Re: kernel panic: Attempted to kill init!

[Yonghong Song <yhs@meta.com>]

On 12/21/22 8:35 PM, Hao Sun wrote:
> Hi,
> 
> This crash can be triggered by executing the C reproducer for
> multiple times, which just keep loading the following prog as
> raw tracepoint into kmem_cache_free().
> 
> The prog send SIGSEGV to current via bpf_send_signal_thread(),
> after load this, whoever tries to free mem would trigger this,
> kernel crashed when this happens to init.
> 
> Seems we should filter init out in bpf_send_signal_common() by
> is_global_init(current), or maybe we should check this in the
> verifier?

The helper is just to send a particular signal to *current*
thread. In typical use case, it is never a good idea to send
the signal to a *random* thread. In certain cases, maybe user
indeed wants to send the signal to init thread to observe
something. Note that such destructive side effect already
exists in the bpf land. For example, for a xdp program,
it could drop all packets to make machine not responsive
to ssh etc. Therefore, I recommend to keep the existing
bpf_send_signal_common() helper behavior.

> 
> This can be reproduced on:
> 
> HEAD commit: 59fe41b5255f selftests/bpf: Add verifier test exercising jit PROBE_MEM logic
> git tree: bpf-next
> console output: https://pastebin.com/raw/FMgyvEnH
> kernel config : https://pastebin.com/raw/XeF6jU43
> C reproducer  : https://pastebin.com/raw/Tag5N893
> 
> func#0 @0
> 0: R1=ctx(off=0,imm=0) R10=fp0
> 0: (18) r0 = 0x0                      ; R0_w=0
> 2: (18) r6 = 0x0                      ; R6_w=0
> 4: (18) r7 = 0x0                      ; R7_w=0
> 6: (18) r8 = 0x0                      ; R8_w=0
> 8: (18) r9 = 0x0                      ; R9_w=0
> 10: (2d) if r0 > r0 goto pc+2
> last_idx 10 first_idx 0
> regs=1 stack=0 before 8: (18) r9 = 0x0
> regs=1 stack=0 before 6: (18) r8 = 0x0
> regs=1 stack=0 before 4: (18) r7 = 0x0
> regs=1 stack=0 before 2: (18) r6 = 0x0
> regs=1 stack=0 before 0: (18) r0 = 0x0
> last_idx 10 first_idx 0
> regs=1 stack=0 before 8: (18) r9 = 0x0
> regs=1 stack=0 before 6: (18) r8 = 0x0
> regs=1 stack=0 before 4: (18) r7 = 0x0
> regs=1 stack=0 before 2: (18) r6 = 0x0
> regs=1 stack=0 before 0: (18) r0 = 0x0
> 11: R0_w=0
> 11: (b7) r1 = 11                      ; R1_w=11
> 12: (85) call bpf_send_signal_thread#117      ; R0=scalar()
> 13: (95) exit
> processed 9 insns (limit 1000000) max_states_per_insn 0 total_states 1 peak_states 1 mark_read 1
> 
> Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
> CPU: 3 PID: 1 Comm: systemd Not tainted 6.1.0-09652-g59fe41b5255f #148
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
> 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
> RIP: 0033:0x55e738964df0
> Code: 00 31 f6 89 ef 4c 8d 05 be 1b 0d 00 48 8d 15 b0 85 0c 00 31 c0 e8 f0 c3 ff ff e9 1c ff ff ff 66 66 2e 0f 1f 84 00 00 00 00 00 <41> 57 41 56 41 55 41 54 41 89 fc 55 53 48 81 ec 48 01 00 00 64 48
> RSP: 002b:00007ffeb8e87bb8 EFLAGS: 00000246 ORIG_RAX: 00000000000000f7
> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 00007f29dc8a6bc1
> RDX: 00007ffeb8e87bc0 RSI: 00007ffeb8e87cf0 RDI: 000000000000000b
> RBP: 00007ffeb90b73c0 R08: 0000000000000000 R09: 0000000000000002
> R10: 0000000000000004 R11: 0000000000000246 R12: 00007f29dc3f76c8
> R13: 000000000000294d R14: 0000000000000000 R15: 00007ffeb9686870
>   </TASK>
> Kernel Offset: disabled
> Rebooting in 86400 seconds..