Re: [PATCH] kprobes: Update ftrace_ops when clearing ftrace-based aggrprobe's post_handler

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Fri, 11 Nov 2022 18:10:06 +0800
Li Huafei <lihuafei1@huawei.com> wrote:

> In __unregister_kprobe_top(), if the currently unregistered probe has
> post_handler but other child probes of the aggrprobe do not have
> post_handler, the post_handler of the aggrprobe is cleared. If this is
> a ftrace-based probe, there is a problem. In later calls to
> disarm_kprobe(), we will use kprobe_ftrace_ops because post_handler is
> NULL. But we're armed with kprobe_ipmodify_ops. This triggers a WARN in
> __disarm_kprobe_ftrace() and may even cause use-after-free:
> 
>   Failed to disarm kprobe-ftrace at kernel_clone+0x0/0x3c0 (error -2)
>   WARNING: CPU: 5 PID: 137 at kernel/kprobes.c:1135 __disarm_kprobe_ftrace.isra.21+0xcf/0xe0
>   Modules linked in: testKprobe_007(-)
>   CPU: 5 PID: 137 Comm: rmmod Not tainted 6.1.0-rc4-dirty #18
>   [...]
>   Call Trace:
>    <TASK>
>    __disable_kprobe+0xcd/0xe0
>    __unregister_kprobe_top+0x12/0x150
>    ? mutex_lock+0xe/0x30
>    unregister_kprobes.part.23+0x31/0xa0
>    unregister_kprobe+0x32/0x40
>    __x64_sys_delete_module+0x15e/0x260
>    ? do_user_addr_fault+0x2cd/0x6b0
>    do_syscall_64+0x3a/0x90
>    entry_SYSCALL_64_after_hwframe+0x63/0xcd
>    [...]

Ah, good catch! :D

> 
> For ftrace kprobe, update post_handler at the same time update
> ftrace_ops, moving it from kprobe_ipmodify_ops to kprobe_ftrace_ops.

Hmm, but I would not like this because there can be a time
window when it can miss an event. What about just skipping
clearing ap->post_handler in kprobe-on-ftrace case?

> 
> Fixes: 0bc11ed5ab60 ("kprobes: Allow kprobes coexist with livepatch")
> Reported-by: Zhao Gongyi <zhaogongyi@huawei.com>
> Signed-off-by: Li Huafei <lihuafei1@huawei.com>
> ---
>  kernel/kprobes.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index cd9f5a66a690..f8bec48a9cf9 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -1766,7 +1766,17 @@ static int __unregister_kprobe_top(struct kprobe *p)
>  				if ((list_p != p) && (list_p->post_handler))
>  					goto noclean;
>  			}
> -			ap->post_handler = NULL;
> +			/*
> +			 * For ftrace kprobe, we need to update ftrace_ops
> +			 * at the same time as we update post_handler, moving
> +			 * it from kprobe_ipmodify_ops to kprobe_ftrace_ops.
> +			 */
> +			if (unlikely(kprobe_ftrace(ap))) {
> +				disarm_kprobe(ap, false);
> +				ap->post_handler = NULL;
> +				arm_kprobe(ap);
> +			} else

So here, just add; (also, don't use unlikely/likely for this case, this
depends on where the user probes, not a systematically rare case.)

			if (!kprobe_ftrace(ap))

> +				ap->post_handler = NULL;

Thank you!

>  		}
>  noclean:
>  		/*
> -- 
> 2.17.1
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>