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Date:   Thu, 19 Jan 2023 22:41:36 +0530
From:   Kumar Kartikeya Dwivedi <memxor@gmail.com>
To:     Hao Sun <sunhao.th@gmail.com>
Cc:     bpf@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net,
        john.fastabend@gmail.com, andrii@kernel.org, martin.lau@linux.dev,
        song@kernel.org, yhs@fb.com, kpsingh@kernel.org, sdf@google.com,
        haoluo@google.com, jolsa@kernel.org, davem@davemloft.net,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] bpf: skip task with pid=1 in send_signal_common()
Message-ID: <20230119171136.3i25lucvhij2tr76@apollo>
References: <20230106084838.12690-1-sunhao.th@gmail.com>
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On Fri, Jan 06, 2023 at 02:18:38PM IST, Hao Sun wrote:
> The following kernel panic can be triggered when a task with pid=1
> attach a prog that attempts to send killing signal to itself, also
> see [1] for more details:
>
> Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
> CPU: 3 PID: 1 Comm: systemd Not tainted 6.1.0-09652-g59fe41b5255f #148
> Call Trace:
> <TASK>
> __dump_stack lib/dump_stack.c:88 [inline]
> dump_stack_lvl+0x100/0x178 lib/dump_stack.c:106
> panic+0x2c4/0x60f kernel/panic.c:275
> do_exit.cold+0x63/0xe4 kernel/exit.c:789
> do_group_exit+0xd4/0x2a0 kernel/exit.c:950
> get_signal+0x2460/0x2600 kernel/signal.c:2858
> arch_do_signal_or_restart+0x78/0x5d0 arch/x86/kernel/signal.c:306
> exit_to_user_mode_loop kernel/entry/common.c:168 [inline]
> exit_to_user_mode_prepare+0x15f/0x250 kernel/entry/common.c:203
> __syscall_exit_to_user_mode_work kernel/entry/common.c:285 [inline]
> syscall_exit_to_user_mode+0x1d/0x50 kernel/entry/common.c:296
> do_syscall_64+0x44/0xb0 arch/x86/entry/common.c:86
> entry_SYSCALL_64_after_hwframe+0x63/0xcd
>
> So skip task with pid=1 in bpf_send_signal_common() to avoid the panic.
>
> [1] https://lore.kernel.org/bpf/20221222043507.33037-1-sunhao.th@gmail.com
>
> Signed-off-by: Hao Sun <sunhao.th@gmail.com>
> ---
>  kernel/trace/bpf_trace.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 23ce498bca97..ed21ab9fe846 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -844,6 +844,9 @@ static int bpf_send_signal_common(u32 sig, enum pid_type type)
>  	 */
>  	if (unlikely(current->flags & (PF_KTHREAD | PF_EXITING)))
>  		return -EPERM;
> +	/* Task should not be pid=1 to avoid kernel panic. */
> +	if (unlikely(is_global_init(current)))
> +		return -EPERM;

Sorry for chiming in late.
This seems a bit too restrictive, there are a lot of legitimate signals that can
be sent to the init process (like the ones listed in man 1 systemd). Maybe you
should only prevent sig == SIGKILL and not others?