[PATCH v2] inet: frags: invalidate queues before flushing them

fqdir_pre_exit() flushes the skbs from incomplete queues without
changing their completion state. A fragment which found a queue before
high_thresh was cleared can then acquire the queue lock and reuse stale
reassembly metadata. A queue concurrently killed after fqdir->dead is
set can instead become INET_FRAG_COMPLETE|INET_FRAG_HASH_DEAD while
still holding its old skbs; skipping it because it is complete leaves
those references behind until asynchronous fqdir teardown.

For IPv6, stale metadata can make ip6_frag_reasm() use the old
nhoffset with a new skb and access memory out of bounds. The resulting
heap corruption can be leveraged for local privilege escalation when
unprivileged network namespaces are available. Unflushed fragments can
also keep conntrack references alive after the conntrack per-net
cleanup point.

Kill each incomplete queue, then flush every queue still owned by the
dying rhashtable. HASH_DEAD identifies that ownership, while complete
queues without it are already owned by another destroy path and must be
left alone. Releasing a timer reference removed by inet_frag_kill() is
deferred to inet_frag_putn(), after the queue lock is dropped.

KASAN report:

  BUG: KASAN: slab-out-of-bounds in ipv6_frag_rcv (net/ipv6/reassembly.c:289 (discriminator 2) net/ipv6/reassembly.c:229 (discriminator 2) net/ipv6/reassembly.c:391 (discriminator 2))
  Write of size 1 at addr ff110001039c6e00 by task poc/771
  Call Trace:
  ? ipv6_frag_rcv (net/ipv6/reassembly.c:289 (discriminator 2) net/ipv6/reassembly.c:229 (discriminator 2) net/ipv6/reassembly.c:391 (discriminator 2))
  ipv6_frag_rcv (net/ipv6/reassembly.c:289 (discriminator 2) net/ipv6/reassembly.c:229 (discriminator 2) net/ipv6/reassembly.c:391 (discriminator 2))
  ip6_protocol_deliver_rcu (net/ipv6/ip6_input.c:479 (discriminator 5))
  ip6_input_finish (net/ipv6/ip6_input.c:534)
  ipv6_rcv (include/net/dst.h:480 (discriminator 3) net/ipv6/ip6_input.c:119 (discriminator 3) net/ipv6/ip6_input.c:109 (discriminator 3) include/linux/netfilter.h:325 (discriminator 3) include/linux/netfilter.h:319 (discriminator 3) net/ipv6/ip6_input.c:351 (discriminator 3))
  packet_sendmsg (net/packet/af_packet.c:3110 net/packet/af_packet.c:3142)
  __x64_sys_sendmmsg (net/socket.c:2883 net/socket.c:2880 net/socket.c:2880)
  The buggy address belongs to the object at ff110001039c6b40
  which belongs to the cache skbuff_small_head of size 704
  The buggy address is located 0 bytes to the right of
  allocated 704-byte region [ff110001039c6b40, ff110001039c6e00)

  BUG: KASAN: slab-out-of-bounds in ip6_protocol_deliver_rcu (net/ipv6/ip6_input.c:423 (discriminator 1))
  Read of size 1 at addr ff110001039c6e08 by task poc/771

24 more lines

diff --git a/net/ipv4/inet_fragment.c b/net/ipv4/inet_fragment.c
--- a/net/ipv4/inet_fragment.c
+++ b/net/ipv4/inet_fragment.c

@@ -235,15 +235,21 @@ void fqdir_pre_exit(struct fqdir *fqdir)
 	rhashtable_walk_start(&hti);
 
 	while ((fq = rhashtable_walk_next(&hti))) {
+		int refs = 0;
+
 		if (IS_ERR(fq)) {
 			if (PTR_ERR(fq) != -EAGAIN)
 				break;
 			continue;
 		}
 		spin_lock_bh(&fq->lock);
-		if (!(fq->flags & INET_FRAG_COMPLETE))
+		if (!(fq->flags & INET_FRAG_COMPLETE))
+			inet_frag_kill(fq, &refs);
+
+		if (fq->flags & INET_FRAG_HASH_DEAD)
 			inet_frag_queue_flush(fq, 0);
 		spin_unlock_bh(&fq->lock);
+		inet_frag_putn(fq, refs);
 	}
 
 	rhashtable_walk_stop(&hti);

-- 
2.43.0

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski [off-list ref]:

On Sat,  5 Sep 2026 00:28:00 +0800 you wrote:

fqdir_pre_exit() flushes the skbs from incomplete queues without
changing their completion state. A fragment which found a queue before
high_thresh was cleared can then acquire the queue lock and reuse stale
reassembly metadata. A queue concurrently killed after fqdir->dead is
set can instead become INET_FRAG_COMPLETE|INET_FRAG_HASH_DEAD while
still holding its old skbs; skipping it because it is complete leaves
those references behind until asynchronous fqdir teardown.

[...]

Here is the summary with links:
  - [v2] inet: frags: invalidate queues before flushing them
    https://git.kernel.org/netdev/net/c/b824476c56a1

You are awesome, thank you!
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