Re: [Phishing Risk] [PATCH] kprobe: fix kretprobe stack backtrace

[Muchun Song <songmuchun@bytedance.com>]

On Fri, Jun 25, 2021 at 4:49 PM Qiang Wang
<wangqiang.wq.frank@bytedance.com> wrote:
>
> We found that we couldn't get the correct kernel stack from
> kretprobe. For example:
>
> bpftrace -e 'kr:submit_bio {print(kstack)}'
> Attaching 1 probe...
>
>         kretprobe_trampoline+0
>
>         kretprobe_trampoline+0
>
> The problem is caused by the wrong instruction register which
> points to the address of kretprobe_trampoline in regs.
> So we set the real return address in instruction register.
> Finally, we tested and successfully fixed it.
>
> bpftrace -e 'kr:submit_bio {print(kstack)}'
> Attaching 1 probe...
>
>         ext4_mpage_readpages+475
>         read_pages+139
>         page_cache_ra_unbounded+417
>         filemap_get_pages+245
>         filemap_read+169
>         __kernel_read+327
>         bprm_execve+648
>         do_execveat_common.isra.39+409
>         __x64_sys_execve+50
>         do_syscall_64+54
>         entry_SYSCALL_64_after_hwframe+68
>
> Reported-by: Chengming Zhou <zhouchengming@bytedance.com>
> Signed-off-by: Qiang Wang <wangqiang.wq.frank@bytedance.com>

Seems like a bug. Maybe we should add a "Fixes" tag here.

> ---
>  kernel/kprobes.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 745f08fdd..1130381ca 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -1899,6 +1899,9 @@ unsigned long __kretprobe_trampoline_handler(struct pt_regs *regs,
>         current->kretprobe_instances.first = node->next;
>         node->next = NULL;
>
> +       /* Kretprobe handler expects address is the real return address */
> +       instruction_pointer_set(regs, (unsigned long)correct_ret_addr);
> +
>         /* Run them..  */
>         while (first) {
>                 ri = container_of(first, struct kretprobe_instance, llist);
> --
> 2.20.1
>