100 lines
3.7 KiB
Diff
100 lines
3.7 KiB
Diff
From: Christian Brauner <brauner@kernel.org>
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Date: Thu, 23 Apr 2026 11:56:09 +0200
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Subject: [PATCH] eventpoll: fix ep_remove struct eventpoll / struct file UAF
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CVE: CVE-2026-46242
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Upstream commit a6dc643c69311677c574a0f17a3f4d66a5f3744b, adapted to the
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pre-e9e5cd40d7c4 ("eventpoll: kill __ep_remove()") code shape still used
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by this kernel, where the removal path is the monolithic __ep_remove(ep,
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epi, force) rather than upstream's ep_remove()/ep_remove_epi()/
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ep_remove_file() split.
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ep_remove() used epi->ffd.file across the file->f_lock critical section
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without holding a reference. A concurrent __fput() taking the
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eventpoll_release() fastpath could free the file (and, for the
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epoll-watches-epoll case, the watched struct eventpoll) mid-section,
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leading to a use-after-free / attacker-controllable kmem_cache_free().
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Pin the file with epi_fget() before entering the critical section and
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release it afterwards. A failed pin means __fput() is already in flight
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and will remove the epi via eventpoll_release_file() (which blocks on
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ep->mtx), so it is safe to bail. The pin is taken only on the !force
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path: the force path is reached from eventpoll_release_file() itself,
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i.e. from within __fput() where f_count has already dropped to zero and
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epi_fget() would necessarily fail; there @file is still valid and no
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concurrent __fput() can race, so it keeps using epi->ffd.file directly.
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A successful pin also makes the in-lock epi->dying recheck redundant; the
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cheap lockless READ_ONCE(epi->dying) fast-path bailout is kept.
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Fixes: 58c9b016e128 ("epoll: use refcount to reduce ep_mutex contention")
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Reported-by: Jaeyoung Chung <jjy600901@snu.ac.kr>
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Signed-off-by: Christian Brauner <brauner@kernel.org>
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---
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diff --git a/fs/eventpoll.c b/fs/eventpoll.c
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index f4efb5b..68e58d0 100644
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--- a/fs/eventpoll.c
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+++ b/fs/eventpoll.c
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@@ -821,6 +821,8 @@ static void ep_free(struct eventpoll *ep)
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kfree(ep);
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}
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+static struct file *epi_fget(const struct epitem *epi);
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+
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/*
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* Removes a "struct epitem" from the eventpoll RB tree and deallocates
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* all the associated resources. Must be called with "mtx" held.
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@@ -832,6 +834,7 @@ static void ep_free(struct eventpoll *ep)
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static bool __ep_remove(struct eventpoll *ep, struct epitem *epi, bool force)
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{
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struct file *file = epi->ffd.file;
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+ struct file *to_put = NULL;
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struct epitems_head *to_free;
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struct hlist_head *head;
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@@ -842,13 +845,27 @@ static bool __ep_remove(struct eventpoll *ep, struct epitem *epi, bool force)
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*/
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ep_unregister_pollwait(ep, epi);
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- /* Remove the current item from the list of epoll hooks */
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- spin_lock(&file->f_lock);
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- if (epi->dying && !force) {
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- spin_unlock(&file->f_lock);
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- return false;
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+ if (!force) {
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+ /* cheap sync with eventpoll_release_file() */
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+ if (unlikely(READ_ONCE(epi->dying)))
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+ return false;
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+ /*
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+ * Grabbing a reference proves we are not racing
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+ * eventpoll_release_file() and are not going to: a concurrent
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+ * __fput() cannot free @file (nor, for the epoll-on-epoll
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+ * case, the watched eventpoll) across the f_lock section
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+ * below. A failed pin means @file already reached refcount
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+ * zero and __fput() is in flight; it will remove this epi via
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+ * eventpoll_release_file(), so bail out here.
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+ */
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+ file = epi_fget(epi);
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+ if (!file)
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+ return false;
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+ to_put = file;
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}
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+ /* Remove the current item from the list of epoll hooks */
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+ spin_lock(&file->f_lock);
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to_free = NULL;
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head = file->f_ep;
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if (head->first == &epi->fllink && !epi->fllink.next) {
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@@ -882,6 +899,8 @@ static bool __ep_remove(struct eventpoll *ep, struct epitem *epi, bool force)
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kfree_rcu(epi, rcu);
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percpu_counter_dec(&ep->user->epoll_watches);
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+ if (to_put)
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+ fput(to_put);
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return true;
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}
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--
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2.43.0
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