CVE-2023-54019

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

sched/psi: use kernfs polling functions for PSI trigger polling

Destroying psi trigger in cgroup_file_release causes UAF issues when

a cgroup is removed from under a polling process. This is happening

because cgroup removal causes a call to cgroup_file_release while the

actual file is still alive. Destroying the trigger at this point would

also destroy its waitqueue head and if there is still a polling process

on that file accessing the waitqueue, it will step on the freed pointer:

do_select

vfs_poll

do_rmdir

cgroup_rmdir

kernfs_drain_open_files

cgroup_file_release

cgroup_pressure_release

psi_trigger_destroy

wake_up_pollfree(&t->event_wait)

// vfs_poll is unblocked

synchronize_rcu

kfree(t)

poll_freewait -> UAF access to the trigger's waitqueue head

Patch [1] fixed this issue for epoll() case using wake_up_pollfree(),

however the same issue exists for synchronous poll() case.

The root cause of this issue is that the lifecycles of the psi trigger's

waitqueue and of the file associated with the trigger are different. Fix

this by using kernfs_generic_poll function when polling on cgroup-specific

psi triggers. It internally uses kernfs_open_node->poll waitqueue head

with its lifecycle tied to the file's lifecycle. This also renders the

fix in [1] obsolete, so revert it.

[1] commit c2dbe32d5db5 ("sched/psi: Fix use-after-free in ep_remove_wait_queue()")