[PATCH net v2] inet: frags: fix use-after-free caused by the fqdir_pre_exit() flush

[PATCH net v2] inet: frags: fix use-after-free caused by the fqdir_pre_exit() flush

[Hyunwoo Kim]

On netns teardown, fqdir_pre_exit() walks the fqdir rhashtable and
flushes every fragment queue that is not yet complete using
inet_frag_queue_flush(). That helper frees all the skbs queued on the
fragment queue but does not set INET_FRAG_COMPLETE, and leaves
q->fragments_tail and q->last_run_head pointing at the freed skbs.
The queue itself stays in the rhashtable.

fqdir_pre_exit() first lowers high_thresh to 0 to stop new queue lookups,
but it cannot stop a fragment that already obtained the queue through
inet_frag_find() earlier and stalled just before taking the queue lock.
Once that fragment resumes after the flush and takes the queue lock,
it passes the INET_FRAG_COMPLETE check and then dereferences the freed
fragments_tail. inet_frag_queue_insert() reads FRAG_CB() and ->len of
that pointer and, on the append path, writes ->next_frag, causing a
slab use-after-free. IPv6, nf_conntrack_reasm6 and 6lowpan reassembly
share the same flush path and are affected as well.

Reset rb_fragments, fragments_tail and last_run_head in
inet_frag_queue_flush() so a flushed queue no longer points at the
freed skbs. A fragment that resumes after the flush and takes the
queue lock then finds an empty queue and starts a new run instead of
dereferencing the freed fragments_tail. ip_frag_reinit() already
performed this reset after its own flush, so drop the now duplicate
code there.

Cc: stable@vger.kernel.org
Fixes: 006a5035b495 ("inet: frags: flush pending skbs in fqdir_pre_exit()")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
---
Changes in v2:
- Move the queue pointer reset into inet_frag_queue_flush() to remove the
duplicate reset in ip_frag_reinit().
- Drop the INET_FRAG_COMPLETE setting since it leaks the queue on the
fqdir_pre_exit() path.
- v1: https://lore.kernel.org/all/ah1Sw2g-I89BRRiT@v4bel/
---
 net/ipv4/inet_fragment.c | 3 +++
 net/ipv4/ip_fragment.c   | 3 ---
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/inet_fragment.c b/net/ipv4/inet_fragment.c
index 393770920abd..1127519b8416 100644
--- a/net/ipv4/inet_fragment.c
+++ b/net/ipv4/inet_fragment.c
@@ -328,6 +328,9 @@ void inet_frag_queue_flush(struct inet_frag_queue *q,
 	reason = reason ?: SKB_DROP_REASON_FRAG_REASM_TIMEOUT;
 	sum = inet_frag_rbtree_purge(&q->rb_fragments, reason);
 	sub_frag_mem_limit(q->fqdir, sum);
+	q->rb_fragments = RB_ROOT;
+	q->fragments_tail = NULL;
+	q->last_run_head = NULL;
 }
 EXPORT_SYMBOL(inet_frag_queue_flush);
 
diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
index 56b0f738d2f2..c790d2f49487 100644
--- a/net/ipv4/ip_fragment.c
+++ b/net/ipv4/ip_fragment.c
@@ -250,9 +250,6 @@ static int ip_frag_reinit(struct ipq *qp)
 	qp->q.flags = 0;
 	qp->q.len = 0;
 	qp->q.meat = 0;
-	qp->q.rb_fragments = RB_ROOT;
-	qp->q.fragments_tail = NULL;
-	qp->q.last_run_head = NULL;
 	qp->iif = 0;
 	qp->ecn = 0;
 
-- 
2.43.0

Re: [PATCH net v2] inet: frags: fix use-after-free caused by the fqdir_pre_exit() flush

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Tue, 2 Jun 2026 19:21:05 +0900 you wrote:
> On netns teardown, fqdir_pre_exit() walks the fqdir rhashtable and
> flushes every fragment queue that is not yet complete using
> inet_frag_queue_flush(). That helper frees all the skbs queued on the
> fragment queue but does not set INET_FRAG_COMPLETE, and leaves
> q->fragments_tail and q->last_run_head pointing at the freed skbs.
> The queue itself stays in the rhashtable.
> 
> [...]

Here is the summary with links:
  - [net,v2] inet: frags: fix use-after-free caused by the fqdir_pre_exit() flush
    https://git.kernel.org/netdev/net/c/32594b098549

You are awesome, thank you!
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