Re: Re: [PATCH v2] ipv6: fix memory leak in __ip6_make_skb() when queue is empty

[王明煜 <25181214217@stu.xidian.edu.cn>]

Hi,

Thank you so much for the review and for pointing me to the correct Fixes tag!

You hit the nail on the head regarding `__ip6_append_data()`. After re-evaluating the code path based on your question, I realize my assumption in v2 was incorrect. `__ip6_append_data()` does indeed guarantee that an skb is queued upon success, making the `skb == NULL` path dead code in this context.

I traced the `failslab` memory leak back to its true origin: the lockless fast path wrapper `ip6_make_skb()`.

Sabrina previously noted that `ip6_setup_cork()` failures correctly release the dst. That is absolutely true for the slow path, where `udp_v6_flush_pending_frames()` eventually handles the cleanup. 

However, in the fast path, `ip6_make_skb()` calls `ip6_setup_cork()`. Inside `ip6_setup_cork()`, `cork->base.dst` is assigned early. If a subsequent memory allocation fails (e.g., `v6_cork->opt = kzalloc(...)` failing due to failslab), it returns an error. `ip6_make_skb()` then directly returns `ERR_PTR(err)` WITHOUT calling `ip6_cork_release(cork)`.

Since `udpv6_sendmsg()` assumes the `dst` reference is stolen by `ip6_make_skb()` and unconditionally jumps to `out_no_dst`, the `dst` is completely leaked.

The fix is simply to add `ip6_cork_release(cork)` in the `ip6_setup_cork()` error path inside `ip6_make_skb()`.

I will submit a v3 patch shortly addressing this true root cause and using your suggested Fixes tag. Thank you again for steering me in the exact right direction!

Best regards,
Mingyu Wang


> -----原始邮件-----
> 发件人: "Willem de Bruijn" <willemdebruijn.kernel@gmail.com>
> 发送时间:2026-04-23 22:59:45 (星期四)
> 收件人: "Mingyu Wang" <25181214217@stu.xidian.edu.cn>, willemdebruijn.kernel@gmail.com, davem@davemloft.net, dsahern@kernel.org, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com
> 抄送: sd@queasysnail.net, horms@kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, "Mingyu Wang" <25181214217@stu.xidian.edu.cn>, syzbot+e5d6936b9f4545fd88ab@syzkaller.appspotmail.com
> 主题: Re: [PATCH v2] ipv6: fix memory leak in __ip6_make_skb() when queue is empty
> 
> Mingyu Wang wrote:
> > During fuzzing with failslab enabled, a memory leak was observed in the
> > IPv6 UDP send path.
> > 
> > The root cause resides in __ip6_make_skb(). In extremely rare cases
> > (such as fault injection or specific empty payload conditions),
> 
> Can you elaborate on this? Which fault injection lets
> __ip6_append_data succeed without writing data?
> 
> > __ip6_append_data() may succeed but leave the socket's write queue
> > empty.
> > 
> > When __ip6_make_skb() is subsequently called, __skb_dequeue(queue)
> > returns NULL. The previous logic handled this by executing a 'goto out;',
> > which completely bypassed the call to ip6_cork_release(cork).
> > 
> > Since the 'cork' structure actively holds a reference to the routing
> > entry (dst_entry) and potentially other allocated options, skipping
> > the release cleanly leaks these resources.
> > 
> > Fix this by introducing an 'out_cork_release' label and jumping to it
> > when skb is NULL, ensuring the cork state is always properly cleaned up.
> > The now-unused 'out' label is also removed to prevent compiler warnings.
> > 
> > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> 
> I think this is 
> 
> Fixes: 6422398c2ab0 ("ipv6: introduce ipv6_make_skb")
> 
> > Reported-by: syzbot+e5d6936b9f4545fd88ab@syzkaller.appspotmail.com
> > Signed-off-by: Mingyu Wang <25181214217@stu.xidian.edu.cn>