[Devel] [PATCH vz9 2/3] eventpoll: defer struct eventpoll free to RCU grace period

[Eva Kurchatova]

From: Nicholas Carlini <nicholas at carlini.com>

[ Upstream commit 07712db80857d5d09ae08f3df85a708ecfc3b61f ]

In certain situations, ep_free() in eventpoll.c will kfree the epi->ep
eventpoll struct while it still being used by another concurrent thread.
Defer the kfree() to an RCU callback to prevent UAF.

Fixes: f2e467a48287 ("eventpoll: Fix semi-unbounded recursion")
Signed-off-by: Nicholas Carlini <nicholas at carlini.com>
Signed-off-by: Christian Brauner <brauner at kernel.org>
Signed-off-by: Sasha Levin <sashal at kernel.org>
(cherry picked from commit 5b1173b165421561db29f30afc7e97d940a398a9)

Signed-off-by: Eva Kurchatova <eva.kurchatova at virtuozzo.com>

https://virtuozzo.atlassian.net/browse/VSTOR-137490
Feature: fix epoll cve
---
 fs/eventpoll.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 2dda4b4dc671..7ac22aadaf8b 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -225,6 +225,9 @@ struct eventpoll {
 	 */
 	refcount_t refcount;
 
+	/* used to defer freeing past ep_get_upwards_depth_proc() RCU walk */
+	struct rcu_head rcu;
+
 #ifdef CONFIG_NET_RX_BUSY_POLL
 	/* used to track busy poll napi_id */
 	unsigned int napi_id;
@@ -708,7 +711,8 @@ static void ep_free(struct eventpoll *ep)
 	mutex_destroy(&ep->mtx);
 	free_uid(ep->user);
 	wakeup_source_unregister(ep->ws);
-	kfree(ep);
+	/* ep_get_upwards_depth_proc() may still hold epi->ep under RCU */
+	kfree_rcu(ep, rcu);
 }
 
 /*
-- 
2.55.0