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Subject: Re: [PATCH] bpf: skip task with pid=1 in send_signal_common()
To:     Stanislav Fomichev <sdf@google.com>, Hao Sun <sunhao.th@gmail.com>
Cc:     bpf@vger.kernel.org, ast@kernel.org, john.fastabend@gmail.com,
        andrii@kernel.org, martin.lau@linux.dev, song@kernel.org,
        yhs@fb.com, kpsingh@kernel.org, haoluo@google.com,
        jolsa@kernel.org, davem@davemloft.net, linux-kernel@vger.kernel.org
References: <20230106084838.12690-1-sunhao.th@gmail.com>
 <CAKH8qBtAubqGg42+QgNv5nTHeHke=OWskfR1bxX0TG4yGb-FAg@mail.gmail.com>
From:   Daniel Borkmann <daniel@iogearbox.net>
Message-ID: <f262fb2a-a173-a979-f34f-d8a7eba49441@iogearbox.net>
Date:   Fri, 6 Jan 2023 22:54:32 +0100
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On 1/6/23 8:44 PM, Stanislav Fomichev wrote:
> On Fri, Jan 6, 2023 at 12:48 AM Hao Sun <sunhao.th@gmail.com> 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>
> 
> Acked-by: Stanislav Fomichev <sdf@google.com>

Looks good, agree & applied. These three tests could actually be refactored and also
reused in bpf_probe_write_user() given also the latter should have no business to
mess with pid 1.

>>   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;
>>          if (unlikely(!nmi_uaccess_okay()))
>>                  return -EPERM;
>>
>>
>> base-commit: 4aea86b4033f92f01547e6d4388d4451ae9b0980
>> --
>> 2.39.0
>>