Re: [PATCH] fprobe: Release rethook after the ftrace_ops is unregistered

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

Hi Jiri,

On Thu, 15 Jun 2023 13:52:36 +0200
Jiri Olsa <jolsa@kernel.org> wrote:

> While running bpf selftests it's possible to get following fault:
> 
>   general protection fault, probably for non-canonical address \
>   0x6b6b6b6b6b6b6b6b: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC NOPTI
>   ...
>   Call Trace:
>    <TASK>
>    fprobe_handler+0xc1/0x270
>    ? __pfx_bpf_testmod_init+0x10/0x10
>    ? __pfx_bpf_testmod_init+0x10/0x10
>    ? bpf_fentry_test1+0x5/0x10
>    ? bpf_fentry_test1+0x5/0x10
>    ? bpf_testmod_init+0x22/0x80
>    ? do_one_initcall+0x63/0x2e0
>    ? rcu_is_watching+0xd/0x40
>    ? kmalloc_trace+0xaf/0xc0
>    ? do_init_module+0x60/0x250
>    ? __do_sys_finit_module+0xac/0x120
>    ? do_syscall_64+0x37/0x90
>    ? entry_SYSCALL_64_after_hwframe+0x72/0xdc
>    </TASK>
> 
> In unregister_fprobe function we can't release fp->rethook while it's
> possible there are some of its users still running on another cpu.

Ah, OK. rethook_free() invoked call_rcu(rethook_free_rcu) to free the
rethook, and it is possible rethook_free_rcu() is called before disabling
all fprobe, then `rethook_try_get(fp->rethook)` will access fp->rethook
which has been freed.

> 
> Moving rethook_free call after fp->ops is unregistered with
> unregister_ftrace_function call.
> 
> Fixes: 5b0ab78998e3 ("fprobe: Add exit_handler support")
> Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>

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

Thank you!


> ---
>  kernel/trace/fprobe.c | 12 +++---------
>  1 file changed, 3 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> index 18d36842faf5..0121e8c0d54e 100644
> --- a/kernel/trace/fprobe.c
> +++ b/kernel/trace/fprobe.c
> @@ -364,19 +364,13 @@ int unregister_fprobe(struct fprobe *fp)
>  		    fp->ops.saved_func != fprobe_kprobe_handler))
>  		return -EINVAL;
>  
> -	/*
> -	 * rethook_free() starts disabling the rethook, but the rethook handlers
> -	 * may be running on other processors at this point. To make sure that all
> -	 * current running handlers are finished, call unregister_ftrace_function()
> -	 * after this.
> -	 */
> -	if (fp->rethook)
> -		rethook_free(fp->rethook);
> -
>  	ret = unregister_ftrace_function(&fp->ops);
>  	if (ret < 0)
>  		return ret;
>  
> +	if (fp->rethook)
> +		rethook_free(fp->rethook);
> +
>  	ftrace_free_filter(&fp->ops);
>  
>  	return ret;
> -- 
> 2.40.1
> 


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