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

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Date:   Tue, 29 Nov 2016 13:03:13 +0200
From:   Amir Vadai <amir@...ai.me>
To:     zhuyj <zyjzyj2000@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Or Gerlitz <ogerlitz@...lanox.com>,
        Hadar Har-Zion <hadarh@...lanox.com>,
        Jiri Pirko <jiri@...lanox.com>
Subject: Re: [PATCH net V2] net/sched: pedit: make sure that offset is valid

On Tue, Nov 29, 2016 at 05:27:05PM +0800, zhuyj wrote:
>  Thanks a lot.
> When will offset become -1?
offset is supplied by userspace. For example iproute2 tc tool.
It is valid to supply a negative value, since offset is relative to the
networking layer, one might want to edit a MAC layer field.

Please use bottom posting when replying to mails...

> 
> On Tue, Nov 29, 2016 at 3:14 PM, Amir Vadai <amir@...ai.me> wrote:
> > 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@...ai.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@...ai.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
> >> >