CVE-2022-48989

In the Linux kernel, the following vulnerability has been resolved:

fscache: Fix oops due to race with cookielru and usecookie

If a cookie expires from the LRU and the LRUDISCARD flag is set, but the state machine has not run yet, it's possible another thread can call fscacheuse_cookie and begin to use it.

When the cookieworker finally runs, it will see the LRUDISCARD flag set, transition the cookie->state to LRU_DISCARDING, which will then withdraw the cookie. Once the cookie is withdrawn the object is removed the below oops will occur because the object associated with the cookie is now NULL.

Fix the oops by clearing the LRUDISCARD bit if another thread uses the cookie before the cookieworker runs.

BUG: kernel NULL pointer dereference, address: 0000000000000008 ... CPU: 31 PID: 44773 Comm: kworker/u130:1 Tainted: G E 6.0.0-5.dneg.x8664 #1 Hardware name: Google Compute Engine/Google Compute Engine, BIOS Google 08/26/2022 Workqueue: eventsunbound netfsrreqwritetocachework [netfs] RIP: 0010:cachefilespreparewrite+0x28/0x90 [cachefiles] ... Call Trace: netfsrreqwritetocachework+0x11c/0x320 [netfs] processonework+0x217/0x3e0 worker_thread+0x4a/0x3b0 kthread+0xd6/0x100