sock *sk,
>  		return ip_dst_mtu_maybe_forward(dst, forwarding);
>  	}
>  
> -	mtu = min(READ_ONCE(dst_dev(dst)->mtu), IP_MAX_MTU);
> +	rcu_read_lock();
> +	mtu = min(READ_ONCE(dst_dev_rcu(dst)->mtu), IP_MAX_MTU);
> +	rcu_read_unlock();
>  	return mtu - lwtunnel_headroom(dst->lwtstate, mtu);
>  }
>  
> -- 
> 2.43.0
> 

Reviewed-by: Hangbin Liu <liuhangbin@xxxxxxxxxx>