The following commit has been merged into the perf/core branch of tip:

Commit-ID: 3a5465418f5fd970e86a86c7f4075be262682840 Gitweb: https://git.kernel.org/tip/3a5465418f5fd970e86a86c7f4075be262682840 Author: Frederic Weisbecker frederic@kernel.org AuthorDate: Fri, 21 Jun 2024 11:16:01 +02:00 Committer: Peter Zijlstra peterz@infradead.org CommitterDate: Tue, 09 Jul 2024 13:26:33 +02:00

perf: Fix event leak upon exec and file release

The perf pending task work is never waited upon the matching event release. In the case of a child event, released via free_event() directly, this can potentially result in a leaked event, such as in the following scenario that doesn't even require a weak IRQ work implementation to trigger:

schedule() prepare_task_switch() =======> <NMI> perf_event_overflow() event->pending_sigtrap = ... irq_work_queue(&event->pending_irq) <======= </NMI> perf_event_task_sched_out() event_sched_out() event->pending_sigtrap = 0; atomic_long_inc_not_zero(&event->refcount) task_work_add(&event->pending_task) finish_lock_switch() =======> <IRQ> perf_pending_irq() //do nothing, rely on pending task work <======= </IRQ>

begin_new_exec() perf_event_exit_task() perf_event_exit_event() // If is child event free_event() WARN(atomic_long_cmpxchg(&event->refcount, 1, 0) != 1) // event is leaked

Similar scenarios can also happen with perf_event_remove_on_exec() or simply against concurrent perf_event_release().

Fix this with synchonizing against the possibly remaining pending task work while freeing the event, just like is done with remaining pending IRQ work. This means that the pending task callback neither need nor should hold a reference to the event, preventing it from ever beeing freed.

Fixes: 517e6a301f34 ("perf: Fix perf_pending_task() UaF") Signed-off-by: Frederic Weisbecker frederic@kernel.org Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/20240621091601.18227-5-frederic@kernel.org --- include/linux/perf_event.h | 1 +- kernel/events/core.c | 38 +++++++++++++++++++++++++++++++++---- 2 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h index a5304ae..393fb13 100644 --- a/include/linux/perf_event.h +++ b/include/linux/perf_event.h @@ -786,6 +786,7 @@ struct perf_event { struct irq_work pending_irq; struct callback_head pending_task; unsigned int pending_work; + struct rcuwait pending_work_wait;

atomic_t event_limit;

diff --git a/kernel/events/core.c b/kernel/events/core.c index 576400d..32c7996 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -2288,7 +2288,6 @@ event_sched_out(struct perf_event *event, struct perf_event_context *ctx) if (state != PERF_EVENT_STATE_OFF && !event->pending_work && !task_work_add(current, &event->pending_task, TWA_RESUME)) { - WARN_ON_ONCE(!atomic_long_inc_not_zero(&event->refcount)); event->pending_work = 1; } else { local_dec(&event->ctx->nr_pending); @@ -5203,9 +5202,35 @@ static bool exclusive_event_installable(struct perf_event *event, static void perf_addr_filters_splice(struct perf_event *event, struct list_head *head);

+static void perf_pending_task_sync(struct perf_event *event) +{ + struct callback_head *head = &event->pending_task; + + if (!event->pending_work) + return; + /* + * If the task is queued to the current task's queue, we + * obviously can't wait for it to complete. Simply cancel it. + */ + if (task_work_cancel(current, head)) { + event->pending_work = 0; + local_dec(&event->ctx->nr_pending); + return; + } + + /* + * All accesses related to the event are within the same + * non-preemptible section in perf_pending_task(). The RCU + * grace period before the event is freed will make sure all + * those accesses are complete by then. + */ + rcuwait_wait_event(&event->pending_work_wait, !event->pending_work, TASK_UNINTERRUPTIBLE); +} + static void _free_event(struct perf_event *event) { irq_work_sync(&event->pending_irq); + perf_pending_task_sync(event);

unaccount_event(event);

@@ -6818,23 +6843,27 @@ static void perf_pending_task(struct callback_head *head) int rctx;

/* + * All accesses to the event must belong to the same implicit RCU read-side + * critical section as the ->pending_work reset. See comment in + * perf_pending_task_sync(). + */ + preempt_disable_notrace(); + /* * If we 'fail' here, that's OK, it means recursion is already disabled * and we won't recurse 'further'. */ - preempt_disable_notrace(); rctx = perf_swevent_get_recursion_context();

if (event->pending_work) { event->pending_work = 0; perf_sigtrap(event); local_dec(&event->ctx->nr_pending); + rcuwait_wake_up(&event->pending_work_wait); }

if (rctx >= 0) perf_swevent_put_recursion_context(rctx); preempt_enable_notrace(); - - put_event(event); }

#ifdef CONFIG_GUEST_PERF_EVENTS @@ -11948,6 +11977,7 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu, init_waitqueue_head(&event->waitq); init_irq_work(&event->pending_irq, perf_pending_irq); init_task_work(&event->pending_task, perf_pending_task); + rcuwait_init(&event->pending_work_wait);

mutex_init(&event->mmap_mutex); raw_spin_lock_init(&event->addr_filters.lock);