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

[Jiri Olsa]

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

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;