Re: extended bpf_send_signal_thread with argument

[Dmitry Vyukov]

On Mon, Feb 1, 2021 at 10:22 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Sun, Jan 31, 2021 at 12:14:02PM +0100, Dmitry Vyukov wrote:
> > Hi,
> >
> > I would like to send a signal from a bpf program invoked from a
> > perf_event. There is:
>
> You can't. Sending signals requires sighand lock, and you're not allowed
> to take locks from perf_event context.


Then we just found a vulnerability because there is
bpf_send_signal_thread which can be attached to perf and it passes the
verifier :)
https://elixir.bootlin.com/linux/v5.11-rc5/source/kernel/trace/bpf_trace.c#L1145

It can defer sending the signal to the exit of irq context:
https://elixir.bootlin.com/linux/v5.11-rc5/source/kernel/trace/bpf_trace.c#L1108
Perhaps this is what makes it work?