On 12/04, Song Liu wrote:
> On Mon, Dec 3, 2018 at 8:01 PM Stanislav Fomichev <s...@google.com> wrote:
> >
> > We want to make sure that the following condition holds:
> > 0 <= nhoff <= thoff <= skb->len
> >
> > BPF program can set out-of-bounds nhoff and thoff, which is dangerous, see
> > recent commit d0c081b49137 ("flow_dissector: properly cap thoff field")'.
> >
> > Signed-off-by: Stanislav Fomichev <s...@google.com>
> > ---
> > net/core/flow_dissector.c | 6 ++++--
> > 1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
> > index ac19da6f390b..bb1a54747d64 100644
> > --- a/net/core/flow_dissector.c
> > +++ b/net/core/flow_dissector.c
> > @@ -716,6 +716,10 @@ bool __skb_flow_bpf_dissect(struct bpf_prog *prog,
> > /* Restore state */
> > memcpy(cb, &cb_saved, sizeof(cb_saved));
> >
> > + flow_keys->nhoff = clamp_t(u16, flow_keys->nhoff, 0, skb->len);
> > + flow_keys->thoff = clamp_t(u16, flow_keys->thoff,
> > + flow_keys->nhoff, skb->len);
> > +
> > return result == BPF_OK;
> > }
> >
> > @@ -808,8 +812,6 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
> > &flow_keys);
> > __skb_flow_bpf_to_target(&flow_keys, flow_dissector,
> > target_container);
> > - key_control->thoff = min_t(u16, key_control->thoff,
> > - skb->len);
> > rcu_read_unlock();
> > return ret;
> > }
> > --
> > 2.20.0.rc1.387.gf8505762e3-goog
> >
>
> Same question as 3/5:
>
> Do we need this fix without this set? If yes, do we need it for bpf
> tree as well?
No, for the older versions we do this capping when copying to key_control:
key_control->thoff = min_t(u16, key_control->thoff,
skb->len);
I just moved this logic to the flow_keys and made it more approachable (use
actual clamping with min/max boundary, not cryptic min).
I think my commit message might be confusing. There is no real issue
here, it's done mostly for testing (so we see the result of clamping).
>
> Thanks,
> Song
