[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Message-ID: <20240623045141.78101-1-kuniyu@amazon.com>
Date: Sat, 22 Jun 2024 21:51:41 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <zacecob@...tonmail.com>
CC: <bpf@...r.kernel.org>, <netdev@...r.kernel.org>, <kuniyu@...zon.com>
Subject: Re: Returning negative values repeatedly from a SOCK_FILTER ebpf prog stalls kernel thread
From: Zac Ecob <zacecob@...tonmail.com>
Date: Sat, 22 Jun 2024 12:20:05 +0000
> Problem is title.
>
> To trigger, I attached an EBPF prof that just returned -1, and send ~1000
> packets through it.
If you want to drop the packet, the prog must return 0.
You can see sk_filter_trim_cap() where the returned value from bpf prog
is cast to unsigned int. Then, pskb_trim() does nothing because skb->len
is smaller than (unsigned int)-1, and 0 is set to err.
unsigned int pkt_len;
pkt_len = bpf_prog_run_save_cb(filter->prog, skb);
err = pkt_len ? pskb_trim(skb, max(cap, pkt_len)) : -EPERM;
After calling sk_filter() from unix_dgram_sendmsg(), the skb is just queued
to the peer.
>
> After doing some investigation, the `sk_wmem_alloc` member of `struct sk`
> seems to only be increasing, presumably missing some refcnt_dec somewhere.
So, no refcnt is leaked.
What is missing is recv() on the peer side.
>
> At a certain point, in `sock_alloc_send_pskb`, we fail the check:
>
> `
> if (sk_wmem_alloc_get(sk) < READ_ONCE(sk->sk_sndbuf))
> `
>
> Upon which we enter `sock_wait_for_wmem` and schedule a massive timeout
> (at least that's what happened in my tests).
>
> Not sure where the missing refcnt subs are, must admit unfamiliarity with
> the network code.
The paired sub is sock_wfree() in unix_destruct_scm(), which is set
to skb->destructor() in unix_scm_to_skb() and called from kfree_skb().
>
> Please let me know if I need to add anything.
>
> Thanks
Powered by blists - more mailing lists