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 without clearing the queue and reference 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 can keep having the last reference to the
pressure file that is tied to the freed waitqueue until explicit close or
exit later.
fput
ep_eventpoll_release
ep_free
ep_remove_wait_queue
remove_wait_queue
Then, the thread accesses to the already-freed waitqueue when dropping the
reference and results in use-after-free as pasted below.
The fundamental problem here is that the lifetime of the waitqueue is not
tied to the file's real lifetime as shown above. 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
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..6e66c15f6450 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 otherwise.
*/
- wake_up_interruptible(&t->event_wait);
+ wake_up_pollfree(&t->event_wait);
mutex_lock(&group->trigger_lock);
--
2.38.1
