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

[Yonghong Song]

On 5/10/23 10:27 AM, Jiri Olsa wrote:
> On Wed, May 10, 2023 at 07:13:58AM -0700, Yonghong Song wrote:
> > 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@xxxxxxxxxxx>
> > > ---
> > >    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.
>
> also fprobe uses ftrace drectly without paths in kprobe, so I wonder
> some of the kprobe blacklisted functions are actually safe

Could you give a pointer about 'some of the kprobe blacklisted
functions are actually safe'?

> jirka
>
> > > +			/* 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;