epoll: fix use-after-free in eventpoll_release_file

This fixes use-after-free of epi->fllink.next inside list loop macro.
This loop actually releases elements in the body.  The list is
rcu-protected but here we cannot hold rcu_read_lock because we need to
lock mutex inside.

The 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.

The bug was introduced by ae10b2b4eb01 ("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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index b73e0621ce9e..b10b48c2a7af 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);