Re: [PATCH bpf-next 1/2] bpf: permit map_ptr arithmetic with opcode add and offset 0

[Daniel Borkmann <daniel@iogearbox.net>]

On 9/5/20 2:10 AM, Alexei Starovoitov wrote:
> On Fri, Sep 4, 2020 at 5:08 PM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
>> On Fri, Sep 4, 2020 at 4:20 PM Yonghong Song <yhs@fb.com> wrote:
[...]
>>> for scalar constant, reg->var_off.mask should be 0. so we will have
>>> reg->smin_value = reg->smax_value = (s64)reg->var_off.value.
>>>
>>> The smin_val is also used below, e.g., BPF_ADD, for a known value.
>>> That is why I am using smin_val here.
>>>
>>> Will add a comment and submit v2.
>>
>> it would be way-way more obvious (and reliable in the long run,
>> probably) if you just used (known && reg->var_off.value == 0). or just
>> tnum_equals_const(reg->var_off, 0)?
> 
> Pls dont. smin_val == 0 is a standard way to do this.
> Just check all other places in this function and everywhere else.

Also, we taint the reg earlier in that function if its known and min != max:

         if ((known && (smin_val != smax_val || umin_val != umax_val)) ||
             smin_val > smax_val || umin_val > umax_val) {
                 /* Taint dst register if offset had invalid bounds derived from
                  * e.g. dead branches.
                  */
                 __mark_reg_unknown(env, dst_reg);
                 return 0;
         }