linux-ext4 - Re: [syzbot] possible deadlock in p9_write

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Date:   Wed, 30 Mar 2022 08:51:21 +0900
From:   Dominique Martinet <asmadeus@...ewreck.org>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
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

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.
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.
-- 
Dominique