Re: [RFC nf-next v3 1/2] netfilter: bpf: support prog update

[D. Wythe]

On 12/21/23 5:11 AM, Alexei Starovoitov wrote:
> On Wed, Dec 20, 2023 at 6:09 AM D. Wythe <alibuda@xxxxxxxxxxxxxxxxx> wrote:
> > From: "D. Wythe" <alibuda@xxxxxxxxxxxxxxxxx>
> >
> > To support the prog update, we need to ensure that the prog seen
> > within the hook is always valid. Considering that hooks are always
> > protected by rcu_read_lock(), which provide us the ability to
> > access the prog under rcu.
> >
> > Signed-off-by: D. Wythe <alibuda@xxxxxxxxxxxxxxxxx>
> > ---
> >   net/netfilter/nf_bpf_link.c | 63 ++++++++++++++++++++++++++++++++++-----------
> >   1 file changed, 48 insertions(+), 15 deletions(-)
> >
> > diff --git a/net/netfilter/nf_bpf_link.c b/net/netfilter/nf_bpf_link.c
> > index e502ec0..9bc91d1 100644
> > --- a/net/netfilter/nf_bpf_link.c
> > +++ b/net/netfilter/nf_bpf_link.c
> > @@ -8,17 +8,8 @@
> >   #include <net/netfilter/nf_bpf_link.h>
> >   #include <uapi/linux/netfilter_ipv4.h>
> >
> > -static unsigned int nf_hook_run_bpf(void *bpf_prog, struct sk_buff *skb,
> > -                                   const struct nf_hook_state *s)
> > -{
> > -       const struct bpf_prog *prog = bpf_prog;
> > -       struct bpf_nf_ctx ctx = {
> > -               .state = s,
> > -               .skb = skb,
> > -       };
> > -
> > -       return bpf_prog_run(prog, &ctx);
> > -}
> > +/* protect link update in parallel */
> > +static DEFINE_MUTEX(bpf_nf_mutex);
> >
> >   struct bpf_nf_link {
> >          struct bpf_link link;
> > @@ -26,8 +17,20 @@ struct bpf_nf_link {
> >          struct net *net;
> >          u32 dead;
> >          const struct nf_defrag_hook *defrag_hook;
> > +       struct rcu_head head;
> I have to point out the same issues as before, but
> will ask them differently...
>
> Why do you think above rcu_head is necessary?
>
> >   };
> >
> > +static unsigned int nf_hook_run_bpf(void *bpf_link, struct sk_buff *skb,
> > +                                   const struct nf_hook_state *s)
> > +{
> > +       const struct bpf_nf_link *nf_link = bpf_link;
> > +       struct bpf_nf_ctx ctx = {
> > +               .state = s,
> > +               .skb = skb,
> > +       };
> > +       return bpf_prog_run(rcu_dereference_raw(nf_link->link.prog), &ctx);
> > +}
> > +
> >   #if IS_ENABLED(CONFIG_NF_DEFRAG_IPV4) || IS_ENABLED(CONFIG_NF_DEFRAG_IPV6)
> >   static const struct nf_defrag_hook *
> >   get_proto_defrag_hook(struct bpf_nf_link *link,
> > @@ -126,8 +129,7 @@ static void bpf_nf_link_release(struct bpf_link *link)
> >   static void bpf_nf_link_dealloc(struct bpf_link *link)
> >   {
> >          struct bpf_nf_link *nf_link = container_of(link, struct bpf_nf_link, link);
> > -
> > -       kfree(nf_link);
> > +       kfree_rcu(nf_link, head);
> Why is this needed ?
> Have you looked at tcx_link_lops ?

Introducing rcu_head/kfree_rcu is to address the situation where the 
netfilter hooks might
still access the link after bpf_nf_link_dealloc.

                                                     nf_hook_run_bpf
                                                     const struct 
bpf_nf_link *nf_link = bpf_link;

bpf_nf_link_release
    nf_unregister_net_hook(nf_link->net, &nf_link->hook_ops);

bpf_nf_link_dealloc
    free(link)
bpf_prog_run(link->prog);


I had checked the tcx_link_lops ,it's seems it use the synchronize_rcu() 
to solve the
same problem, which is also the way we used in the first version.

https://lore.kernel.org/bpf/1702467945-38866-1-git-send-email-alibuda@xxxxxxxxxxxxxxxxx/

However, we have received some opposing views, believing that this is a 
bit overkill,
so we decided to use kfree_rcu.

https://lore.kernel.org/bpf/20231213222415.GA13818@xxxxxxxxxxxxx/

> >   }
> >
> >   static int bpf_nf_link_detach(struct bpf_link *link)
> > @@ -162,7 +164,34 @@ static int bpf_nf_link_fill_link_info(const struct bpf_link *link,
> >   static int bpf_nf_link_update(struct bpf_link *link, struct bpf_prog *new_prog,
> >                                struct bpf_prog *old_prog)
> >   {
> > -       return -EOPNOTSUPP;
> > +       struct bpf_nf_link *nf_link = container_of(link, struct bpf_nf_link, link);
> > +       int err = 0;
> > +
> > +       mutex_lock(&bpf_nf_mutex);
> Why do you need this mutex?
> What race does it solve?

To avoid user update a link with differ prog at the same time. I noticed 
that sys_bpf()
doesn't seem to prevent being invoked by user at the same time. Have I 
missed something?

Best wishes,
D. Wythe
> > +
> > +       if (nf_link->dead) {
> > +               err = -EPERM;
> > +               goto out;
> > +       }
> > +
> > +       /* target old_prog mismatch */
> > +       if (old_prog && link->prog != old_prog) {
> > +               err = -EPERM;
> > +               goto out;
> > +       }
> > +
> > +       old_prog = link->prog;
> > +       if (old_prog == new_prog) {
> > +               /* don't need update */
> > +               bpf_prog_put(new_prog);
> > +               goto out;
> > +       }
> > +
> > +       old_prog = xchg(&link->prog, new_prog);
> > +       bpf_prog_put(old_prog);
> > +out:
> > +       mutex_unlock(&bpf_nf_mutex);
> > +       return err;
> >   }
> >
> >   static const struct bpf_link_ops bpf_nf_link_lops = {
> > @@ -226,7 +255,11 @@ int bpf_nf_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
> >
> >          link->hook_ops.hook = nf_hook_run_bpf;
> >          link->hook_ops.hook_ops_type = NF_HOOK_OP_BPF;
> > -       link->hook_ops.priv = prog;
> > +
> > +       /* bpf_nf_link_release & bpf_nf_link_dealloc() can ensures that link remains
> > +        * valid at all times within nf_hook_run_bpf().
> > +        */
> > +       link->hook_ops.priv = link;
> >
> >          link->hook_ops.pf = attr->link_create.netfilter.pf;
> >          link->hook_ops.priority = attr->link_create.netfilter.priority;
> > --
> > 1.8.3.1