linux-ext4 - Re: [PATCH] jbd2: Fix use after free in jbd2_log_do

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Date:   Thu, 4 Oct 2018 18:33:11 +0200
From:   Lukas Czerner <lczerner@...hat.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Greg KH <greg@...ah.com>, Ted Tso <tytso@....edu>,
        linux-ext4@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] jbd2: Fix use after free in jbd2_log_do_checkpoint()

On Thu, Oct 04, 2018 at 06:05:46PM +0200, Jan Kara wrote:
> On Thu 04-10-18 08:44:02, Greg KH wrote:
> > On Thu, Oct 04, 2018 at 12:46:40PM +0200, Jan Kara wrote:
> > > The code cleaning transaction's lists of checkpoint buffers has a bug
> > > where it increases bh refcount only after releasing
> > > journal->j_list_lock. Thus the following race is possible:
> > > 
> > > CPU0					CPU1
> > > jbd2_log_do_checkpoint()
> > > 					jbd2_journal_try_to_free_buffers()
> > > 					  __journal_try_to_free_buffer(bh)
> > >   ...
> > >   while (transaction->t_checkpoint_io_list)
> > >   ...
> > >     if (buffer_locked(bh)) {
> > > 
> > > <-- IO completes now, buffer gets unlocked -->
> > > 
> > >       spin_unlock(&journal->j_list_lock);
> > > 					    spin_lock(&journal->j_list_lock);
> > > 					    __jbd2_journal_remove_checkpoint(jh);
> > > 					    spin_unlock(&journal->j_list_lock);
> > > 					  try_to_free_buffers(page);
> > >       get_bh(bh) <-- accesses freed bh
> > > 
> > > Fix the problem by grabbing bh reference before unlocking
> > > journal->j_list_lock.
> > > 
> > > Fixes: dc6e8d669cf5cb3ff84707c372c0a2a8a5e80845
> > > Fixes: be1158cc615fd723552f0d9912087423c7cadda5
> > 
> > Nit, this normally looks like:
> > 
> > Fixes: dc6e8d669cf5 ("jbd2: don't call get_bh() before calling __jbd2_journal_remove_checkpoint()")
> > 
> > And this is created by:
> > 	git show -s --abbrev-commit --abbrev=12 --pretty=format:"%h (\"%s\")%n"
> 
> OK, ok, will do next time. The full git commit ID is faster to cut-n-paste
> which shows how lazy I'm ;) 

I have this in the .gitconfig

[pretty]
	fixes = Fixes: %h (\"%s\")

which helps. I think I found it in the docs somewhere.

-Lukas

> 
> 								Honza
> -- 
> Jan Kara <jack@...e.com>
> SUSE Labs, CR