linux-kernel - Re: linux-next: manual merge of the net-next tree with the net tree

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Date:   Fri, 19 Mar 2021 16:38:26 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Yonghong Song <yhs@...com>
Cc:     Piotr Krysiuk <piotras@...il.com>,
        David Miller <davem@...emloft.net>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: linux-next: manual merge of the net-next tree with the net tree

On 3/19/21 4:33 PM, Alexei Starovoitov wrote:
> On Fri, Mar 19, 2021 at 8:17 AM Yonghong Song <yhs@...com> wrote:
>> On 3/19/21 12:21 AM, Daniel Borkmann wrote:
>>> On 3/19/21 3:11 AM, Piotr Krysiuk wrote:
>>>> Hi Daniel,
>>>>
>>>> On Fri, Mar 19, 2021 at 12:16 AM Stephen Rothwell <sfr@...b.auug.org.au>
>>>> wrote:
>>>>
>>>>> diff --cc kernel/bpf/verifier.c
>>>>> index 44e4ec1640f1,f9096b049cd6..000000000000
>>>>> --- a/kernel/bpf/verifier.c
>>>>> +++ b/kernel/bpf/verifier.c
>>>>> @@@ -5876,10 -6056,22 +6060,23 @@@ static int
>>>>> retrieve_ptr_limit(const str
>>>>>                   if (mask_to_left)
>>>>>                           *ptr_limit = MAX_BPF_STACK + off;
>>>>>                   else
>>>>>    -                      *ptr_limit = -off;
>>>>>    -              return 0;
>>>>>    +                      *ptr_limit = -off - 1;
>>>>>    +              return *ptr_limit >= max ? -ERANGE : 0;
>>>>> +       case PTR_TO_MAP_KEY:
>>>>> +               /* Currently, this code is not exercised as the only use
>>>>> +                * is bpf_for_each_map_elem() helper which requires
>>>>> +                * bpf_capble. The code has been tested manually for
>>>>> +                * future use.
>>>>> +                */
>>>>> +               if (mask_to_left) {
>>>>> +                       *ptr_limit = ptr_reg->umax_value + ptr_reg->off;
>>>>> +               } else {
>>>>> +                       off = ptr_reg->smin_value + ptr_reg->off;
>>>>> +                       *ptr_limit = ptr_reg->map_ptr->key_size - off;
>>>>> +               }
>>>>> +               return 0;
>>>>
>>>> PTR_TO_MAP_VALUE logic above looks like copy-paste of old
>>>> PTR_TO_MAP_VALUE
>>>> code from before "bpf: Fix off-by-one for area size in creating mask to
>>>> left" and is apparently affected by the same off-by-one, except this time
>>>> on "key_size" area and not "value_size".
>>>>
>>>> This needs to be fixed in the same way as we did with PTR_TO_MAP_VALUE.
>>>> What is the best way to proceed?
>>>
>>> Hm, not sure why PTR_TO_MAP_KEY was added by 69c087ba6225 in the first
>>> place, I
>>> presume noone expects this to be used from unprivileged as the comment
>>> says.
>>> Resolution should be to remove the PTR_TO_MAP_KEY case entirely from
>>> that switch
>>> until we have an actual user.
>>
>> Alexei suggested so that we don't forget it in the future if
>> bpf_capable() requirement is removed.
>>      https://lore.kernel.org/bpf/c837ae55-2487-2f39-47f6-a18781dc6fcc@fb.com/
>>
>> I am okay with either way, fix it or remove it.
> 
> I prefer to fix it.

If the bpf_capable() is removed, the verifier would bail out on PTR_TO_MAP_KEY
if not covered in the switch given the recent fixes we did. I can fix it up after
merge if we think bpf_for_each_map_elem() will be used by unpriv in future..