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

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Date:   Wed, 30 Mar 2022 11:29:37 +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 10:57:15AM +0900:
> >> 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.

Sorry, I hadn't noticed.
9p is the one calling schedule_work, so ultimately it really is the
combinaison of the two, and not just ext4 that's wrong here.

> > 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?

Yes, I can't think of any valid usage that would involve non-socket fd
there.
It might be useful to leave as a test vector, but if it causes problems
I think it's perfectly OK to just refuse these.

> Then, it's something you can check. You can use S_ISSOCK() like
> e.g. netlink_getsockbyfilp() does

Thanks for the example
-- 
Dominique