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@xxxxxxxxxx>
Btw, do you by any chance have a reproducer for this ?
-Lukas
>
> Fixes: dc6e8d669cf5cb3ff84707c372c0a2a8a5e80845
> Fixes: be1158cc615fd723552f0d9912087423c7cadda5
> Reported-by: syzbot+7f4a27091759e2fe7453@xxxxxxxxxxxxxxxxxxxxxxxxx
> CC: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Jan Kara <jack@xxxxxxx>
> ---
> 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
>
