Re: [PATCH] bpf: skip task with pid=1 in send_signal_common()

[Daniel Borkmann]

On 1/6/23 8:44 PM, Stanislav Fomichev wrote:
> On Fri, Jan 6, 2023 at 12:48 AM Hao Sun <sunhao.th@xxxxxxxxx> 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@xxxxxxxxx
> >
> > Signed-off-by: Hao Sun <sunhao.th@xxxxxxxxx>
>
> Acked-by: Stanislav Fomichev <sdf@xxxxxxxxxx>

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