[PATCH 5.15 13/14] perf: Fix perf_pending_task() UaF

Greg Kroah-Hartman
2022-12-15T18:12:40Z

From: Peter Zijlstra <peterz@infradead.org>

[ Upstream commit 517e6a301f34613bff24a8e35b5455884f2d83d8 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/events/core.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 44f982b73640..5422bd77c7d4 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2367,6 +2367,7 @@ 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)
@@ -2412,6 +2413,7 @@ 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
@@ -2432,12 +2434,20 @@ __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)
@@ -5212,9 +5222,7 @@ 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.
@@ -5226,8 +5234,7 @@ 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);

@@ -6662,6 +6669,8 @@ static void perf_pending_task(struct callback_head *head)
if (rctx >= 0)
perf_swevent_put_recursion_context(rctx);
preempt_enable_notrace();
+
+ put_event(event);
}

/*
--
2.35.1