linux-kernel - Re: [PATCH] ext4: fix memory leak in ext4_fill

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Date:   Thu, 29 Apr 2021 22:20:23 +0300
From:   Pavel Skripkin <paskripkin@...il.com>
To:     "Theodore Ts'o" <tytso@....edu>
Cc:     Vegard Nossum <vegard.nossum@...cle.com>,
        akpm@...ux-foundation.org, peterz@...radead.org, axboe@...nel.dk,
        pmladek@...e.com, adilger.kernel@...ger.ca,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzbot+d9e482e303930fa4f6ff@...kaller.appspotmail.com
Subject: Re: [PATCH] ext4: fix memory leak in ext4_fill_super

Hi! Thanks for your reply.

On Thu, 29 Apr 2021 13:05:01 -0400
"Theodore Ts'o" <tytso@....edu> wrote:
> On Thu, Apr 29, 2021 at 02:33:54PM +0300, Pavel Skripkin wrote:
> > 
> > There is a chance, that kthread_stop() call will happen before
> > threadfn call. It means, that kthread_stop() return value must be
> > checked everywhere, isn't it? Otherwise, there are a lot of
> > potential memory leaks, because some developers rely on the fact,
> > that data allocated for the thread will be freed _inside_ thread
> > function.
> 
> That's not the only potential way that we could leak memory.  Earlier
> in kthread(), if this memory allocation fails,
> 
> 	self = kzalloc(sizeof(*self), GFP_KERNEL);
> 
> we will exit with -ENOMEM.  So at the very least all callers of
> kthread_stop() also need to check for -ENOMEM as well as -EINTR ---
> or, be somehow sure that the thread function was successfully called
> and started.  In this particular case, the ext4 mount code had just
> started the kmmpd thread, and then detected that something else had
> gone wrong, and failed the mount before the kmmpd thread ever had a
> chance to run.
> 
> I think if we want to fix this more generally across the whole kernel,
> we would need to have a variant of kthread_run which supplies two
> functions --- one which is the thread function, and the other which is
> a cleanup function.  The cleanup function could just be kfree, but
> there will be other cases where the cleanup function will need to do
> other work before freeing the data structure (e.g., brelse((struct
> mmpd_data *)data->bh)).

I skimmed through kernel code and I didn't find any code
examples, except ext4, where kthread is freeing something. Maybe, this
API isn't required, but, as Vegard said, comment over
kthread_stop() should be changed, because it's confusing.

I have already added kthread.c developers (I hope, I chose
the right emails) to CC. Maybe, they will think about this API. 

> 
> Is it worth it to provide such a cleanup function, which if present
> would be called any time the thread exits or is killed?  I dunno.
> It's probably simpler to just strongly recommend that the cleanup work
> should never be done in the thread function, but after kthread_stop()
> is called, whether it returns an error or not.  That's probably the
> right fix for ext4, I think.
> 
> (Although note that kthread_stop(sbi->s_mmp_task) is called in
> multiple places in fs/ext4/super.c, not just in the single location
> which this patch touches.)
> 

Good point, I'll add this and -ENOMEM checks and will send v2.

Thanks!

> 						- Ted



With regards,
Pavel Skripkin