FAILED: patch "[PATCH] eventpoll: fix ep_remove struct eventpoll / struct file UAF" failed to apply to 6.1-stable tree

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x a6dc643c69311677c574a0f17a3f4d66a5f3744b
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026051522-phobia-rebate-b7c8@gregkh' --subject-prefix 'PATCH 6.1.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From a6dc643c69311677c574a0f17a3f4d66a5f3744b Mon Sep 17 00:00:00 2001
From: Christian Brauner <[email protected]>
Date: Thu, 23 Apr 2026 11:56:09 +0200
Subject: [PATCH] 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.

index 5ee4398a6cb8..0f785c0a1544 100644
@@ -912,22 +912,26 @@ 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))