netdev - Re: [PATCH v1 net] tcp/dccp: Don't use timer_pending() in reqsk_queue

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Message-ID: <20241008144205.83199-1-kuniyu@amazon.com>
Date: Tue, 8 Oct 2024 07:42:05 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <edumazet@...gle.com>
CC: <davem@...emloft.net>, <dsahern@...nel.org>, <kuba@...nel.org>,
	<kuni1840@...il.com>, <kuniyu@...zon.com>, <martin.lau@...nel.org>,
	<netdev@...r.kernel.org>, <pabeni@...hat.com>
Subject: Re: [PATCH v1 net] tcp/dccp: Don't use timer_pending() in reqsk_queue_unlink().

From: Eric Dumazet <edumazet@...gle.com>
Date: Tue, 8 Oct 2024 16:28:53 +0200
> On Tue, Oct 8, 2024 at 4:21 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
> >
> > From: Eric Dumazet <edumazet@...gle.com>
> > Date: Tue, 8 Oct 2024 11:54:21 +0200
> > > On Tue, Oct 8, 2024 at 1:53 AM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
> > > >
> > > > From: Jakub Kicinski <kuba@...nel.org>
> > > > Date: Mon, 7 Oct 2024 16:26:10 -0700
> > > > > On Mon, 7 Oct 2024 07:15:57 -0700 Kuniyuki Iwashima wrote:
> > > > > > Martin KaFai Lau reported use-after-free [0] in reqsk_timer_handler().
> > > > > >
> > > > > >   """
> > > > > >   We are seeing a use-after-free from a bpf prog attached to
> > > > > >   trace_tcp_retransmit_synack. The program passes the req->sk to the
> > > > > >   bpf_sk_storage_get_tracing kernel helper which does check for null
> > > > > >   before using it.
> > > > > >   """
> > > > >
> > > > > I think this crashes a bunch of selftests, example:
> > > > >
> > > > > https://netdev-3.bots.linux.dev/vmksft-nf-dbg/results/805581/8-nft-queue-sh/stderr
> > > >
> > > > Oops, sorry, I copy-and-pasted __inet_csk_reqsk_queue_drop()
> > > > for different reqsk.  I'll squash the diff below.
> > > >
> > > > ---8<---
> > > > diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
> > > > index 36f03d51356e..433c80dc57d5 100644
> > > > --- a/net/ipv4/inet_connection_sock.c
> > > > +++ b/net/ipv4/inet_connection_sock.c
> > > > @@ -1188,7 +1190,7 @@ static void reqsk_timer_handler(struct timer_list *t)
> > > >         }
> > > >
> > > >  drop:
> > > > -       __inet_csk_reqsk_queue_drop(sk_listener, nreq, true);
> > > > +       __inet_csk_reqsk_queue_drop(sk_listener, oreq, true);
> > > >         reqsk_put(req);
> > > >  }
> > > >
> > > > ---8<---
> > > >
> > > > Thanks!
> > >
> > > Just to clarify. In the old times rsk_timer was pinned, right ?
> > >
> > > 83fccfc3940c4 ("inet: fix potential deadlock in reqsk_queue_unlink()")
> > > was fine I think.
> > >
> > > So the bug was added recently ?
> > >
> > > Can we give a precise Fixes: tag ?
> >
> > TIMER_PINNED was used in reqsk_queue_hash_req() in v6.4 mentioned
> > by Martin and still used in the latest net-next.
> >
> > $ git blame -L:reqsk_queue_hash_req net/ipv4/inet_connection_sock.c v6.4
> > 079096f103fac (Eric Dumazet             2015-10-02 11:43:32 -0700 1095) static void reqsk_queue_hash_req(struct request_sock *req,
> > 079096f103fac (Eric Dumazet             2015-10-02 11:43:32 -0700 1096)                                  unsigned long timeout)
> > fa76ce7328b28 (Eric Dumazet             2015-03-19 19:04:20 -0700 1097) {
> > 59f379f9046a9 (Kees Cook                2017-10-16 17:29:19 -0700 1098)         timer_setup(&req->rsk_timer, reqsk_timer_handler, TIMER_PINNED);
> >
> > Maybe the connection was localhost, or unlikely but RPS was
> > configured after SYN+ACK, or setup like ff46e3b44219 was used ??
> 
> I do not really understand the issue.
> How a sk can be 'closed' with outstanding request sock ?
> They hold a refcount on the listener.

My understanding is

1. inet_csk_complete_hashdance() calls inet_csk_reqsk_queue_drop(),
   but del_timer_sync() is missed

2. reqsk timer is executed and scheduled again

3. req->sk is accept()ed, but inet_csk_accept() does not clear
   req->sk for non-TFO sockets, and reqsk_put() decrements one
   refcnt, but still reqsk timer has another one

4. sk is close()d

5. reqsk timer is executed again, and BPF touches req->sk

reqsk timer will run for 63s by default, so I think it's possible
that sk is close()d earlier than the timer expiration.