netdev - Re: [PATCH net-next] udp: expand SKB_DROP_REASON_UDP

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Message-ID: <67ca0a2dea753_212e62946f@willemb.c.googlers.com.notmuch>
Date: Thu, 06 Mar 2025 15:48:45 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Eric Dumazet <edumazet@...gle.com>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: "David S . Miller" <davem@...emloft.net>, 
 Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, 
 Willem de Bruijn <willemb@...gle.com>, 
 David Ahern <dsahern@...nel.org>, 
 Simon Horman <horms@...nel.org>, 
 netdev@...r.kernel.org, 
 eric.dumazet@...il.com
Subject: Re: [PATCH net-next] udp: expand SKB_DROP_REASON_UDP_CSUM use

Eric Dumazet wrote:
> On Thu, Mar 6, 2025 at 8:57 PM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > Eric Dumazet wrote:
> > > On Thu, Mar 6, 2025 at 8:01 PM Willem de Bruijn
> > > <willemdebruijn.kernel@...il.com> wrote:
> > > >
> > > > Eric Dumazet wrote:
> > > > > Use SKB_DROP_REASON_UDP_CSUM in __first_packet_length()
> > > > > and udp_read_skb() when dropping a packet because of
> > > > > a wrong UDP checksum.
> > > > >
> > > > > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > > > > ---
> > > > >  net/ipv4/udp.c | 4 ++--
> > > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> > > > > index 17c7736d8349433ad2d4cbcc9414b2f8112610af..39c3adf333b5f02ca53f768c918c75f2fc7f93ac 100644
> > > > > --- a/net/ipv4/udp.c
> > > > > +++ b/net/ipv4/udp.c
> > > > > @@ -1848,7 +1848,7 @@ static struct sk_buff *__first_packet_length(struct sock *sk,
> > > > >                       atomic_inc(&sk->sk_drops);
> > > > >                       __skb_unlink(skb, rcvq);
> > > > >                       *total += skb->truesize;
> > > > > -                     kfree_skb(skb);
> > > > > +                     kfree_skb_reason(skb, SKB_DROP_REASON_UDP_CSUM);
> > > > >               } else {
> > > > >                       udp_skb_csum_unnecessary_set(skb);
> > > > >                       break;
> > > > > @@ -2002,7 +2002,7 @@ int udp_read_skb(struct sock *sk, skb_read_actor_t recv_actor)
> > > > >               __UDP_INC_STATS(net, UDP_MIB_CSUMERRORS, is_udplite);
> > > > >               __UDP_INC_STATS(net, UDP_MIB_INERRORS, is_udplite);
> > > > >               atomic_inc(&sk->sk_drops);
> > > > > -             kfree_skb(skb);
> > > > > +             kfree_skb_reason(skb, SKB_DROP_REASON_UDP_CSUM);
> > > > >               goto try_again;
> > > > >       }
> > > >
> > > > From a quick search for UDP_MIB_CSUMERRORS, one more case with regular
> > > > kfree_skb:
> > > >
> > > > csum_copy_err:
> > > >         if (!__sk_queue_drop_skb(sk, &udp_sk(sk)->reader_queue, skb, flags,
> > > >                                  udp_skb_destructor)) {
> > > >                 UDP_INC_STATS(sock_net(sk), UDP_MIB_CSUMERRORS, is_udplite);
> > > >                 UDP_INC_STATS(sock_net(sk), UDP_MIB_INERRORS, is_udplite);
> > > >         }
> > > >         kfree_skb(skb);
> > >
> > > Right, I was unsure because of the conditional SNMP updates.
> >
> > That seems to only suppress the update if peeking and the skb was
> > already dequeued (ENOENT).
> >
> > Frankly, we probably never intended to increment this counter when
> > peeking, as it is intended to be a per-datagram counter, not a
> > per-recvmsg counter.
> >
> > I think ever erring on the side of extra increments in the unlikely
> > case of ENOENT is fine.
> >
> > Cleaner is perhaps to have a kfree_skb_reason inside that branch and
> > a consume_skb in the else.
> 
> I think it should be a  kfree_skb_reason(skb, SKB_DROP_REASON_UDP_CSUM),
> because only one real 'free' will happen, if two or more threads were
> using MSG_PEEK for this packet.

Oh right.
 
> Using a consume_skb() would be racy, because if the skb refcount
> reaches 0, a wrong event would be generated.
> 
> I will squash in v2 :
> 
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index 39c3adf333b5f02ca53f768c918c75f2fc7f93ac..d0bffcfa56d8deb14f38cc48a4a2b1d899ad6af4
> 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -2117,7 +2117,7 @@ int udp_recvmsg(struct sock *sk, struct msghdr
> *msg, size_t len, int flags,
>                 UDP_INC_STATS(sock_net(sk), UDP_MIB_CSUMERRORS, is_udplite);
>                 UDP_INC_STATS(sock_net(sk), UDP_MIB_INERRORS, is_udplite);
>         }
> -       kfree_skb(skb);
> +       kfree_skb_reason(skb, SKB_DROP_REASON_UDP_CSUM);
> 
>         /* starting over for a new packet, but check if we need to yield */
>         cond_resched();
> diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
> index 3a0d6c5a8286b1685e8a1dec50365fe392ab9a87..024458ef163c9e24dfb37aea2690b2030f6a0fbc
> 100644
> --- a/net/ipv6/udp.c
> +++ b/net/ipv6/udp.c
> @@ -586,7 +586,7 @@ int udpv6_recvmsg(struct sock *sk, struct msghdr
> *msg, size_t len,
>                 SNMP_INC_STATS(mib, UDP_MIB_CSUMERRORS);
>                 SNMP_INC_STATS(mib, UDP_MIB_INERRORS);
>         }
> -       kfree_skb(skb);
> +       kfree_skb_reason(skb, SKB_DROP_REASON_UDP_CSUM);
> 
>         /* starting over for a new packet, but check if we need to yield */
>         cond_resched();

Sounds good. Thanks Eric.