Re: [PATCH] bpf: reject blacklisted symbols in kprobe_multi to avoid recursive trap

[Yonghong Song <yhs@meta.com>]

On 5/10/23 5:20 AM, Ze Gao wrote:
> BPF_LINK_TYPE_KPROBE_MULTI attaches kprobe programs through fprobe,
> however it does not takes those kprobe blacklisted into consideration,
> which likely introduce recursive traps and blows up stacks.
> 
> this patch adds simple check and remove those are in kprobe_blacklist
> from one fprobe during bpf_kprobe_multi_link_attach. And also
> check_kprobe_address_safe is open for more future checks.
> 
> note that ftrace provides recursion detection mechanism, but for kprobe
> only, we can directly reject those cases early without turning to ftrace.
> 
> Signed-off-by: Ze Gao <zegao@tencent.com>
> ---
>   kernel/trace/bpf_trace.c | 37 +++++++++++++++++++++++++++++++++++++
>   1 file changed, 37 insertions(+)
> 
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 9a050e36dc6c..44c68bc06bbd 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -2764,6 +2764,37 @@ static int get_modules_for_addrs(struct module ***mods, unsigned long *addrs, u3
>   	return arr.mods_cnt;
>   }
>   
> +static inline int check_kprobe_address_safe(unsigned long addr)
> +{
> +	if (within_kprobe_blacklist(addr))
> +		return -EINVAL;
> +	else
> +		return 0;
> +}
> +
> +static int check_bpf_kprobe_addrs_safe(unsigned long *addrs, int num)
> +{
> +	int i, cnt;
> +	char symname[KSYM_NAME_LEN];
> +
> +	for (i = 0; i < num; ++i) {
> +		if (check_kprobe_address_safe((unsigned long)addrs[i])) {
> +			lookup_symbol_name(addrs[i], symname);
> +			pr_warn("bpf_kprobe: %s at %lx is blacklisted\n", symname, addrs[i]);

So user request cannot be fulfilled and a warning is issued and some
of user requests are discarded and the rest is proceeded. Does not
sound a good idea.

Maybe we should do filtering in user space, e.g., in libbpf, check
/sys/kernel/debug/kprobes/blacklist and return error
earlier? bpftrace/libbpf-tools/bcc-tools all do filtering before
requesting kprobe in the kernel.

> +			/* mark blacklisted symbol for remove */
> +			addrs[i] = 0;
> +		}
> +	}
> +
> +	/* remove blacklisted symbol from addrs */
> +	for (i = 0, cnt = 0; i < num; ++i) {
> +		if (addrs[i])
> +			addrs[cnt++]  = addrs[i];
> +	}
> +
> +	return cnt;
> +}
> +
>   int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
>   {
>   	struct bpf_kprobe_multi_link *link = NULL;
> @@ -2859,6 +2890,12 @@ int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *pr
>   	else
>   		link->fp.entry_handler = kprobe_multi_link_handler;
>   
> +	cnt = check_bpf_kprobe_addrs_safe(addrs, cnt);
> +	if (!cnt) {
> +		err = -EINVAL;
> +		goto error;
> +	}
> +
>   	link->addrs = addrs;
>   	link->cookies = cookies;
>   	link->cnt = cnt;