Re: [PATCH] kprobes: fix NULL pointer dereference at kprobe_ftrace_handler

[Muchun Song <songmuchun@bytedance.com>]

Ping guys. Any comments or suggestions?

On Tue, Jul 28, 2020 at 2:45 PM Muchun Song <songmuchun@bytedance.com> wrote:
>
> We found a case of kernel panic on our server. The stack trace is as
> follows(omit some irrelevant information):
>
>   BUG: kernel NULL pointer dereference, address: 0000000000000080
>   RIP: 0010:kprobe_ftrace_handler+0x5e/0xe0
>   RSP: 0018:ffffb512c6550998 EFLAGS: 00010282
>   RAX: 0000000000000000 RBX: ffff8e9d16eea018 RCX: 0000000000000000
>   RDX: ffffffffbe1179c0 RSI: ffffffffc0535564 RDI: ffffffffc0534ec0
>   RBP: ffffffffc0534ec1 R08: ffff8e9d1bbb0f00 R09: 0000000000000004
>   R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
>   R13: ffff8e9d1f797060 R14: 000000000000bacc R15: ffff8e9ce13eca00
>   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>   CR2: 0000000000000080 CR3: 00000008453d0005 CR4: 00000000003606e0
>   DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>   DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>   Call Trace:
>    <IRQ>
>    ftrace_ops_assist_func+0x56/0xe0
>    ftrace_call+0x5/0x34
>    tcpa_statistic_send+0x5/0x130 [ttcp_engine]
>
> The tcpa_statistic_send is the function being kprobed. After analysis,
> the root cause is that the fourth parameter regs of kprobe_ftrace_handler
> is NULL. Why regs is NULL? We use the crash tool to analyze the kdump.
>
>   crash> dis tcpa_statistic_send -r
>          <tcpa_statistic_send>: callq 0xffffffffbd8018c0 <ftrace_caller>
>
> The tcpa_statistic_send calls ftrace_caller instead of ftrace_regs_caller.
> So it is reasonable that the fourth parameter regs of kprobe_ftrace_handler
> is NULL. In theory, we should call the ftrace_regs_caller instead of the
> ftrace_caller. After in-depth analysis, we found a reproducible path.
>
>   Writing a simple kernel module which starts a periodic timer. The
>   timer's handler is named 'kprobe_test_timer_handler'. The module
>   name is kprobe_test.ko.
>
>   1) insmod kprobe_test.ko
>   2) bpftrace -e 'kretprobe:kprobe_test_timer_handler {}'
>   3) echo 0 > /proc/sys/kernel/ftrace_enabled
>   4) rmmod kprobe_test
>   5) stop step 2) kprobe
>   6) insmod kprobe_test.ko
>   7) bpftrace -e 'kretprobe:kprobe_test_timer_handler {}'
>
> We mark the kprobe as GONE but not disarm the kprobe in the step 4).
> The step 5) also do not disarm the kprobe when unregister kprobe. So
> we do not remove the ip from the filter. In this case, when the module
> loads again in the step 6), we will replace the code to ftrace_caller
> via the ftrace_module_enable(). When we register kprobe again, we will
> not replace ftrace_caller to ftrace_regs_caller because the ftrace is
> disabled in the step 3). So the step 7) will trigger kernel panic. Fix
> this problem by disarming the kprobe when the module is going away.
>
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> Co-developed-by: Chengming Zhou <zhouchengming@bytedance.com>
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> ---
>  kernel/kprobes.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 146c648eb943..503add629599 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -2148,6 +2148,13 @@ static void kill_kprobe(struct kprobe *p)
>          * the original probed function (which will be freed soon) any more.
>          */
>         arch_remove_kprobe(p);
> +
> +       /*
> +        * The module is going away. We should disarm the kprobe which
> +        * is using ftrace.
> +        */
> +       if (kprobe_ftrace(p))
> +               disarm_kprobe_ftrace(p);
>  }
>
>  /* Disable one kprobe */
> --
> 2.11.0
>


-- 
Yours,
Muchun