On Fri, Mar 19, 2021 at 8:17 AM Yonghong Song <yhs@xxxxxx> 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@xxxxxxxxxxxxxxxx>
> >> 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@xxxxxx/
>
> I am okay with either way, fix it or remove it.
I prefer to fix it.
