netdev - Re: [PATCH ipsec 4/7] xfrm: policy: delete inexact policies from inexact list on hash rebuild

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Date:   Sat, 5 Jan 2019 10:53:56 +0100
From:   Florian Westphal <fw@...len.de>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Florian Westphal <fw@...len.de>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH ipsec 4/7] xfrm: policy: delete inexact policies from
 inexact list on hash rebuild

Cong Wang <xiyou.wangcong@...il.com> wrote:
> On Fri, Jan 4, 2019 at 5:19 AM Florian Westphal <fw@...len.de> wrote:
> >
> > An xfrm hash rebuild has to reset the inexact policy list before the
> > policies get re-inserted: A change of hash thresholds will result in
> > policies to get moved from inexact tree to the policy hash table.
> >
> > If the thresholds are increased again later, they get moved from hash
> > table to inexact tree.
> >
> > We must unlink all policies from the inexact tree before re-insertion.
> >
> > Otherwise 'migrate' may find policies that are in main hash table a
> > second time, when it searches the inexact lists.
> >
> > Furthermore, re-insertion without deletion can cause elements ->next to
> > point back to itself, causing soft lockups or double-frees.
> >
> > Reported-by: syzbot+9d971dd21eb26567036b@...kaller.appspotmail.com
> > Fixes: 9cf545ebd591da ("xfrm: policy: store inexact policies in a tree ordered by destination address")
> > Signed-off-by: Florian Westphal <fw@...len.de>
> > ---
> >  net/xfrm/xfrm_policy.c | 23 ++++++++++-------------
> >  1 file changed, 10 insertions(+), 13 deletions(-)
> >
> > diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
> > index d8fba27a4bfb..24dfd1e47cf0 100644
> > --- a/net/xfrm/xfrm_policy.c
> > +++ b/net/xfrm/xfrm_policy.c
> > @@ -680,16 +680,6 @@ static void xfrm_hash_resize(struct work_struct *work)
> >         mutex_unlock(&hash_resize_mutex);
> >  }
> >
> > -static void xfrm_hash_reset_inexact_table(struct net *net)
> > -{
> > -       struct xfrm_pol_inexact_bin *b;
> > -
> > -       lockdep_assert_held(&net->xfrm.xfrm_policy_lock);
> > -
> > -       list_for_each_entry(b, &net->xfrm.inexact_bins, inexact_bins)
> > -               INIT_HLIST_HEAD(&b->hhead);
> > -}
> > -
> >  /* Make sure *pol can be inserted into fastbin.
> >   * Useful to check that later insert requests will be sucessful
> >   * (provided xfrm_policy_lock is held throughout).
> > @@ -1279,10 +1269,14 @@ static void xfrm_hash_rebuild(struct work_struct *work)
> >         }
> >
> >         /* reset the bydst and inexact table in all directions */
> 
> 
> Does this comment need to update?

I don't think so, it predates my changes and we still reset the
inexact trees.

What would you suggest the comment should read?
 
> > +               hlist_del_rcu(&policy->bydst);
> 
> 
> Need to test hlist_unhashed()?

No, all policies are in a hlist, either via the exact hash table or
an inexact tree list.

Also, the _rcu version leaves entry in undefined state so hlist_unhashed
doesn't return true.