Re: [kernel-hardening] [PATCH v4 02/10] asm/nospec, array_ptr: sanitize speculative array de-references

[Linus Torvalds]

On Fri, Jan 19, 2018 at 2:20 AM, Jann Horn <jannh@xxxxxxxxxx> wrote:
>> +                                                                       \
>> +       __u._ptr = _arr + (_i & _mask);                                 \
>> +       __u._bit &= _mask;                                              \
>
> AFAICS, if `idx` is out of bounds, you first zero out the index
> (`_i & _mask`) and then immediately afterwards zero out
> the whole pointer (`_u._bit &= _mask`).
> Is there a reason for the `_i & _mask`, and if so, can you
> add a comment explaining that?

I think that's just leftovers from my original (untested) thing that
also did the access itself. So that __u._bit masking wasn't masking
the pointer, it was masking the value that was *read* from the
pointer, so that you could know that an invalid access returned
0/NULL, not just the first value in the array.

                  Linus