jbd2: fix use after free in jbd2_log_do_checkpoint()

jankara · tytso · commit ccd3c4373eac · 2018-10-05T18:44:40.000-04:00
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: dc6e8d6 ("jbd2: don't call get_bh() before calling __jbd2_journal_remove_checkpoint()") Fixes: be1158c ("jbd2: fold __process_buffer() into jbd2_log_do_checkpoint()") Reported-by: syzbot+7f4a27091759e2fe7453@syzkaller.appspotmail.com CC: stable@vger.kernel.org Reviewed-by: Lukas Czerner <lczerner@redhat.com> Signed-off-by: Jan Kara <jack@suse.cz> Signed-off-by: Theodore Ts'o <tytso@mit.edu>
diff --git 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");
