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Subject[PATCH] perf: Fix race between close() and fork()

Syzcaller reported the following Use-after-Free issue:

close() clone()

copy_process()
perf_event_init_task()
perf_event_init_context()
mutex_lock(parent_ctx->mutex)
inherit_task_group()
inherit_group()
inherit_event()
mutex_lock(event->child_mutex)
// expose event on child list
list_add_tail()
mutex_unlock(event->child_mutex)
mutex_unlock(parent_ctx->mutex)

...
goto bad_fork_*

bad_fork_cleanup_perf:
perf_event_free_task()

perf_release()
perf_event_release_kernel()
list_for_each_entry()
mutex_lock(ctx->mutex)
mutex_lock(event->child_mutex)
// event is from the failing inherit
// on the other CPU
perf_remove_from_context()
list_move()
mutex_unlock(event->child_mutex)
mutex_unlock(ctx->mutex)

mutex_lock(ctx->mutex)
list_for_each_entry_safe()
// event already stolen
mutex_unlock(ctx->mutex)

delayed_free_task()
free_task()

list_for_each_entry_safe()
list_del()
free_event()
_free_event()
// and so event->hw.target
// is the already freed failed clone()
if (event->hw.target)
put_task_struct(event->hw.target)
// WHOOPSIE, already quite dead


Which puts the lie to the the comment on perf_event_free_task():
'unexposed, unused context' not so much.

Which is a 'fun' confluence of fail; copy_process() doing an
unconditional free_task() and not respecting refcounts, and perf having
creative locking. In particular:

82d94856fa22 ("perf/core: Fix lock inversion between perf,trace,cpuhp")

seems to have overlooked this 'fun' parade.

Solve it by using the fact that detached events still have a reference
count on their (previous) context. With this perf_event_free_task()
can detect when events have escaped and wait for their destruction.

Cc: Mark Rutland <mark.rutland@arm.com>
Fixes: 82d94856fa22 ("perf/core: Fix lock inversion between perf,trace,cpuhp")
Reported-by: syzbot+a24c397a29ad22d86c98@syzkaller.appspotmail.com
Debugged-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/events/core.c | 49 +++++++++++++++++++++++++++++++++++++++++--------
1 file changed, 41 insertions(+), 8 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 10c1dba9068c..5302c19e9892 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4463,12 +4463,20 @@ static void _free_event(struct perf_event *event)
if (event->destroy)
event->destroy(event);

- if (event->ctx)
- put_ctx(event->ctx);
-
+ /*
+ * Must be after ->destroy(), due to uprobe_perf_close() using
+ * hw.target.
+ */
if (event->hw.target)
put_task_struct(event->hw.target);

+ /*
+ * perf_event_free_task() relies on put_ctx() being 'last', in particular
+ * all task references must be cleaned up.
+ */
+ if (event->ctx)
+ put_ctx(event->ctx);
+
exclusive_event_destroy(event);
module_put(event->pmu->module);

@@ -4648,8 +4656,17 @@ int perf_event_release_kernel(struct perf_event *event)
mutex_unlock(&event->child_mutex);

list_for_each_entry_safe(child, tmp, &free_list, child_list) {
+ void *var = &child->ctx->refcount;
+
list_del(&child->child_list);
free_event(child);
+
+ /*
+ * Wake any perf_event_free_task() waiting for this event to be
+ * freed.
+ */
+ smp_mb(); /* pairs with wait_var_event() */
+ wake_up_var(var);
}

no_ctx:
@@ -11512,11 +11529,11 @@ static void perf_free_event(struct perf_event *event,
}

/*
- * Free an unexposed, unused context as created by inheritance by
- * perf_event_init_task below, used by fork() in case of fail.
+ * Free a context as created by inheritance by perf_event_init_task() below,
+ * used by fork() in case of fail.
*
- * Not all locks are strictly required, but take them anyway to be nice and
- * help out with the lockdep assertions.
+ * Even though the task has never lived, the context and events have been
+ * exposed through the child_list, so we must take care tearing it all down.
*/
void perf_event_free_task(struct task_struct *task)
{
@@ -11546,7 +11563,23 @@ void perf_event_free_task(struct task_struct *task)
perf_free_event(event, ctx);

mutex_unlock(&ctx->mutex);
- put_ctx(ctx);
+
+ /*
+ * perf_event_release_kernel() could've stolen some of our
+ * child events and still have them on its free_list. In that
+ * case we must wait for these events to have been freed (in
+ * particular all their references to this task must've been
+ * dropped).
+ *
+ * Without this copy_process() will unconditionally free this
+ * task (irrespective of its reference count) and
+ * _free_event()'s put_task_struct(event->hw.target) will be a
+ * use-after-free.
+ *
+ * Wait for all events to drop their context reference.
+ */
+ wait_var_event(&ctx->refcount, refcount_read(&ctx->refcount) == 1);
+ put_ctx(ctx); /* must be last */
}
}

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