linux-kernel - [PATCH] bpf: skip task with pid=1 in send_signal

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Message-Id: <20230106084838.12690-1-sunhao.th@gmail.com>
Date:   Fri,  6 Jan 2023 16:48:38 +0800
From:   Hao Sun <sunhao.th@...il.com>
To:     bpf@...r.kernel.org
Cc:     ast@...nel.org, daniel@...earbox.net, john.fastabend@...il.com,
        andrii@...nel.org, martin.lau@...ux.dev, song@...nel.org,
        yhs@...com, kpsingh@...nel.org, sdf@...gle.com, haoluo@...gle.com,
        jolsa@...nel.org, davem@...emloft.net,
        linux-kernel@...r.kernel.org, Hao Sun <sunhao.th@...il.com>
Subject: [PATCH] bpf: skip task with pid=1 in send_signal_common()

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@...il.com>
---
 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