linux.git/kernel/sched/psi.c, branch v6.5

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

2023-07-10T07:52:30+00:00

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()")

Fixes: 0e94682b73bf ("psi: introduce psi monitor")
Closes: https://lore.kernel.org/all/20230613062306.101831-1-lujialin4@huawei.com/
Reported-by: Lu Jialin 
Signed-off-by: Suren Baghdasaryan 
Signed-off-by: Peter Zijlstra (Intel) 
Link: https://lkml.kernel.org/r/20230630005612.1014540-1-surenb@google.com

sched/psi: Avoid resetting the min update period when it is unnecessary

2023-05-20T10:53:16+00:00

Psi_group's poll_min_period is determined by the minimum window size of
psi_trigger when creating new triggers. While destroying a psi_trigger,
there is no need to reset poll_min_period if the psi_trigger being
destroyed did not have the minimum window size, since in this condition
poll_min_period will remain the same as before.

Signed-off-by: Yang Yang 
Signed-off-by: Peter Zijlstra (Intel) 
Acked-by: Suren Baghdasaryan 
Link: https://lkml.kernel.org/r/20230514163338.834345-1-surenb@google.com

psi: remove 500ms min window size limitation for triggers

2023-05-08T08:58:38+00:00

Current 500ms min window size for psi triggers limits polling interval
to 50ms to prevent polling threads from using too much cpu bandwidth by
polling too frequently. However the number of cgroups with triggers is
unlimited, so this protection can be defeated by creating multiple
cgroups with psi triggers (triggers in each cgroup are served by a single
"psimon" kernel thread).
Instead of limiting min polling period, which also limits the latency of
psi events, it's better to limit psi trigger creation to authorized users
only, like we do for system-wide psi triggers (/proc/pressure/* files can
be written only by processes with CAP_SYS_RESOURCE capability). This also
makes access rules for cgroup psi files consistent with system-wide ones.
Add a CAP_SYS_RESOURCE capability check for cgroup psi file writers and
remove the psi window min size limitation.

Suggested-by: Sudarshan Rajagopalan 
Signed-off-by: Suren Baghdasaryan 
Signed-off-by: Peter Zijlstra (Intel) 
Acked-by: Michal Hocko 
Acked-by: Johannes Weiner 
Link: https://lore.kernel.org/all/cover.1676067791.git.quic_sudaraja@quicinc.com/

sched/psi: Allow unprivileged polling of N*2s period

2023-04-05T07:58:50+00:00

PSI offers 2 mechanisms to get information about a specific resource
pressure. One is reading from /proc/pressure/, which gives
average pressures aggregated every 2s. The other is creating a pollable
fd for a specific resource and cgroup.

The trigger creation requires CAP_SYS_RESOURCE, and gives the
possibility to pick specific time window and threshold, spawing an RT
thread to aggregate the data.

Systemd would like to provide containers the option to monitor pressure
on their own cgroup and sub-cgroups. For example, if systemd launches a
container that itself then launches services, the container should have
the ability to poll() for pressure in individual services. But neither
the container nor the services are privileged.

This patch implements a mechanism to allow unprivileged users to create
pressure triggers. The difference with privileged triggers creation is
that unprivileged ones must have a time window that's a multiple of 2s.
This is so that we can avoid unrestricted spawning of rt threads, and
use instead the same aggregation mechanism done for the averages, which
runs independently of any triggers.

Suggested-by: Johannes Weiner 
Signed-off-by: Domenico Cerasuolo 
Signed-off-by: Peter Zijlstra (Intel) 
Acked-by: Johannes Weiner 
Link: https://lore.kernel.org/r/20230330105418.77061-5-cerasuolodomenico@gmail.com

sched/psi: Extract update_triggers side effect

2023-04-05T07:58:49+00:00

This change moves update_total flag out of update_triggers function,
currently called only in psi_poll_work.
In the next patch, update_triggers will be called also in psi_avgs_work,
but the total update information is specific to psi_poll_work.
Returning update_total value to the caller let us avoid differentiating
the implementation of update_triggers for different aggregators.

Suggested-by: Johannes Weiner 
Signed-off-by: Domenico Cerasuolo 
Signed-off-by: Peter Zijlstra (Intel) 
Acked-by: Johannes Weiner 
Link: https://lore.kernel.org/r/20230330105418.77061-4-cerasuolodomenico@gmail.com

sched/psi: Rename existing poll members in preparation

2023-04-05T07:58:49+00:00

Renaming in PSI implementation to make a clear distinction between
privileged and unprivileged triggers code to be implemented in the
next patch.

Suggested-by: Johannes Weiner 
Signed-off-by: Domenico Cerasuolo 
Signed-off-by: Peter Zijlstra (Intel) 
Acked-by: Johannes Weiner 
Link: https://lore.kernel.org/r/20230330105418.77061-3-cerasuolodomenico@gmail.com

sched/psi: Rearrange polling code in preparation

2023-04-05T07:58:48+00:00

Move a few functions up in the file to avoid forward declaration needed
in the patch implementing unprivileged PSI triggers.

Suggested-by: Johannes Weiner 
Signed-off-by: Domenico Cerasuolo 
Signed-off-by: Peter Zijlstra (Intel) 
Acked-by: Johannes Weiner 
Link: https://lore.kernel.org/r/20230330105418.77061-2-cerasuolodomenico@gmail.com

sched/psi: Fix use-after-free in ep_remove_wait_queue()

2023-02-15T13:19:16+00:00

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 is in line
with 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:
	
	dump_stack_lvl+0x73/0xa0
	print_report+0x16c/0x4e0
	kasan_report+0xc3/0xf0
	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
	do_group_exit+0x100/0x100
	get_signal+0xd67/0xde0
	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
	

 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

Fixes: 0e94682b73bf ("psi: introduce psi monitor")
Signed-off-by: Munehisa Kamata 
Signed-off-by: Mengchi Cheng 
Signed-off-by: Ingo Molnar 
Acked-by: Suren Baghdasaryan 
Acked-by: Peter Zijlstra 
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/lkml/20230106224859.4123476-1-kamatam@amazon.com/
Link: https://lore.kernel.org/r/20230214212705.4058045-1-kamatam@amazon.com

sched/psi: Stop relying on timer_pending() for poll_work rescheduling

2022-10-30T09:12:14+00:00

Psi polling mechanism is trying to minimize the number of wakeups to
run psi_poll_work and is currently relying on timer_pending() to detect
when this work is already scheduled. This provides a window of opportunity
for psi_group_change to schedule an immediate psi_poll_work after
poll_timer_fn got called but before psi_poll_work could reschedule itself.
Below is the depiction of this entire window:

poll_timer_fn
  wake_up_interruptible(&group->poll_wait);

psi_poll_worker
  wait_event_interruptible(group->poll_wait, ...)
  psi_poll_work
    psi_schedule_poll_work
      if (timer_pending(&group->poll_timer)) return;
      ...
      mod_timer(&group->poll_timer, jiffies + delay);

Prior to 461daba06bdc we used to rely on poll_scheduled atomic which was
reset and set back inside psi_poll_work and therefore this race window
was much smaller.
The larger window causes increased number of wakeups and our partners
report visible power regression of ~10mA after applying 461daba06bdc.
Bring back the poll_scheduled atomic and make this race window even
narrower by resetting poll_scheduled only when we reach polling expiration
time. This does not completely eliminate the possibility of extra wakeups
caused by a race with psi_group_change however it will limit it to the
worst case scenario of one extra wakeup per every tracking window (0.5s
in the worst case).
This patch also ensures correct ordering between clearing poll_scheduled
flag and obtaining changed_states using memory barrier. Correct ordering
between updating changed_states and setting poll_scheduled is ensured by
atomic_xchg operation.
By tracing the number of immediate rescheduling attempts performed by
psi_group_change and the number of these attempts being blocked due to
psi monitor being already active, we can assess the effects of this change:

Before the patch:
                                           Run#1    Run#2      Run#3
Immediate reschedules attempted:           684365   1385156    1261240
Immediate reschedules blocked:             682846   1381654    1258682
Immediate reschedules (delta):             1519     3502       2558
Immediate reschedules (% of attempted):    0.22%    0.25%      0.20%

After the patch:
                                           Run#1    Run#2      Run#3
Immediate reschedules attempted:           882244   770298    426218
Immediate reschedules blocked:             881996   769796    426074
Immediate reschedules (delta):             248      502       144
Immediate reschedules (% of attempted):    0.03%    0.07%     0.03%

The number of non-blocked immediate reschedules dropped from 0.22-0.25%
to 0.03-0.07%. The drop is attributed to the decrease in the race window
size and the fact that we allow this race only when psi monitors reach
polling window expiration time.

Fixes: 461daba06bdc ("psi: eliminate kthread_worker from psi trigger scheduling mechanism")
Reported-by: Kathleen Chang 
Reported-by: Wenju Xu 
Reported-by: Jonathan Chen 
Signed-off-by: Suren Baghdasaryan 
Signed-off-by: Peter Zijlstra (Intel) 
Reviewed-by: Chengming Zhou 
Acked-by: Johannes Weiner 
Tested-by: SH Chen 
Link: https://lore.kernel.org/r/20221028194541.813985-1-surenb@google.com

sched/psi: Fix avgs_work re-arm in psi_avgs_work()

2022-10-30T09:12:14+00:00

Pavan reported a problem that PSI avgs_work idle shutoff is not
working at all. Because PSI_NONIDLE condition would be observed in
psi_avgs_work()->collect_percpu_times()->get_recent_times() even if
only the kworker running avgs_work on the CPU.

Although commit 1b69ac6b40eb ("psi: fix aggregation idle shut-off")
avoided the ping-pong wake problem when the worker sleep, psi_avgs_work()
still will always re-arm the avgs_work, so shutoff is not working.

This patch changes to use PSI_STATE_RESCHEDULE to flag whether to
re-arm avgs_work in get_recent_times(). For the current CPU, we re-arm
avgs_work only when (NR_RUNNING > 1 || NR_IOWAIT > 0 || NR_MEMSTALL > 0),
for other CPUs we can just check PSI_NONIDLE delta. The new flag
is only used in psi_avgs_work(), so we check in get_recent_times()
that current_work() is avgs_work.

One potential problem is that the brief period of non-idle time
incurred between the aggregation run and the kworker's dequeue will
be stranded in the per-cpu buckets until avgs_work run next time.
The buckets can hold 4s worth of time, and future activity will wake
the avgs_work with a 2s delay, giving us 2s worth of data we can leave
behind when shut off the avgs_work. If the kworker run other works after
avgs_work shut off and doesn't have any scheduler activities for 2s,
this maybe a problem.

Reported-by: Pavan Kondeti 
Signed-off-by: Chengming Zhou 
Signed-off-by: Peter Zijlstra (Intel) 
Acked-by: Johannes Weiner 
Acked-by: Suren Baghdasaryan 
Tested-by: Chengming Zhou 
Link: https://lore.kernel.org/r/20221014110551.22695-1-zhouchengming@bytedance.com