Subject: Re: general protection fault in sock_close

From: Marc Zyngier <[email-masked]>
Date: Mon, 31 Aug 2020 21:39:27 +0100

On 2020-08-31 21:03, Al Viro wrote:
> On Mon, Aug 31, 2020 at 12:48:13PM -0700, syzbot wrote:
>> Hello,
>> 
>> syzbot found the following issue on:
>> 
>> HEAD commit:    15bc20c6 Merge tag 'tty-5.9-rc3' of 
>> git://git.kernel.org/p..
>> git tree:       upstream
>> console output: 
>> https://syzkaller.appspot.com/x/log.txt?x=16a85669900000
>> kernel config:  
>> https://syzkaller.appspot.com/x/.config?x=891ca5711a9f1650
>> dashboard link: 
>> https://syzkaller.appspot.com/bug?extid=e24baf53dc389927a7c3
>> compiler:       clang version 10.0.0 
>> (https://github.com/llvm/llvm-project/ 
>> c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
>> syz repro:      
>> https://syzkaller.appspot.com/x/repro.syz?x=127d3c99900000
> 
>> The issue was bisected to:
>> 
>> commit a9ed4a6560b8562b7e2e2bed9527e88001f7b682
>> Author: Marc Zyngier <[email-masked]>
>> Date:   Wed Aug 19 16:12:17 2020 +0000
>> 
>>     epoll: Keep a reference on files added to the check list
> 
> All of those are essentially duplicates.
> 
> The minimal fix is below; I'm not happy with it long-term, but I'm 
> still
> digging through the eventpoll locking, and there's a good chance that 
> this
> is the least intrusive variant for -stable.  Folks, could you check if 
> the
> following patch fixes those suckers?  Again, all reports bisected to 
> that
> commit are essentially the same.
> 
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index e0decff22ae2..8107e06d7f6f 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -1995,9 +1995,9 @@ static int ep_loop_check_proc(void *priv, void
> *cookie, int call_nests)
>  			 * during ep_insert().
>  			 */
>  			if (list_empty(&epi->ffd.file->f_tfile_llink)) {
> -				get_file(epi->ffd.file);
> -				list_add(&epi->ffd.file->f_tfile_llink,
> -					 &tfile_check_list);
> +				if (get_file_rcu(epi->ffd.file))
> +					list_add(&epi->ffd.file->f_tfile_llink,
> +						 &tfile_check_list);
>  			}
>  		}
>  	}

I've managed to reproduce the issue using [1] (throw a few in a VM,
see things explode like clockwork).

With this patch on top of -rc3, the VM keep ticking away. FWIW:

Tested-by: Marc Zyngier <[email-masked]>
Fixes: a9ed4a6560b8 ("epoll: Keep a reference on files added to the 
check list")

Thanks,

         M.

[1] https://syzkaller.appspot.com/x/repro.c?x=140a19a9900000
-- 
Jazz is not dead. It just smells funny...