Re: [PATCH] net: sched: fix memory leak in tcindex_set_parms

[Hawkins Jiawei <yin31149@gmail.com>]

On Fri, 4 Nov 2022 at 10:23, Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Fri,  4 Nov 2022 00:07:00 +0800 Hawkins Jiawei wrote:
> > > Can't you localize all the changes to this if block?
> > >
> > > Maybe add a function called tcindex_filter_result_reinit()
> > > which will act more appropriately? 
> >
> > I think we shouldn't put the tcf_exts_destroy(&old_e)
> > into this if block, or other RCU readers may derefer the
> > freed memory (Please correct me If I am wrong).
> >
> > So I put the tcf_exts_destroy(&old_e) near the tcindex
> > destroy work, after the RCU updateing.
>
> I'm not sure what this code is trying to do, to be honest.
> Your concern that there may be a concurrent reader is valid,
> but then again tcindex_filter_result_init() just wipes the
> entire structure with a memset() so concurrent readers are
> already likely broken?
>
> Maybe tcindex_filter_result_init() dates back to times when
> exts were a list (see commit 22dc13c837c) and calling
> tcf_exts_init() wasn't that different than cleaning it up?
> In other words this code is trying to destroy old_r, not
> reinitialize it?
Yes, I also think this code is just trying to destroy the old_r.

In my opinion, the context here is a bit like, this filter's some
properties has been changed, so kernel should drop its old filter
result and update a new one.

Before kernel finishes RCU updating, concurrent readers should
see an empty result(or a valid old result), cleaned by
tcindex_filter_result_init().

This won't trigger the memory leak before commit b9a24bb76bf6
("net_sched: properly handle failure case of tcf_exts_init()"),
I think. Because the new filter result still uses the old_r->exts.

Yet after this commit, kernel allocates the new struct tcf_exts for
new filter result in tcindex_alloc_perfect_hash(), which triggers
the memory leak if kernel cleans the old_r without destroying its
newly allocted struct tcf_exts.

As for the patch, I think we'd better free this struct tcf_exts
after RCU updating, to make sure that concurrent readers can only
see an empty result or a valid old result, before finishing updating
(Please correct me if I am wrong).
>
> > > 
> > > >               err = tcindex_filter_result_init(old_r, cp, net);