Re: [PATCH] epoll: fix use-after-free in eventpoll_release_file

Jason Baron
2014-06-17T16:40:21Z

Hi,

Indeed, eventpoll_release_file() is on the 'write' side of the rcu
here (the read-side being in reverse_path_check_proc()). It's a race,
in that 'struct epitem' has to be freed by rcu before the next loop
iteration such that epi->fllink.next pointer is no longer valid. So,
I suspect that makes this more difficult to hit, but obviously it
happens.

Thanks for finding/fixing this!

Acked-by: Jason Baron <jbaron@akamai.com>

On 06/16/2014 10:58 PM, Konstantin Khlebnikov wrote:
> This fixes use-after-free of epi->fllink.next inside list loop macro.
> This loop actually releases elements in the body. List is rcu-protected
> but here we cannot hold rcu_read_lock because we need to lock mutex inside.
>
> Obvious solution is to use list_for_each_entry_safe(). RCU-ness isn't essential
> because nobody can change this list under us, it's final fput for this file.
>
> Bug is here since ae10b2b4eb01bedc91d29d5c5bb9e416fd806c40
> ("epoll: optimize EPOLL_CTL_DEL using rcu")
>
> Signed-off-by: Konstantin Khlebnikov <koct9i@gmail.com>
> Reported-by: Cyrill Gorcunov <gorcunov@openvz.org>
> Cc: Stable <stable@vger.kernel.org> # 3.13+
> Cc: Sasha Levin <sasha.levin@oracle.com>
> Cc: Jason Baron <jbaron@akamai.com>
> ---
> fs/eventpoll.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index b73e062..b10b48c 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -910,7 +910,7 @@ static const struct file_operations eventpoll_fops = {
> void eventpoll_release_file(struct file *file)
> {
> struct eventpoll *ep;
> - struct epitem *epi;
> + struct epitem *epi, *next;
>
> /*
> * We don't want to get "file->f_lock" because it is not
> @@ -926,7 +926,7 @@ void eventpoll_release_file(struct file *file)
> * Besides, ep_remove() acquires the lock, so we can't hold it here.
> */
> mutex_lock(&epmutex);
> - list_for_each_entry_rcu(epi, &file->f_ep_links, fllink) {
> + list_for_each_entry_safe(epi, next, &file->f_ep_links, fllink) {
> ep = epi->ep;
> mutex_lock_nested(&ep->mtx, 0);
> ep_remove(ep, epi);
>