[PATCH] ptrace: disable single step in __ptrace_unlink for protecting init task

[chen zhang]

I got below panic when doing fuzz test:

Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000005
CPU: 2 PID: 1 Comm: systemd Kdump: loaded Not tainted 6.1.0-rc1 #1
Hardware name: LENOVO 20L5A07XCD/20L5A07XCD, BIOS N24ET56W (1.31 ) 02/19/2020
Call Trace:
[  157.210356]  dump_stack_lvl+0x49/0x63
[  157.210364]  dump_stack+0x10/0x16
[  157.210368]  panic+0x10c/0x299
[  157.210375]  do_exit.cold+0x15/0x15
[  157.210381]  do_group_exit+0x35/0x90
[  157.210386]  get_signal+0x910/0x960
[  157.210390]  ? signal_wake_up_state+0x2e/0x40
[  157.210396]  ? complete_signal+0xd0/0x2c0
[  157.210402]  arch_do_signal_or_restart+0x37/0x7c0
[  157.210408]  ? send_signal_locked+0xf5/0x140
[  157.210416]  exit_to_user_mode_prepare+0x133/0x180
[  157.210423]  irqentry_exit_to_user_mode+0x9/0x20
[  157.210428]  noist_exc_debug+0xea/0x150
[  157.210433]  asm_exc_debug+0x34/0x40
[  157.210438] RIP: 0033:0x7fcf2a8e51c9
[  157.210442] Code: ff ff 73 01 c3 48 8b 0d c5 7c 0c 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa b8 ba 00 00 00 <0f> 05 c3 0f 1f 40 00 f3 0f 1e fa b8 ea 00 00 00 0f 05 48 3d 01 f0
[  157.210446] RSP: 002b:00007ffd7dc44678 EFLAGS: 00000302
[  157.210451] RAX: 00000000000000ba RBX: 000055f7c0363170 RCX: 000055f7c04d2820
[  157.210454] RDX: 00000000ffffffff RSI: ffffffffffffffff RDI: 000055f7c0363170
[  157.210457] RBP: 0000000000000001 R08: 0000000000000000 R09: 0000000000001dd0
[  157.210460] R10: 00007ffd7ddc9090 R11: 000000000000d7da R12: 0000000000000001
[  157.210463] R13: ffffffffffffffff R14: 000055f7bf3557c1 R15: 0000000000000000

If a task attaches init task and is single stepping it, when this task
exits, ptrace value will be cleaned. It causes SIGNAL_UNKILLABLE flag
cleaned, and init task will lose the protection. Init task maybe be killed
by SIGTRAP signal because of stepping enabled. So syscall tracing and
stepping should be turned off for protecting init task before ptrace value
is cleaned.

Signed-off-by: chen zhang <chenzhang@xxxxxxxxxx>
---
 kernel/ptrace.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 54482193e1ed..e7c41154b31e 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -130,6 +130,9 @@ void __ptrace_unlink(struct task_struct *child)
 	put_cred(old_cred);
 
 	spin_lock(&child->sighand->siglock);
+	if (unlikely(child->signal->flags & SIGNAL_UNKILLABLE) &&
+	    unlikely(task_thread_info(child)->flags & _TIF_SINGLESTEP))
+		user_disable_single_step(child);
 	child->ptrace = 0;
 	/*
 	 * Clear all pending traps and TRAPPING.  TRAPPING should be
-- 
2.25.1