On Tue 31-01-17 20:08:57, Sahitya Tummala wrote:
> Below is the synchronization issue between unmount and kjournald2
> contexts, which results into use after free issue in kjournald2().
> Fix this issue by using journal->j_state_lock to synchronize the
> wait_event() done in journal_kill_thread() and the wake_up() done
> in kjournald2().
>
> TASK 1:
> umount cmd:
> |--jbd2_journal_destroy() {
> |--journal_kill_thread() {
> write_lock(&journal->j_state_lock);
> journal->j_flags |= JBD2_UNMOUNT;
> ...
> write_unlock(&journal->j_state_lock);
> wake_up(&journal->j_wait_commit); TASK 2 wakes up here:
> kjournald2() {
> ...
> checks JBD2_UNMOUNT flag and calls goto end-loop;
> ...
> end_loop:
> write_unlock(&journal->j_state_lock);
> journal->j_task = NULL; --> If this thread gets
> pre-empted here, then TASK 1 wait_event will
> exit even before this thread is completely
> done.
> wait_event(journal->j_wait_done_commit, journal->j_task == NULL);
> ...
> write_lock(&journal->j_state_lock);
> write_unlock(&journal->j_state_lock);
> }
> |--kfree(journal);
> }
> }
> wake_up(&journal->j_wait_done_commit); --> this step
> now results into use after free issue.
> }
>
> Signed-off-by: Sahitya Tummala <stummala@xxxxxxxxxxxxxx>
Yeah, what you write looks possible (although rather unlikely). Thanks for
catching this. One small nit below:
> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> index a097048..f5cd3c0 100644
> --- a/fs/jbd2/journal.c
> +++ b/fs/jbd2/journal.c
> @@ -278,9 +278,11 @@ static int kjournald2(void *arg)
> end_loop:
> write_unlock(&journal->j_state_lock);
> del_timer_sync(&journal->j_commit_timer);
> + write_lock(&journal->j_state_lock);
There's no good reason to do del_timer_sync() outside of j_state_lock. This
is not performance critical code and commit_timeout is trivial and cannot
block on anything. So just keep j_state_lock locked upto the place where
you unlock it now...
Honza
> journal->j_task = NULL;
> wake_up(&journal->j_wait_done_commit);
> jbd_debug(1, "Journal thread exiting.\n");
> + write_unlock(&journal->j_state_lock);
> return 0;
> }
>
> --
> Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
>
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
