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Message-ID: <20181005225705.GA2548@thunk.org> Date: Fri, 5 Oct 2018 18:57:05 -0400 From: "Theodore Y. Ts'o" <tytso@....edu> To: Jan Kara <jack@...e.cz> Cc: 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. > > Fixes: dc6e8d669cf5cb3ff84707c372c0a2a8a5e80845 > Fixes: be1158cc615fd723552f0d9912087423c7cadda5 > Reported-by: syzbot+7f4a27091759e2fe7453@...kaller.appspotmail.com > CC: stable@...r.kernel.org > Signed-off-by: Jan Kara <jack@...e.cz> Applied, thanks (with the Fixes field adjusted). - Ted
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