[SRU][N:hwe-6.17][PATCH 1/1] eventpoll: fix ep_remove struct eventpoll / struct file UAF

[Cengiz Can]

From: Christian Brauner <brauner at kernel.org>

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 at snu.ac.kr>
Link: https://patch.msgid.link/20260423-work-epoll-uaf-v1-6-2470f9eec0f5@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner at kernel.org>

(backported from commit a6dc643c69311677c574a0f17a3f4d66a5f3744b)
[cengizcan: adapt to 6.17's monolithic __ep_remove(ep, epi, force); pin
 via epi_fget() only on the !force path, keep the raw unpinned file on
 the force (eventpoll_release_file) path, drop the redundant under-lock
 epi->dying recheck, forward-declare epi_fget(), and drop the pin right
 after the f_lock section rather than at function exit (equivalent here;
 upstream uses the __free(fput) cleanup attribute)]
CVE-2026-46242
Signed-off-by: Cengiz Can <cengiz.can at canonical.com>
---
 fs/eventpoll.c | 29 +++++++++++++++++++++++++----
 1 file changed, 25 insertions(+), 4 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index ee7c4b683ec3..86ea62c4f8ea 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -822,6 +822,9 @@ static void ep_free(struct eventpoll *ep)
 	kfree(ep);
 }
 
+/* epi_fget() is defined further down; forward-declared for __ep_remove(). */
+static struct file *epi_fget(const struct epitem *epi);
+
 /*
  * Removes a "struct epitem" from the eventpoll RB tree and deallocates
  * all the associated resources. Must be called with "mtx" held.
@@ -832,6 +835,8 @@ static void ep_free(struct eventpoll *ep)
  */
 static bool __ep_remove(struct eventpoll *ep, struct epitem *epi, bool force)
 {
+	/* Raw pointer used by the force path; the !force path overwrites it
+	 * below with a pinned reference from epi_fget(). */
 	struct file *file = epi->ffd.file;
 	struct epitems_head *to_free;
 	struct hlist_head *head;
@@ -843,12 +848,26 @@ static bool __ep_remove(struct eventpoll *ep, struct epitem *epi, bool force)
 	 */
 	ep_unregister_pollwait(ep, epi);
 
+	if (!force) {
+		/* cheap sync with eventpoll_release_file() */
+		if (unlikely(READ_ONCE(epi->dying)))
+			return false;
+		/*
+		 * If we manage to grab a reference it means we're not in
+		 * eventpoll_release_file() and aren't going to be.  Pinning
+		 * @file keeps it, and transitively the watched struct
+		 * eventpoll, alive across the f_lock critical section below,
+		 * closing the UAF.  A successful pin also proves we are not
+		 * racing eventpoll_release_file() on this epi, so the old
+		 * under-lock epi->dying recheck is dropped.
+		 */
+		file = epi_fget(epi);
+		if (!file)
+			return false;
+	}
+
 	/* Remove the current item from the list of epoll hooks */
 	spin_lock(&file->f_lock);
-	if (epi->dying && !force) {
-		spin_unlock(&file->f_lock);
-		return false;
-	}
 
 	to_free = NULL;
 	head = file->f_ep;
@@ -864,6 +883,8 @@ static bool __ep_remove(struct eventpoll *ep, struct epitem *epi, bool force)
 	}
 	hlist_del_rcu(&epi->fllink);
 	spin_unlock(&file->f_lock);
+	if (!force)
+		fput(file);
 	free_ephead(to_free);
 
 	rb_erase_cached(&epi->rbn, &ep->rbr);
-- 
2.43.0