[PATCH v3] sched/psi: fix use-after-free in ep_remove_wait_queue()

[Munehisa Kamata]

If a non-root cgroup gets removed when there is a thread that registered
trigger and is polling on a pressure file within the cgroup, the polling
waitqueue gets freed in the following path.

 do_rmdir
   cgroup_rmdir
     kernfs_drain_open_files
       cgroup_file_release
         cgroup_pressure_release
           psi_trigger_destroy

However, the polling thread still has a reference to the pressure file and
will access the freed waitqueue when the file is closed or upon exit.

 fput
   ep_eventpoll_release
     ep_free
       ep_remove_wait_queue
         remove_wait_queue

This results in use-after-free as pasted below.

The fundamental problem here is that cgroup_file_release() (and
consequently waitqueue's lifetime) is not tied to the file's real lifetime.
Using wake_up_pollfree() here might be less than ideal, but it also is not
fully contradicting the comment at commit 42288cb44c4b ("wait: add
wake_up_pollfree()") since the waitqueue's lifetime is not tied to file's
one and can be considered as another special case. While this would be
fixable by somehow making cgroup_file_release() be tied to the fput(), it
would require sizable refactoring at cgroups or higher layer which might be
more justifiable if we identify more cases like this.

 BUG: KASAN: use-after-free in _raw_spin_lock_irqsave+0x60/0xc0
 Write of size 4 at addr ffff88810e625328 by task a.out/4404

 CPU: 19 PID: 4404 Comm: a.out Not tainted 6.2.0-rc6 #38
 Hardware name: Amazon EC2 c5a.8xlarge/, BIOS 1.0 10/16/2017
 Call Trace:
 <TASK>
 dump_stack_lvl+0x73/0xa0
 print_report+0x16c/0x4e0
 ? _printk+0x59/0x80
 ? __virt_addr_valid+0xb8/0x130
 ? _raw_spin_lock_irqsave+0x60/0xc0
 kasan_report+0xc3/0xf0
 ? _raw_spin_lock_irqsave+0x60/0xc0
 kasan_check_range+0x2d2/0x310
 _raw_spin_lock_irqsave+0x60/0xc0
 remove_wait_queue+0x1a/0xa0
 ep_free+0x12c/0x170
 ep_eventpoll_release+0x26/0x30
 __fput+0x202/0x400
 task_work_run+0x11d/0x170
 do_exit+0x495/0x1130
 ? update_cfs_rq_load_avg+0x2c2/0x2e0
 do_group_exit+0x100/0x100
 get_signal+0xd67/0xde0
 ? finish_task_switch+0x15f/0x3a0
 arch_do_signal_or_restart+0x2a/0x2b0
 exit_to_user_mode_prepare+0x94/0x100
 syscall_exit_to_user_mode+0x20/0x40
 do_syscall_64+0x52/0x90
 entry_SYSCALL_64_after_hwframe+0x63/0xcd
 RIP: 0033:0x7f8e392bfb91
 Code: Unable to access opcode bytes at 0x7f8e392bfb67.
 RSP: 002b:00007fff261e08d8 EFLAGS: 00000246 ORIG_RAX: 0000000000000022
 RAX: fffffffffffffdfe RBX: 0000000000000000 RCX: 00007f8e392bfb91
 RDX: 0000000000000001 RSI: 00007fff261e08e8 RDI: 0000000000000004
 RBP: 00007fff261e0920 R08: 0000000000400780 R09: 00007f8e3960f240
 R10: 00000000000003df R11: 0000000000000246 R12: 00000000004005a0
 R13: 00007fff261e0a00 R14: 0000000000000000 R15: 0000000000000000
 </TASK>

 Allocated by task 4404:
 kasan_set_track+0x3d/0x60
 __kasan_kmalloc+0x85/0x90
 psi_trigger_create+0x113/0x3e0
 pressure_write+0x146/0x2e0
 cgroup_file_write+0x11c/0x250
 kernfs_fop_write_iter+0x186/0x220
 vfs_write+0x3d8/0x5c0
 ksys_write+0x90/0x110
 do_syscall_64+0x43/0x90
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

 Freed by task 4407:
 kasan_set_track+0x3d/0x60
 kasan_save_free_info+0x27/0x40
 ____kasan_slab_free+0x11d/0x170
 slab_free_freelist_hook+0x87/0x150
 __kmem_cache_free+0xcb/0x180
 psi_trigger_destroy+0x2e8/0x310
 cgroup_file_release+0x4f/0xb0
 kernfs_drain_open_files+0x165/0x1f0
 kernfs_drain+0x162/0x1a0
 __kernfs_remove+0x1fb/0x310
 kernfs_remove_by_name_ns+0x95/0xe0
 cgroup_addrm_files+0x67f/0x700
 cgroup_destroy_locked+0x283/0x3c0
 cgroup_rmdir+0x29/0x100
 kernfs_iop_rmdir+0xd1/0x140
 vfs_rmdir+0xfe/0x240
 do_rmdir+0x13d/0x280
 __x64_sys_rmdir+0x2c/0x30
 do_syscall_64+0x43/0x90
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

v3: updated commit message and the comment in the code
v2: updated commit message

Link: https://lore.kernel.org/lkml/20230106224859.4123476-1-kamatam@xxxxxxxxxx/
Fixes: 0e94682b73bf ("psi: introduce psi monitor")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Munehisa Kamata <kamatam@xxxxxxxxxx>
Signed-off-by: Mengchi Cheng <mengcc@xxxxxxxxxx>
---
 kernel/sched/psi.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index 8ac8b81bfee6..02e011cabe91 100644
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -1343,10 +1343,11 @@ void psi_trigger_destroy(struct psi_trigger *t)
 
 	group = t->group;
 	/*
-	 * Wakeup waiters to stop polling. Can happen if cgroup is deleted
-	 * from under a polling process.
+	 * Wakeup waiters to stop polling and clear the queue to prevent it from
+	 * being accessed later. Can happen if cgroup is deleted from under a
+	 * polling process.
 	 */
-	wake_up_interruptible(&t->event_wait);
+	wake_up_pollfree(&t->event_wait);
 
 	mutex_lock(&group->trigger_lock);
 
-- 
2.38.1