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

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@vger.kernel.org
Signed-off-by: Chengfeng Ye <redacted>
---
 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