Subject: Re: [PATCH 1/1] net/sched: cls_u32: Fix reference counter leak leading to overflow

From: Lee Jones <[email-masked]>
Date: Thu, 1 Jun 2023 15:06:40 +0100

On Wed, 31 May 2023, Jamal Hadi Salim wrote:

> On Wed, May 31, 2023 at 11:03 AM Eric Dumazet <[email-masked]> wrote:
> >
> > On Wed, May 31, 2023 at 4:16 PM Lee Jones <[email-masked]> wrote:
> > >
> > > In the event of a failure in tcf_change_indev(), u32_set_parms() will
> > > immediately return without decrementing the recently incremented
> > > reference counter.  If this happens enough times, the counter will
> > > rollover and the reference freed, leading to a double free which can be
> > > used to do 'bad things'.
> > >
> > > Cc: [email-masked] # v4.14+
> >
> > Please add a Fixes: tag.

Why?

>From memory, I couldn't identify a specific commit to fix, which is why
I used a Cc tag as per the Stable documentation:

Option 1
********

To have the patch automatically included in the stable tree, add the tag

.. code-block:: none

     Cc: [email-masked]

in the sign-off area. Once the patch is merged it will be applied to
the stable tree without anything else needing to be done by the author
or subsystem maintainer.

> > > Signed-off-by: Lee Jones <[email-masked]>
> > > ---
> > >  net/sched/cls_u32.c | 5 ++++-
> > >  1 file changed, 4 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
> > > index 4e2e269f121f8..fad61ca5e90bf 100644
> > > --- a/net/sched/cls_u32.c
> > > +++ b/net/sched/cls_u32.c
> > > @@ -762,8 +762,11 @@ static int u32_set_parms(struct net *net, struct tcf_proto *tp,
> > >         if (tb[TCA_U32_INDEV]) {
> > >                 int ret;
> > >                 ret = tcf_change_indev(net, tb[TCA_U32_INDEV], extack);
> >
> > This call should probably be done earlier in the function, next to
> > tcf_exts_validate_ex()
> >
> > Otherwise we might ask why the tcf_bind_filter() does not need to be undone.
> >
> > Something like:
> >
> > diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
> > index 4e2e269f121f8a301368b9783753e055f5af6a4e..ac957ff2216ae18bcabdd3af3b0e127447ef8f91
> > 100644
> > --- a/net/sched/cls_u32.c
> > +++ b/net/sched/cls_u32.c
> > @@ -718,13 +718,18 @@ static int u32_set_parms(struct net *net, struct
> > tcf_proto *tp,
> >                          struct nlattr *est, u32 flags, u32 fl_flags,
> >                          struct netlink_ext_ack *extack)
> >  {
> > -       int err;
> > +       int err, ifindex = -1;
> >
> >         err = tcf_exts_validate_ex(net, tp, tb, est, &n->exts, flags,
> >                                    fl_flags, extack);
> >         if (err < 0)
> >                 return err;
> >
> > +       if (tb[TCA_U32_INDEV]) {
> > +               ifindex = tcf_change_indev(net, tb[TCA_U32_INDEV], extack);
> > +               if (ifindex < 0)
> > +                       return -EINVAL;
> > +       }

Thanks for the advice.  Leave it with me.

> >         if (tb[TCA_U32_LINK]) {
> >                 u32 handle = nla_get_u32(tb[TCA_U32_LINK]);
> >                 struct tc_u_hnode *ht_down = NULL, *ht_old;
> > @@ -759,13 +764,9 @@ static int u32_set_parms(struct net *net, struct
> > tcf_proto *tp,
> >                 tcf_bind_filter(tp, &n->res, base);
> >         }
> >
> > -       if (tb[TCA_U32_INDEV]) {
> > -               int ret;
> > -               ret = tcf_change_indev(net, tb[TCA_U32_INDEV], extack);
> > -               if (ret < 0)
> > -                       return -EINVAL;
> > -               n->ifindex = ret;
> > -       }
> > +       if (ifindex >= 0)
> > +               n->ifindex = ifindex;
> > +
> 
> I guess we crossed paths ;->

> Please, add a tdc test as well - it doesnt have to be in this patch,
> can be a followup.

I don't know how to do that, or even what a 'tdc' is.  Is it trivial?

Can you point me towards the documentation please?

-- 
Lee Jones [李琼斯]