Andrew Lukoshko
2042c1b105
- Drop AlmaLinux ahead-of-RHEL eventpoll CVE-2026-46242 fix (1712), superseded by the RHEL eventpoll series in 687.23.1 - Temporarily drop the rtmutex remove_waiter() fixes (1736, 1738) - Add RHEL 687.23.1 backports recreated from CS10/upstream (1739-1755)
105 lines
3.9 KiB
Diff
105 lines
3.9 KiB
Diff
From 5c53e2d149e55d3ee64fb03387d2f81d59477774 Mon Sep 17 00:00:00 2001
|
|
From: Ian Kent <ikent@redhat.com>
|
|
Date: Tue, 16 Jun 2026 16:00:59 +0800
|
|
Subject: [PATCH] eventpoll: fix ep_remove struct eventpoll / struct file UAF
|
|
|
|
JIRA: https://redhat.atlassian.net/browse/RHEL-180777
|
|
Upstream status: Linus
|
|
|
|
CVE: CVE-2026-46242
|
|
|
|
commit a6dc643c69311677c574a0f17a3f4d66a5f3744b
|
|
Author: Christian Brauner <brauner@kernel.org>
|
|
Date: Thu Apr 23 11:56:09 2026 +0200
|
|
|
|
eventpoll: fix ep_remove struct eventpoll / struct file UAF
|
|
|
|
ep_remove() (via ep_remove_file()) cleared file->f_ep under
|
|
file->f_lock but then kept using @file inside the critical section
|
|
(is_file_epoll(), hlist_del_rcu() through the head, spin_unlock).
|
|
A concurrent __fput() taking the eventpoll_release() fastpath in
|
|
that window observed the transient NULL, skipped
|
|
eventpoll_release_file() and ran to f_op->release / file_free().
|
|
|
|
For the epoll-watches-epoll case, f_op->release is
|
|
ep_eventpoll_release() -> ep_clear_and_put() -> ep_free(), which
|
|
kfree()s the watched struct eventpoll. Its embedded ->refs
|
|
hlist_head is exactly where epi->fllink.pprev points, so the
|
|
subsequent hlist_del_rcu()'s "*pprev = next" scribbles into freed
|
|
kmalloc-192 memory.
|
|
|
|
In addition, struct file is SLAB_TYPESAFE_BY_RCU, so the slot
|
|
backing @file could be recycled by alloc_empty_file() --
|
|
reinitializing f_lock and f_ep -- while ep_remove() is still
|
|
nominally inside that lock. The upshot is an attacker-controllable
|
|
kmem_cache_free() against the wrong slab cache.
|
|
|
|
Pin @file via epi_fget() at the top of ep_remove() and gate the
|
|
critical section on the pin succeeding. With the pin held @file
|
|
cannot reach refcount zero, which holds __fput() off and
|
|
transitively keeps the watched struct eventpoll alive across the
|
|
hlist_del_rcu() and the f_lock use, closing both UAFs.
|
|
|
|
If the pin fails @file has already reached refcount zero and its
|
|
__fput() is in flight. Because we bailed before clearing f_ep,
|
|
that path takes the eventpoll_release() slow path into
|
|
eventpoll_release_file() and blocks on ep->mtx until the waiter
|
|
side's ep_clear_and_put() drops it. The bailed epi's share of
|
|
ep->refcount stays intact, so the trailing ep_refcount_dec_and_test()
|
|
in ep_clear_and_put() cannot free the eventpoll out from under
|
|
eventpoll_release_file(); the orphaned epi is then cleaned up
|
|
there.
|
|
|
|
A successful pin also proves we are not racing
|
|
eventpoll_release_file() on this epi, so drop the now-redundant
|
|
re-check of epi->dying under f_lock. The cheap lockless
|
|
READ_ONCE(epi->dying) fast-path bailout stays.
|
|
|
|
Fixes: 58c9b016e128 ("epoll: use refcount to reduce ep_mutex contention")
|
|
Reported-by: Jaeyoung Chung <jjy600901@snu.ac.kr>
|
|
Link: https://patch.msgid.link/20260423-work-epoll-uaf-v1-6-2470f9eec0f5@kernel.org
|
|
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
|
|
|
|
Signed-off-by: Ian Kent <ikent@redhat.com>
|
|
|
|
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
|
|
index 1246c0517aa6..f46906640a9b 100644
|
|
--- a/fs/eventpoll.c
|
|
+++ b/fs/eventpoll.c
|
|
@@ -911,22 +911,26 @@ static bool ep_remove_epi(struct eventpoll *ep, struct epitem *epi)
|
|
*/
|
|
static void ep_remove(struct eventpoll *ep, struct epitem *epi)
|
|
{
|
|
- struct file *file = epi->ffd.file;
|
|
+ struct file *file __free(fput) = NULL;
|
|
|
|
lockdep_assert_irqs_enabled();
|
|
lockdep_assert_held(&ep->mtx);
|
|
|
|
ep_unregister_pollwait(ep, epi);
|
|
|
|
- /* sync with eventpoll_release_file() */
|
|
+ /* cheap sync with eventpoll_release_file() */
|
|
if (unlikely(READ_ONCE(epi->dying)))
|
|
return;
|
|
|
|
- spin_lock(&file->f_lock);
|
|
- if (epi->dying) {
|
|
- spin_unlock(&file->f_lock);
|
|
+ /*
|
|
+ * If we manage to grab a reference it means we're not in
|
|
+ * eventpoll_release_file() and aren't going to be.
|
|
+ */
|
|
+ file = epi_fget(epi);
|
|
+ if (!file)
|
|
return;
|
|
- }
|
|
+
|
|
+ spin_lock(&file->f_lock);
|
|
ep_remove_file(ep, epi, file);
|
|
|
|
if (ep_remove_epi(ep, epi))
|
|
--
|
|
2.50.1 (Apple Git-155)
|
|
|