linux-kernel - nullptr-deference in perf

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Message-ID: <ZSDxDZR5hoaKTCdP@westworld>
Date:   Fri, 6 Oct 2023 22:47:57 -0700
From:   Kyle Zeng <zengyhkyle@...il.com>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: nullptr-deference in perf

Hi there,

I found a nullptr dereference in perf subsystem and it affects at least
v5.10 and v6.1 stable trees. (the same poc cannot trigger the crash in
the mainline).

I fail to find the root cause the bug. All I know is that it is a race
condition in the logic of moving_groups from pure software-based perf
events to hardware ones. More specifically, when we add a hardware perf
event to a software event group, it will trigger a "move_group" logic in
perf_event_open. When the "move_group" logic happens, it will remove all
existing events from the context first using `perf_remove_from_context`.
And it will invoke `__perf_remove_from_context` through
`event_function_call`.

Notice that `event_function_call` is defined as follow:
~~~
static void event_function_call(struct perf_event *event, event_f func, void *data)
{
	...
	func(event, NULL, ctx, data);
	...
}
~~~
This means `__perf_remove_from_context` will be invoked with
cpuctx==NULL, which leads to invoking `event_sched_out` with cpuctx ==
NULL.
At this moment, as long as the event is active, we are going to invoke
the `if (event->attr.exclusive || !cpuctx->active_oncpu)` logic, which
is a null pointer deference.

I don't know the proper way to patch this bug. So I'm asking for help.

A reproducer is attached to this email.

Best,
Kyle Zeng

View attachment "poc.c" of type "text/x-csrc" (1890 bytes)