Messages in this thread | | | Date | Wed, 10 May 2023 13:20:30 -0700 | Subject | Re: [PATCH] bpf: reject blacklisted symbols in kprobe_multi to avoid recursive trap | From | Yonghong Song <> |
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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@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. > > 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;
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