Re: [PATCH] bpf, sockmap: defer sk_psock_free_link() using RCU

[Hillf Danton <hdanton@sina.com>]

On Wed, 22 May 2024 11:50:49 +0200 Jakub Sitnicki <jakub@cloudflare.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@gmail.com>
> >> On Sun, May 12, 2024 at 12:22=E2=80=AFAM Tetsuo Handa <penguin-kernel@i-love.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/
> 
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/