linux-kernel - Re: [syzbot] possible deadlock in p9_write

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Message-ID: <fb5d20c5-36a6-2c51-288a-7cc1e0a76d3e@I-love.SAKURA.ne.jp>
Date:   Wed, 30 Mar 2022 10:57:15 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Dominique Martinet <asmadeus@...ewreck.org>
Cc:     Andrew Perepechko <andrew.perepechko@....com>,
        Andreas Dilger <adilger@...ger.ca>,
        "Theodore Ts'o" <tytso@....edu>,
        syzbot <syzbot+bde0f89deacca7c765b8@...kaller.appspotmail.com>,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        v9fs-developer@...ts.sourceforge.net,
        "open list:EXT4 FILE SYSTEM" <linux-ext4@...r.kernel.org>
Subject: Re: [syzbot] possible deadlock in p9_write_work

On 2022/03/30 8:51, Dominique Martinet wrote:
> Tetsuo Handa wrote on Wed, Mar 30, 2022 at 07:35:47AM +0900:
>> This seems to be an example of
>> https://lkml.kernel.org/r/49925af7-78a8-a3dd-bce6-cfc02e1a9236@I-love.SAKURA.ne.jp
>> introduced by "ext4: truncate during setxattr leads to kernel panic".
> 
> Thanks for the pointer
> 
>> Please don't use schedule_work() if you need to use flush_scheduled_work().
> 
> In this case we don't call flush_scheduled_work -- ext4 does.

Yes, that's why I changed recipients to ext4 people.

> The problem is mixing in the two subsystems when someone (e.g. syzbot)
> opens an ext4 file and passes that fd to 9p when mounting with e.g.
> mount -t 9p -o rfdno=<no>,wfdno=<no>
> 
> Frankly that's just not something I consider useful, interacting through
> 9p to a local file doesn't make sense except for testing.
> 
> If that is a real problem, the simplest way out would be to just forbid
> non-socket FDs if it's something we can check.

Do you mean that p9_fd_open() in net/9p/trans_fd.c does not need to accept non-socket file descriptors?
Then, it's something you can check. You can use S_ISSOCK() like e.g. netlink_getsockbyfilp() does.