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

[Ze Gao]

Thank yonghong for your sage reviews.
Yes, this is an option I am also considering . I will try this out
later to see if works

But like you said it's not clear whether kprobe blacklist== fprobe blacklist.
And also there are cases I need to investigate on, like how to avoid recursions
when kprobes and fprobes are mixed.

Rejecting symbols  kprobe_blacklisted is kinda brute-force yet a straight way to
avoid kernel crash AFAIK.

Ze

On Thu, May 11, 2023 at 7:54 AM Yonghong Song <yhs@xxxxxxxx> wrote:
>
>
>
> On 5/10/23 1:20 PM, Yonghong Song wrote:
> >
> >
> > 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'?
>
> Thanks Jiri for answering my question. it is not clear whether
> kprobe blacklist == fprobe blacklist, probably not.
>
> You mentioned:
>    note that ftrace provides recursion detection mechanism,
>    but for kprobe only
> Maybe the right choice is to improve ftrace to provide recursion
> detection mechanism for fprobe as well?
>
> >
> >>
> >> 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;