Re: [PATCH net v1] net/sched: cls_api: complement tcf_tfilter_dump_policy

[Jamal Hadi Salim]

On Wed, Dec 27, 2023 at 12:02 PM Jamal Hadi Salim <jhs@xxxxxxxxxxxx> wrote:
>
> On Mon, Dec 25, 2023 at 8:39 PM Lin Ma <linma@xxxxxxxxxx> wrote:
> >
> > Hello Jamal,
> >
> > >
> > > Can you clarify what "heap data leak" you are referring to?
> > > As much as i can see any reference to NLA_TCA_CHAIN is checked for
> > > presence before being put to use. So far that reason I  dont see how
> > > this patch qualifies as "net". It looks like an enhancement to me
> > > which should target net-next, unless i am missing something obvious.
> > >
> >
> > Sure, thanks for your reply, (and merry Christmas :D).
> > I didn't mention the detail as I consider the commit message in
> > `5e2424708da7` could make a point. In short, the code
> >
> > ```
> > if (tca[TCA_CHAIN] && nla_get_u32(tca[TCA_CHAIN])
> > ```
> >
> > only checks if the attribute TCA_CHAIN exists but never checks about
> > the attribute length because that attribute is parsed by the function
> > nlmsg_parse_deprecated which will parse an attribute even not described
> > in the given policy (here, the tcf_tfilter_dump_policy).
> >
> > Moreover, the netlink message is allocated via netlink_alloc_large_skb
> > (see net/netlink/af_netlink.c) that does not clear out the heap buffer.
> > Hence a malicious user could send a malicious TCA_CHAIN attribute here
> > without putting any payload and the above `nla_get_u32` could dereference
> > a dirty data that is sprayed by the user.
> >
> > Other place gets TCA_CHAIN with provide policy rtm_tca_policy that has a
> > description.
> >
> > ```
> > [TCA_CHAIN]             = { .type = NLA_U32 },
> > ```
> >
> > and this patch aims to do so.
> >
> > Unfortunately, I have not opened the exploit for CVE-2023-3773
> > (https://access.redhat.com/security/cve/cve-2023-3773) yet but the idea
> > is similar and you can take it as an example.
> >
>
> Sorry, still trying to follow your reasoning that this is a "net issue":
> As you point out, the skb will have enough space to carry the 32 bit
> value. Worst case is we read garbage. And the dump, using this garbage
> chain index,  will not find the chain or will find some unintended
> chain. Am i missing something?
>
> Can you send me a repro (privately) that actually causes the "heap
> data leak" if you have one?
>

To clarify what triggered me is your tie of this as an exploit and
quoting CVEs. Maybe not so much net vs net-next.

cheers,
jamal

> cheers,
> jamal
>
>
> > > cheers,
> > > jamal
> > >
> >
> > Regards
> > Lin