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Message-ID: <20181004163311.axr6uokqaueeri5m@localhost.localdomain> 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
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