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    Subject[PATCH 5.4 104/107] epoll: Keep a reference on files added to the check list
    Date
    From: Marc Zyngier <maz@kernel.org>

    commit a9ed4a6560b8562b7e2e2bed9527e88001f7b682 upstream.

    When adding a new fd to an epoll, and that this new fd is an
    epoll fd itself, we recursively scan the fds attached to it
    to detect cycles, and add non-epool files to a "check list"
    that gets subsequently parsed.

    However, this check list isn't completely safe when deletions
    can happen concurrently. To sidestep the issue, make sure that
    a struct file placed on the check list sees its f_count increased,
    ensuring that a concurrent deletion won't result in the file
    disapearing from under our feet.

    Cc: stable@vger.kernel.org
    Signed-off-by: Marc Zyngier <maz@kernel.org>
    Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
    Signed-off-by: Marc Zyngier <maz@kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    fs/eventpoll.c | 9 +++++++--
    1 file changed, 7 insertions(+), 2 deletions(-)

    --- a/fs/eventpoll.c
    +++ b/fs/eventpoll.c
    @@ -1991,9 +1991,11 @@ static int ep_loop_check_proc(void *priv
    * not already there, and calling reverse_path_check()
    * during ep_insert().
    */
    - if (list_empty(&epi->ffd.file->f_tfile_llink))
    + if (list_empty(&epi->ffd.file->f_tfile_llink)) {
    + get_file(epi->ffd.file);
    list_add(&epi->ffd.file->f_tfile_llink,
    &tfile_check_list);
    + }
    }
    }
    mutex_unlock(&ep->mtx);
    @@ -2037,6 +2039,7 @@ static void clear_tfile_check_list(void)
    file = list_first_entry(&tfile_check_list, struct file,
    f_tfile_llink);
    list_del_init(&file->f_tfile_llink);
    + fput(file);
    }
    INIT_LIST_HEAD(&tfile_check_list);
    }
    @@ -2196,9 +2199,11 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, in
    clear_tfile_check_list();
    goto error_tgt_fput;
    }
    - } else
    + } else {
    + get_file(tf.file);
    list_add(&tf.file->f_tfile_llink,
    &tfile_check_list);
    + }
    mutex_lock_nested(&ep->mtx, 0);
    if (is_file_epoll(tf.file)) {
    tep = tf.file->private_data;

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