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Subject: Re: kernel panic: Attempted to kill init!
From:   Hao Sun <sunhao.th@gmail.com>
In-Reply-To: <CAADnVQ+EMzAmeKvivFKvqCNj6H9X-1rBtBBnjRzDiNvkocsc6A@mail.gmail.com>
Date:   Tue, 3 Jan 2023 20:46:36 +0800
Cc:     Yonghong Song <yhs@meta.com>, bpf <bpf@vger.kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        John Fastabend <john.fastabend@gmail.com>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        David Miller <davem@davemloft.net>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Message-Id: <2EC740FA-A10B-4208-8080-9A268BD8E675@gmail.com>
References: <20221222043507.33037-1-sunhao.th@gmail.com>
 <ef46c1f6-0939-c2ed-3efb-c3c5f28d1931@meta.com>
 <73E3CA9E-F67E-429F-A2DB-C77649E32D7A@gmail.com>
 <CAADnVQ+EMzAmeKvivFKvqCNj6H9X-1rBtBBnjRzDiNvkocsc6A@mail.gmail.com>
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Precedence: bulk


> On 31 Dec 2022, at 12:55 AM, Alexei Starovoitov =
<alexei.starovoitov@gmail.com> wrote:
>=20
> On Fri, Dec 30, 2022 at 1:54 AM Hao Sun <sunhao.th@gmail.com> wrote:
>>=20
>>=20
>>=20
>>> On 28 Dec 2022, at 2:35 PM, Yonghong Song <yhs@meta.com> wrote:
>>>=20
>>>=20
>>>=20
>>> On 12/21/22 8:35 PM, Hao Sun wrote:
>>>> Hi,
>>>> This crash can be triggered by executing the C reproducer for
>>>> multiple times, which just keep loading the following prog as
>>>> raw tracepoint into kmem_cache_free().
>>>> The prog send SIGSEGV to current via bpf_send_signal_thread(),
>>>> after load this, whoever tries to free mem would trigger this,
>>>> kernel crashed when this happens to init.
>>>> Seems we should filter init out in bpf_send_signal_common() by
>>>> is_global_init(current), or maybe we should check this in the
>>>> verifier?
>>>=20
>>> The helper is just to send a particular signal to *current*
>>> thread. In typical use case, it is never a good idea to send
>>> the signal to a *random* thread. In certain cases, maybe user
>>> indeed wants to send the signal to init thread to observe
>>> something. Note that such destructive side effect already
>>> exists in the bpf land. For example, for a xdp program,
>>> it could drop all packets to make machine not responsive
>>> to ssh etc. Therefore, I recommend to keep the existing
>>> bpf_send_signal_common() helper behavior.
>>=20
>> Sound the two are different cases. Not responsive in XDP seems like
>> an intended behaviour, panic caused by killing init is buggy. If the
>> last thread of global init was killed, kernel panic immediately.
>=20
> I don't get it. How was it possible that this prog was
> executed with current =3D=3D pid 1 ?

The prog is raw trace point and is attached to =E2=80=98kmem_cache_free=E2=
=80=99 event.
When init triggered the event, the prog would be executed with pid 1.
But, the reason of this crash is not very clear to me, because it=E2=80=99=
s
really hard to debug with original C reproducer.

The following is the corresponding Syz prog:

# {Threaded:true Repeat:true RepeatTimes:0 Procs:1 Slowdown:1 =
Sandbox:none SandboxArg:0 Leak:false NetInjection:true NetDevices:true =
NetReset:true Cgroups:true BinfmtMisc:true CloseFDs:true KCSAN:false =
DevlinkPCI:false NicVF:false USB:false VhciInjection:false Wifi:false =
IEEE802154:true Sysctl:true UseTmpDir:true HandleSegv:true Repro:false =
Trace:false LegacyOptions:{Collide:false Fault:false FaultCall:0 =
FaultNth:0}}
r0 =3D bpf$BPF_PROG_RAW_TRACEPOINT_LOAD(0x5, &(0x7f0000000000)=3D{0x11, =
0xe, =
&(0x7f0000000400)=3DANY=3D[@ANYBLOB=3D"18000000000000000000000000000000180=
60000000000000000000000000000180700000000000000000000000000001808000000000=
0000000000000000000180900000000000000000000000000002d00020000000000b701000=
00b000000850000007500000095"], &(0x7f00000000c0)}, 0x80)
bpf$BPF_RAW_TRACEPOINT_OPEN(0x11, =
&(0x7f0000000100)=3D{&(0x7f0000000080)=3D'kmem_cache_free\x00', r0}, =
0x10)