4.9-stable review patch. If anyone has any objections, please let me know. ------------------ From: Jan Kara <jack@xxxxxxx> commit ccd3c4373eacb044eb3832966299d13d2631f66f upstream. 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: dc6e8d669cf5 ("jbd2: don't call get_bh() before calling __jbd2_journal_remove_checkpoint()") Fixes: be1158cc615f ("jbd2: fold __process_buffer() into jbd2_log_do_checkpoint()") Reported-by: syzbot+7f4a27091759e2fe7453@xxxxxxxxxxxxxxxxxxxxxxxxx CC: stable@xxxxxxxxxxxxxxx Reviewed-by: Lukas Czerner <lczerner@xxxxxxxxxx> Signed-off-by: Jan Kara <jack@xxxxxxx> Signed-off-by: Theodore Ts'o <tytso@xxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- fs/jbd2/checkpoint.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) --- a/fs/jbd2/checkpoint.c +++ b/fs/jbd2/checkpoint.c @@ -254,8 +254,8 @@ restart: 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"); @@ -336,8 +336,8 @@ restart2: 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");