Re: [PATCH net V2] net/sched: pedit: make sure that offset is valid

[Amir Vadai <amir@vadai.me>]

On Tue, Nov 29, 2016 at 10:32:05AM +0800, zhuyj wrote:
>  +       if (offset > 0 && offset > skb->len)
> 
> offset > skb->len is enough?
offset is signed and skb->len is unsigned. Therefore for example if
offset=-1 and skb->len=10, the actual comparison is 0xff...>10

> 
> On Mon, Nov 28, 2016 at 6:56 PM, Amir Vadai <amir@vadai.me> wrote:
> > Add a validation function to make sure offset is valid:
> > 1. Not below skb head (could happen when offset is negative).
> > 2. Validate both 'offset' and 'at'.
> >
> > Signed-off-by: Amir Vadai <amir@vadai.me>
> > ---
> > Hi Dave,
> >
> > Please pull to -stable branches.
> >
> > Changes from V0:
> > - Add a validation to the 'at' value (this is used as an offset too)
> > - Instead of validating the output of skb_header_pointer(), make sure that the
> >         offset is good before calling it.
> >
> > Thanks,
> > Amir
> >  net/sched/act_pedit.c | 24 ++++++++++++++++++++----
> >  1 file changed, 20 insertions(+), 4 deletions(-)
> >
> > diff --git a/net/sched/act_pedit.c b/net/sched/act_pedit.c
> > index b54d56d4959b..cf9b2fe8eac6 100644
> > --- a/net/sched/act_pedit.c
> > +++ b/net/sched/act_pedit.c
> > @@ -108,6 +108,17 @@ static void tcf_pedit_cleanup(struct tc_action *a, int bind)
> >         kfree(keys);
> >  }
> >
> > +static bool offset_valid(struct sk_buff *skb, int offset)
> > +{
> > +       if (offset > 0 && offset > skb->len)
> > +               return false;
> > +
> > +       if  (offset < 0 && -offset > skb_headroom(skb))
> > +               return false;
> > +
> > +       return true;
> > +}
> > +
> >  static int tcf_pedit(struct sk_buff *skb, const struct tc_action *a,
> >                      struct tcf_result *res)
> >  {
> > @@ -134,6 +145,11 @@ static int tcf_pedit(struct sk_buff *skb, const struct tc_action *a,
> >                         if (tkey->offmask) {
> >                                 char *d, _d;
> >
> > +                               if (!offset_valid(skb, off + tkey->at)) {
> > +                                       pr_info("tc filter pedit 'at' offset %d out of bounds\n",
> > +                                               off + tkey->at);
> > +                                       goto bad;
> > +                               }
> >                                 d = skb_header_pointer(skb, off + tkey->at, 1,
> >                                                        &_d);
> >                                 if (!d)
> > @@ -146,10 +162,10 @@ static int tcf_pedit(struct sk_buff *skb, const struct tc_action *a,
> >                                         " offset must be on 32 bit boundaries\n");
> >                                 goto bad;
> >                         }
> > -                       if (offset > 0 && offset > skb->len) {
> > -                               pr_info("tc filter pedit"
> > -                                       " offset %d can't exceed pkt length %d\n",
> > -                                      offset, skb->len);
> > +
> > +                       if (!offset_valid(skb, off + offset)) {
> > +                               pr_info("tc filter pedit offset %d out of bounds\n",
> > +                                       offset);
> >                                 goto bad;
> >                         }
> >
> > --
> > 2.10.2
> >