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perf: Fix perf_pending_task() UaF

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Per syzbot it is possible for perf_pending_task() to run after the
event is free()'d. There are two related but distinct cases:

 - the task_work was already queued before destroying the event;
 - destroying the event itself queues the task_work.

The first cannot be solved using task_work_cancel() since
perf_release() itself might be called from a task_work (____fput),
which means the current->task_works list is already empty and
task_work_cancel() won't be able to find the perf_pending_task()
entry.

The simplest alternative is extending the perf_event lifetime to cover
the task_work.

The second is just silly, queueing a task_work while you know the
event is going away makes no sense and is easily avoided by
re-arranging how the event is marked STATE_DEAD and ensuring it goes
through STATE_OFF on the way down.

Reported-by: syzbot+9228d6098455bb209ec8@syzkaller.appspotmail.com
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Marco Elver <elver@google.com>
This commit is contained in:
Peter Zijlstra 2022-11-24 12:49:12 +01:00
parent 030a976efa
commit 517e6a301f
1 changed files with 13 additions and 4 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

17
kernel/events/core.c
View file

@ -2291,6 +2291,7 @@ event_sched_out(struct perf_event *event,
!event->pending_work) {
event->pending_work = 1;
dec = false;
WARN_ON_ONCE(!atomic_long_inc_not_zero(&event->refcount));
task_work_add(current, &event->pending_task, TWA_RESUME);
}
if (dec)
@ -2336,6 +2337,7 @@ group_sched_out(struct perf_event *group_event,
#define DETACH_GROUP 0x01UL
#define DETACH_CHILD 0x02UL
#define DETACH_DEAD 0x04UL
/*
* Cross CPU call to remove a performance event
@ -2356,12 +2358,20 @@ __perf_remove_from_context(struct perf_event *event,
update_cgrp_time_from_cpuctx(cpuctx, false);
}
/*
* Ensure event_sched_out() switches to OFF, at the very least
* this avoids raising perf_pending_task() at this time.
*/
if (flags & DETACH_DEAD)
event->pending_disable = 1;
event_sched_out(event, cpuctx, ctx);
if (flags & DETACH_GROUP)
perf_group_detach(event);
if (flags & DETACH_CHILD)
perf_child_detach(event);
list_del_event(event, ctx);
if (flags & DETACH_DEAD)
event->state = PERF_EVENT_STATE_DEAD;
if (!ctx->nr_events && ctx->is_active) {
if (ctx == &cpuctx->ctx)
@ -5121,9 +5131,7 @@ int perf_event_release_kernel(struct perf_event *event)
ctx = perf_event_ctx_lock(event);
WARN_ON_ONCE(ctx->parent_ctx);
perf_remove_from_context(event, DETACH_GROUP);
raw_spin_lock_irq(&ctx->lock);
/*
* Mark this event as STATE_DEAD, there is no external reference to it
* anymore.
@ -5135,8 +5143,7 @@ int perf_event_release_kernel(struct perf_event *event)
* Thus this guarantees that we will in fact observe and kill _ALL_
* child events.
*/
event->state = PERF_EVENT_STATE_DEAD;
raw_spin_unlock_irq(&ctx->lock);
perf_remove_from_context(event, DETACH_GROUP|DETACH_DEAD);
perf_event_ctx_unlock(event, ctx);
@ -6577,6 +6584,8 @@ static void perf_pending_task(struct callback_head *head)
if (rctx >= 0)
perf_swevent_put_recursion_context(rctx);
preempt_enable_notrace();
put_event(event);
}
#ifdef CONFIG_GUEST_PERF_EVENTS