netdev - Re: [PATCH] bpf, sockmap: fix deadlock in rcu_report_exp_cpu

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Date: Thu, 28 Mar 2024 22:29:36 -0700
From: John Fastabend <john.fastabend@...il.com>
To: Jakub Sitnicki <jakub@...udflare.com>, 
 Alexei Starovoitov <alexei.starovoitov@...il.com>, 
 Edward Adam Davis <eadavis@...com>, 
 John Fastabend <john.fastabend@...il.com>
Cc: syzbot+c4f4d25859c2e5859988@...kaller.appspotmail.com, 
 42.hyeyoo@...il.com, 
 andrii@...nel.org, 
 ast@...nel.org, 
 bpf@...r.kernel.org, 
 daniel@...earbox.net, 
 davem@...emloft.net, 
 edumazet@...gle.com, 
 kafai@...com, 
 kpsingh@...nel.org, 
 kuba@...nel.org, 
 linux-kernel@...r.kernel.org, 
 namhyung@...nel.org, 
 netdev@...r.kernel.org, 
 pabeni@...hat.com, 
 peterz@...radead.org, 
 songliubraving@...com, 
 syzkaller-bugs@...glegroups.com, 
 yhs@...com
Subject: Re: [PATCH] bpf, sockmap: fix deadlock in rcu_report_exp_cpu_mult

Jakub Sitnicki wrote:
> On Mon, Mar 25, 2024 at 01:23 PM +01, Jakub Sitnicki wrote:
> 
> [...]
> 
> > But we also need to cover sock_map_unref->sock_sock_map_del_link called
> > from sock_hash_delete_elem. It also grabs a spin lock.
> 
> On second look, no need to disable interrupts in
> sock_map_unref->sock_sock_map_del_link. Call is enclosed in the critical
> section in sock_hash_delete_elem that has been updated.
> 
> I have a question, though, why are we patching sock_hash_free? It
> doesn't get called unless there are no more existing users of the BPF
> map. So nothing can mutate it from interrupt context.
> 
> [...]

Agree sock_hash_free should be only after all refs are dropped.

Edward, did you want to send a v2 for this? Also if you want fixing the
sockmap case as well would be useful. Also happy to finish up the patches
if you would rather not.

Thanks,
John