bpf: Skip task with pid=1 in send_signal_common()

In the Linux kernel, the following vulnerability has been resolved:

bpf: Skip task with pid=1 in send_signal_common()

The following kernel panic can be triggered when a task with pid=1 attaches

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/[email protected]