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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Mon, 1 Feb 2021 10:42:47 +0100
Message-ID: <CACT4Y+Yq69nvj2KZUQrYqtyu+Low+jCCcH++U_vuiHkhezQHGw@mail.gmail.com>
Subject: Re: extended bpf_send_signal_thread with argument
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>, andrii@kernel.org,
        Martin KaFai Lau <kafai@fb.com>,
        David Miller <davem@davemloft.net>, kpsingh@kernel.org,
        John Fastabend <john.fastabend@gmail.com>,
        netdev <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Feb 1, 2021 at 10:22 AM Peter Zijlstra <peterz@infradead.org> 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?