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Message-ID: <20181004115012.s5isxzw4zt4g26y3@localhost.localdomain> Date: Thu, 4 Oct 2018 13:50:12 +0200 From: Lukas Czerner <lczerner@...hat.com> To: Jan Kara <jack@...e.cz> Cc: 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 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. Hi Jan, nice catch. The patch looks good, you can add Reviewed-by: Lukas Czerner <lczerner@...hat.com> Btw, do you by any chance have a reproducer for this ? -Lukas > > Fixes: dc6e8d669cf5cb3ff84707c372c0a2a8a5e80845 > Fixes: be1158cc615fd723552f0d9912087423c7cadda5 > Reported-by: syzbot+7f4a27091759e2fe7453@...kaller.appspotmail.com > CC: stable@...r.kernel.org > Signed-off-by: Jan Kara <jack@...e.cz> > --- > fs/jbd2/checkpoint.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c > index c125d662777c..26f8d7e46462 100644 > --- a/fs/jbd2/checkpoint.c > +++ b/fs/jbd2/checkpoint.c > @@ -251,8 +251,8 @@ int jbd2_log_do_checkpoint(journal_t *journal) > bh = jh2bh(jh); > > if (buffer_locked(bh)) { > - spin_unlock(&journal->j_list_lock); > get_bh(bh); > + spin_unlock(&journal->j_list_lock); > wait_on_buffer(bh); > /* the journal_head may have gone by now */ > BUFFER_TRACE(bh, "brelse"); > @@ -333,8 +333,8 @@ int jbd2_log_do_checkpoint(journal_t *journal) > jh = transaction->t_checkpoint_io_list; > bh = jh2bh(jh); > if (buffer_locked(bh)) { > - spin_unlock(&journal->j_list_lock); > get_bh(bh); > + spin_unlock(&journal->j_list_lock); > wait_on_buffer(bh); > /* the journal_head may have gone by now */ > BUFFER_TRACE(bh, "brelse"); > -- > 2.16.4 >
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