linux-kernel - Re: [PATCH] bpf, sockmap: defer sk_psock_free

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Date: Wed, 22 May 2024 07:57:48 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Jakub Sitnicki <jakub@...udflare.com>, John Fastabend <john.fastabend@...il.com>, 
	Daniel Borkmann <daniel@...earbox.net>
Cc: Hillf Danton <hdanton@...a.com>, Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>, 
	Eric Dumazet <edumazet@...gle.com>, Linus Torvalds <torvalds@...ux-foundation.org>, 
	bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] bpf, sockmap: defer sk_psock_free_link() using RCU

On Wed, May 22, 2024 at 5:12 AM Jakub Sitnicki <jakub@...udflare.com> wrote:
>
> On Wed, May 22, 2024 at 07:33 PM +08, Hillf Danton wrote:
> > On Wed, 22 May 2024 11:50:49 +0200 Jakub Sitnicki <jakub@...udflare.com>
> > On Wed, May 22, 2024 at 06:59 AM +08, Hillf Danton wrote:
> >> > On Tue, 21 May 2024 08:38:52 -0700 Alexei Starovoitov <alexei.starovoitov@...il.com>
> >> >> On Sun, May 12, 2024 at 12:22=E2=80=AFAM Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp> wrote:
> >> >> > --- a/net/core/sock_map.c
> >> >> > +++ b/net/core/sock_map.c
> >> >> > @@ -142,6 +142,7 @@ static void sock_map_del_link(struct sock *sk,
> >> >> >         bool strp_stop =3D false, verdict_stop =3D false;
> >> >> >         struct sk_psock_link *link, *tmp;
> >> >> >
> >> >> > +       rcu_read_lock();
> >> >> >         spin_lock_bh(&psock->link_lock);
> >> >>
> >> >> I think this is incorrect.
> >> >> spin_lock_bh may sleep in RT and it won't be safe to do in rcu cs.
> >> >
> >> > Could you specify why it won't be safe in rcu cs if you are right?
> >> > What does rcu look like in RT if not nothing?
> >>
> >> RCU readers can't block, while spinlock RT doesn't disable preemption.
> >>
> >> https://docs.kernel.org/RCU/rcu.html
> >> https://docs.kernel.org/locking/locktypes.html#spinlock-t-and-preempt-rt
> >>
> >> I've finally gotten around to testing proposed fix that just disallows
> >> map_delete_elem on sockmap/sockhash from BPF tracing progs
> >> completely. This should put an end to this saga of syzkaller reports.
> >>
> >> https://lore.kernel.org/all/87jzjnxaqf.fsf@cloudflare.com/

Agree. Let's do that. According to John the delete path is not something
that is used in production. It's only a source of trouble with syzbot.


> >>
> > The locking info syzbot reported [2] suggests a known issue that like Alexei
> > you hit the send button earlier than expected.
> >
> > 4 locks held by syz-executor361/5090:
> >  #0: ffffffff8e334d20 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire include/linux/rcupdate.h:329 [inline]
> >  #0: ffffffff8e334d20 (rcu_read_lock){....}-{1:2}, at: rcu_read_lock include/linux/rcupdate.h:781 [inline]
> >  #0: ffffffff8e334d20 (rcu_read_lock){....}-{1:2}, at: map_delete_elem+0x388/0x5e0 kernel/bpf/syscall.c:1695
> >  #1: ffff88807b2af8f8 (&htab->buckets[i].lock){+...}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:356 [inline]
> >  #1: ffff88807b2af8f8 (&htab->buckets[i].lock){+...}-{2:2}, at: sock_hash_delete_elem+0x17c/0x400 net/core/sock_map.c:945
> >  #2: ffff88801c2a4290 (&psock->link_lock){+...}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:356 [inline]
> >  #2: ffff88801c2a4290 (&psock->link_lock){+...}-{2:2}, at: sock_map_del_link net/core/sock_map.c:145 [inline]
> >  #2: ffff88801c2a4290 (&psock->link_lock){+...}-{2:2}, at: sock_map_unref+0xcc/0x5e0 net/core/sock_map.c:180
> >  #3: ffffffff8e334d20 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire include/linux/rcupdate.h:329 [inline]
> >  #3: ffffffff8e334d20 (rcu_read_lock){....}-{1:2}, at: rcu_read_lock include/linux/rcupdate.h:781 [inline]
> >  #3: ffffffff8e334d20 (rcu_read_lock){....}-{1:2}, at: __bpf_trace_run kernel/trace/bpf_trace.c:2380 [inline]
> >  #3: ffffffff8e334d20 (rcu_read_lock){....}-{1:2}, at: bpf_trace_run2+0x114/0x420 kernel/trace/bpf_trace.c:2420
> >
> > [2] https://lore.kernel.org/all/000000000000d0b87206170dd88f@google.com/
> >
> >
> > If CONFIG_PREEMPT_RCU=y rcu_read_lock() does not disable
> > preemption. This is even true for !RT kernels with CONFIG_PREEMPT=y
> >
> > [3] Subject: Re: [patch 30/63] locking/spinlock: Provide RT variant
> > https://lore.kernel.org/all/874kc6rizr.ffs@tglx/
>
> That locking issue is related to my earlier, as it turned out -
> incomplete, fix:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ff91059932401894e6c86341915615c5eb0eca48
>
> We don't expect map_delete_elem to be called from map_update_elem for
> sockmap/sockhash, but that is what syzkaller started doing by attaching
> BPF tracing progs which call map_delete_elem.