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

[Ido Schimmel]

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.

This still leaves other potential UAFs in the ip_mc_output() path.
Better to fix it by adding an RCU read-side critical section in
ip_mc_output() in a similar fashion to its unicast counterpart. See
commit 1dbf1d590d10 ("net: Add locking to protect skb->dev access in
ip_output"). You can blame 4a6ce2b6f2ec ("net: introduce a new function
dst_dev_put()").