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Date:   Thu, 6 May 2021 15:09:30 -0700
From:   Jakub Kicinski <kuba@kernel.org>
To:     Pavel Skripkin <paskripkin@gmail.com>
Cc:     ralf@linux-mips.org, davem@davemloft.net, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: GPF in net sybsystem
Message-ID: <20210506150930.3b677dcb@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
In-Reply-To: <20210505200242.31d58452@gmail.com>
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On Wed, 5 May 2021 20:02:42 +0300 Pavel Skripkin wrote:
> Hi, netdev developers!
> 
> I've spent some time debugging this bug
> https://syzkaller.appspot.com/bug?id=c670fb9da2ce08f7b5101baa9426083b39ee9f90
> and, I believe, I found the root case:
> 
> static int nr_accept(struct socket *sock, struct socket *newsock, int flags,
> 		     bool kern)
> {
> ....
> 	for (;;) {
> 		prepare_to_wait(sk_sleep(sk), &wait, TASK_INTERRUPTIBLE);
> 		...
> 		if (!signal_pending(current)) {
> 			release_sock(sk);
> 			schedule();
> 			lock_sock(sk);
> 			continue;
> 		}
> 		...
> 	}
> ...
> }
> 
> When calling process will be scheduled, another proccess can release
> this socket and set sk->sk_wq to NULL. (In this case nr_release()
> will call sock_orphan(sk)). In this case GPF will happen in
> prepare_to_wait().

How does it get released midway through an accept call?
Is there a reference imbalance somewhere else in the code?

> I came up with this patch, but im not an expect in netdev sybsystem and
> im not sure about this one:
> 
> diff --git a/net/netrom/af_netrom.c b/net/netrom/af_netrom.c
> index 6d16e1ab1a8a..89ceddea48e8 100644
> --- a/net/netrom/af_netrom.c
> +++ b/net/netrom/af_netrom.c
> @@ -803,6 +803,10 @@ static int nr_accept(struct socket *sock, struct socket *newsock, int flags,
>  			release_sock(sk);
>  			schedule();
>  			lock_sock(sk);
> +			if (sock_flag(sk, SOCK_DEAD)) {
> +				err = -ECONNABORTED;
> +				goto out_release;
> +			}
>  			continue;
>  		}
>  		err = -ERESTARTSYS;
> 
> I look forward to hearing your perspective on this :)
> 
> 
> BTW, I found similar code in:
> 
> 1) net/ax25/af_ax25.c
> 2) net/rose/af_rose.c
> 
> 
> I hope, this will help!
> 
> With regards,
> Pavel Skripkin