netdev - Re: [PATCH net 1/3] net/sched: cls_u32: No longer copy tcf

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Date: Sat, 22 Jul 2023 03:38:38 +0700
From: M A Ramdhan <ramdhan@...rlabs.sg>
To: Pedro Tammela <pctammela@...atatu.com>
Cc: valis <sec@...is.email>, netdev@...r.kernel.org, jhs@...atatu.com, 
	xiyou.wangcong@...il.com, jiri@...nulli.us, davem@...emloft.net, 
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com, victor@...atatu.com, 
	billy@...rlabs.sg
Subject: Re: [PATCH net 1/3] net/sched: cls_u32: No longer copy tcf_result on
 update to avoid use-after-free

On Sat, Jul 22, 2023 at 1:59 AM Pedro Tammela <pctammela@...atatu.com> wrote:
>
> On 21/07/2023 15:04, M A Ramdhan wrote:
> > On Sat, Jul 22, 2023 at 12:51 AM valis <sec@...is.email> wrote:
> >>
> >> When u32_change() is called on an existing filter, the whole
> >> tcf_result struct is always copied into the new instance of the filter.
> >>
> >> This causes a problem when updating a filter bound to a class,
> >> as tcf_unbind_filter() is always called on the old instance in the
> >> success path, decreasing filter_cnt of the still referenced class
> >> and allowing it to be deleted, leading to a use-after-free.
> >>
> >> Fix this by no longer copying the tcf_result struct from the old filter.
> >>
> >> Fixes: de5df63228fc ("net: sched: cls_u32 changes to knode must appear atomic to readers")
> >> Reported-by: valis <sec@...is.email>
> >> Reported-by: M A Ramdhan <ramdhan@...rlabs.sg>
> >> Signed-off-by: valis <sec@...is.email>
> >> Cc: stable@...r.kernel.org
> >> ---
> >>   net/sched/cls_u32.c | 1 -
> >>   1 file changed, 1 deletion(-)
> >>
> >> diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
> >> index 5abf31e432ca..19aa60d1eea7 100644
> >> --- a/net/sched/cls_u32.c
> >> +++ b/net/sched/cls_u32.c
> >> @@ -826,7 +826,6 @@ static struct tc_u_knode *u32_init_knode(struct net *net, struct tcf_proto *tp,
> >>
> >>          new->ifindex = n->ifindex;
> >>          new->fshift = n->fshift;
> >> -       new->res = n->res;
> >>          new->flags = n->flags;
> >>          RCU_INIT_POINTER(new->ht_down, ht);
> >>
> >> --
> >> 2.30.2
> >>
> > Hi,
> >
> > We also thought it's also the correct fixes,
> > but we're not sure because it will always remove the already bound
> > qdisc class when we change the filter, even tho we never specify
> > the new TCA_U32_CLASSID in the new filter.
>
> The user should always explicitly tell the classid. Some other
> classifiers are already behaving like this, these were just wrong.
>

Understand, thanks for the clarification.

Regards,
M A Ramdhan
> >
> > I also look at the implementation of cls_tcindex and cls_rsvp which still copy
> > the old tcf_result, but don't call the tcf_unbind_filter when changing
> > the filter.
>
> Both were deprecated and removed as they were beyond saving.
>
> >
> > If it's the intended behaviour, then I'm good with this patch.
> >
> > Thanks & Regards,
> > M A Ramdhan
>