Re: [PATCH bpf-next v6 2/5] bpf: Expose bpf_get_socket_cookie to tracing programs

[Daniel Borkmann <daniel@iogearbox.net>]

On 1/30/21 12:45 PM, Florent Revest wrote:
> On Fri, Jan 29, 2021 at 1:49 PM Daniel Borkmann <daniel@iogearbox.net> wrote:
>> On 1/29/21 11:57 AM, Daniel Borkmann wrote:
>>> On 1/27/21 10:01 PM, Andrii Nakryiko wrote:
>>>> On Tue, Jan 26, 2021 at 10:36 AM Florent Revest <revest@chromium.org> wrote:
>>>>>
>>>>> This needs a new helper that:
>>>>> - can work in a sleepable context (using sock_gen_cookie)
>>>>> - takes a struct sock pointer and checks that it's not NULL
>>>>>
>>>>> Signed-off-by: Florent Revest <revest@chromium.org>
>>>>> Acked-by: KP Singh <kpsingh@kernel.org>
>>>>> ---
>>>>>    include/linux/bpf.h            |  1 +
>>>>>    include/uapi/linux/bpf.h       |  8 ++++++++
>>>>>    kernel/trace/bpf_trace.c       |  2 ++
>>>>>    net/core/filter.c              | 12 ++++++++++++
>>>>>    tools/include/uapi/linux/bpf.h |  8 ++++++++
>>>>>    5 files changed, 31 insertions(+)
>>>>>
>>>>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>>>>> index 1aac2af12fed..26219465e1f7 100644
>>>>> --- a/include/linux/bpf.h
>>>>> +++ b/include/linux/bpf.h
>>>>> @@ -1874,6 +1874,7 @@ extern const struct bpf_func_proto bpf_per_cpu_ptr_proto;
>>>>>    extern const struct bpf_func_proto bpf_this_cpu_ptr_proto;
>>>>>    extern const struct bpf_func_proto bpf_ktime_get_coarse_ns_proto;
>>>>>    extern const struct bpf_func_proto bpf_sock_from_file_proto;
>>>>> +extern const struct bpf_func_proto bpf_get_socket_ptr_cookie_proto;
>>>>>
>>>>>    const struct bpf_func_proto *bpf_tracing_func_proto(
>>>>>           enum bpf_func_id func_id, const struct bpf_prog *prog);
>>>>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>>>>> index 0b735c2729b2..5855c398d685 100644
>>>>> --- a/include/uapi/linux/bpf.h
>>>>> +++ b/include/uapi/linux/bpf.h
>>>>> @@ -1673,6 +1673,14 @@ union bpf_attr {
>>>>>     *     Return
>>>>>     *             A 8-byte long unique number.
>>>>>     *
>>>>> + * u64 bpf_get_socket_cookie(void *sk)
>>>>
>>>> should the type be `struct sock *` then?
>>>
>>> Checking libbpf's generated bpf_helper_defs.h it generates:
>>>
>>> /*
>>>    * bpf_get_socket_cookie
>>>    *
>>>    *      If the **struct sk_buff** pointed by *skb* has a known socket,
>>>    *      retrieve the cookie (generated by the kernel) of this socket.
>>>    *      If no cookie has been set yet, generate a new cookie. Once
>>>    *      generated, the socket cookie remains stable for the life of the
>>>    *      socket. This helper can be useful for monitoring per socket
>>>    *      networking traffic statistics as it provides a global socket
>>>    *      identifier that can be assumed unique.
>>>    *
>>>    * Returns
>>>    *      A 8-byte long non-decreasing number on success, or 0 if the
>>>    *      socket field is missing inside *skb*.
>>>    */
>>> static __u64 (*bpf_get_socket_cookie)(void *ctx) = (void *) 46;
>>>
>>> So in terms of helper comment it's picking up the description from the
>>> `u64 bpf_get_socket_cookie(struct sk_buff *skb)` signature. With that
>>> in mind it would likely make sense to add the actual `struct sock *` type
>>> to the comment to make it more clear in here.
>>
>> One thought that still came to mind when looking over the series again, do
>> we need to blacklist certain functions from bpf_get_socket_cookie() under
>> tracing e.g. when attaching to, say fexit? For example, if sk_prot_free()
>> would be temporary uninlined/exported for testing and bpf_get_socket_cookie()
>> was invoked from a prog upon fexit where sock was already passed back to
>> allocator, I presume there's risk of mem corruption, no?
> 
> Mh, this is interesting. I can try to add a deny list in v7 but I'm
> not sure whether I'll be able to catch them all. I'm assuming that
> __sk_destruct, sk_destruct, __sk_free, sk_free would be other
> problematic functions but potentially there would be more.

I was just looking at bpf_skb_output() from a7658e1a4164 ("bpf: Check types of
arguments passed into helpers") which afaiu has similar issue, back at the time
this was introduced there was no fentry/fexit but rather raw tp progs, so you
could still safely dump skb this way including for kfree_skb() tp. Presumably if
you bpf_skb_output() at 'fexit/kfree_skb' you might be able to similarly crash
your kernel which I don't think is intentional (also given we go above and beyond
in other areas to avoid crashing or destabilizing e.g. [0] to mention one). Maybe
these should really only be for BPF_TRACE_RAW_TP (or BPF_PROG_TYPE_LSM) where it
can be audited that it's safe to use like a7658e1a4164's original intention ...
or have some sort of function annotation like __acquires/__releases but for tracing
e.g. __frees(skb) where use would then be blocked (not sure iff feasible).

   [0] https://lore.kernel.org/bpf/20210126001219.845816-1-yhs@fb.com/