From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	Baokun Li <libaokun@linux.alibaba.com>,
	Miklos Szeredi <mszeredi@redhat.com>,
	Sasha Levin <sashal@kernel.org>
Subject: [PATCH 6.12 1205/1376] cuse: wait for pending RCU callbacks on module exit
Date: Sat, 12 Sep 2026 09:00:31 +0200
Message-ID: <20260912065634.405880088@linuxfoundation.org>
In-Reply-To: <20260912065607.535295758@linuxfoundation.org>
References: <20260912065607.535295758@linuxfoundation.org>
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6.12-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Baokun Li <libaokun@linux.alibaba.com>

[ Upstream commit 4deb3edead0c0e172cc7349e8855d741d3c5e162 ]

Since commit 053fc4f755ad ("fuse: fix UAF in rcu pathwalks"),
fuse_conn_put() frees the fuse_conn through call_rcu() rather than
synchronously.  For cuse, fc->release is cuse_fc_release(), which
lives in the cuse module.  If the module is removed before the RCU
grace period ends, the callback jumps into freed module memory:

      userspace / module unload      |        RCU softirq
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 close(/dev/cuse)                    |
  cuse_channel_release()             |
   fuse_dev_release()                |
    fuse_conn_put(fch->conn)         |
     call_rcu(delayed_release) ------+---> callback queued
                                     |
 rmmod cuse                          |
  cuse_exit()                        |
   cuse_channel_destroy()            |
   ...                               |
   return                            |
                                     |
 <module text freed>                 |
                                     |  rcu_do_batch()
                                     |   delayed_release()
                                     |    fc->release()
                                     |     -> cuse_fc_release()
                                     |        ^^^ freed text!

The freed module text is unmapped by vfree(), so the jump into the
stale callback triggers a page-fault Oops.  If the virtual address
is subsequently reused, the callback could execute unrelated code
(undefined behaviour).

Fix this by calling rcu_barrier() in cuse_exit() so that any pending
fuse_conn release callback completes before the module is removed.

Fixes: 053fc4f755ad ("fuse: fix UAF in rcu pathwalks")
Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/fuse/cuse.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/fs/fuse/cuse.c b/fs/fuse/cuse.c
index 22e82dae6c6bb..47c8e17cc0286 100644
--- a/fs/fuse/cuse.c
+++ b/fs/fuse/cuse.c
@@ -647,6 +647,11 @@ static void __exit cuse_exit(void)
 {
 	misc_deregister(&cuse_miscdev);
 	class_destroy(cuse_class);
+	/*
+	 * Wait for pending call_rcu() callbacks that call back into
+	 * this module via fc->release (cuse_fc_release).
+	 */
+	rcu_barrier();
 }
 
 module_init(cuse_init);
-- 
2.53.0