[PATCH AUTOSEL 5.4 2/6] bpf: Skip task with pid=1 in send_signal_common()

[Sasha Levin]

From: Hao Sun <sunhao.th@xxxxxxxxx>

[ Upstream commit a3d81bc1eaef48e34dd0b9b48eefed9e02a06451 ]

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/20221222043507.33037-1-sunhao.th@xxxxxxxxx

Signed-off-by: Hao Sun <sunhao.th@xxxxxxxxx>
Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Acked-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
Link: https://lore.kernel.org/bpf/20230106084838.12690-1-sunhao.th@xxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 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 4d9f81802911..1e1345cd21b4 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -649,6 +649,9 @@ BPF_CALL_1(bpf_send_signal, u32, sig)
 		return -EPERM;
 	if (unlikely(!nmi_uaccess_okay()))
 		return -EPERM;
+	/* Task should not be pid=1 to avoid kernel panic. */
+	if (unlikely(is_global_init(current)))
+		return -EPERM;
 
 	if (irqs_disabled()) {
 		/* Do an early check on signal validity. Otherwise,
-- 
2.39.0