[PATCH 3.13 149/198] epoll: fix use-after-free in eventpoll_release_file

From: Kamal Mostafa
Date: Tue Jul 15 2014 - 18:02:13 EST -stable review patch. If anyone has any objections, please let me know.


From: Konstantin Khlebnikov <koct9i@xxxxxxxxx>

commit ebe06187bf2aec10d537ce4595e416035367d703 upstream.

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@xxxxxxxxx>
Reported-by: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
Cc: Sasha Levin <sasha.levin@xxxxxxxxxx>
Cc: Jason Baron <jbaron@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
fs/eventpoll.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index af90312..ead0046 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.
- 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);

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