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    Subject[PATCH 3.18 08/24] jbd2: fix use after free in kjournald2()
    Date
    3.18-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Sahitya Tummala <stummala@codeaurora.org>

    commit dbfcef6b0f4012c57bc0b6e0e660d5ed12a5eaed upstream.

    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@codeaurora.org>
    Signed-off-by: Theodore Ts'o <tytso@mit.edu>
    Cc: Amit Pundir <amit.pundir@linaro.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    fs/jbd2/journal.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/fs/jbd2/journal.c
    +++ b/fs/jbd2/journal.c
    @@ -275,11 +275,11 @@ loop:
    goto loop;

    end_loop:
    - write_unlock(&journal->j_state_lock);
    del_timer_sync(&journal->j_commit_timer);
    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;
    }


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