On Mon, Sep 23, 2024 at 01:08 PM GMT, David Laight wrote:
> From: Tiago Lam <tiagolam@xxxxxxxxxxxxxx>
[...]
>> To limit its usage, a reverse socket lookup is performed to check if the
>> configured egress source address and/or port have any ingress sk_lookup
>> match. If it does, traffic is allowed to proceed, otherwise it falls
>> back to the regular egress path.
>
> Is that really useful/necessary?
We've been asking ourselves the same question during Plumbers with
Martin.
Unprivileges processes can already source UDP traffic from (almost) any
IP & port by binding a socket to the desired source port and passing
IP_PKTINFO. So perhaps having a reverse socket lookup is an overkill.
We should probably respect net.ipv4.ip_local_reserved_ports and
net.ipv4.ip_unprivileged_port_start system settings, though, or check
for relevant caps.
Open question if it is acceptable to disregard exclusive UDP port
ownership by sockets binding to a wildcard address without SO_REUSEADDR?
[...]
> The check (but not the commit message) implies that some 'bpf thingy'
> also needs to be enabled.
> Any check would need to include the test that the program sending the packet
> has the ability to send a packet through the ingress socket.
> Additionally a check for the sending process having (IIRC) CAP_NET_ADMIN
> (which would let the process send the message by other means) would save the
> slow path.
>
> The code we have sends a lot of UDP RTP (typically 160 bytes of audio every 20ms).
> There is actually no reason for there to be a valid matching ingress path.
> (That code would benefit from being able to bind a lot of ports to the same
> UDP socket.)
>
> David
>
>>
>> Suggested-by: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx>
>> Signed-off-by: Tiago Lam <tiagolam@xxxxxxxxxxxxxx>
>> ---
>> net/ipv6/datagram.c | 79 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>> net/ipv6/udp.c | 8 ++++--
>> 2 files changed, 85 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/ipv6/datagram.c b/net/ipv6/datagram.c
>> index fff78496803d..369c64a478ec 100644
>> --- a/net/ipv6/datagram.c
>> +++ b/net/ipv6/datagram.c
>> @@ -756,6 +756,29 @@ void ip6_datagram_recv_ctl(struct sock *sk, struct msghdr *msg,
>> }
>> EXPORT_SYMBOL_GPL(ip6_datagram_recv_ctl);
>>
>> +static inline bool reverse_sk_lookup(struct flowi6 *fl6, struct sock *sk,
>> + struct in6_addr *saddr, __be16 sport)
>> +{
>> + if (static_branch_unlikely(&bpf_sk_lookup_enabled) &&
>> + (saddr && sport) &&
>> + (ipv6_addr_cmp(&sk->sk_v6_rcv_saddr, saddr) ||
>> + inet_sk(sk)->inet_sport != sport)) {
>> + struct sock *sk_egress;
>> +
>> + bpf_sk_lookup_run_v6(sock_net(sk), IPPROTO_UDP, &fl6->daddr,
>> + fl6->fl6_dport, saddr, ntohs(sport), 0,
>> + &sk_egress);
>> + if (!IS_ERR_OR_NULL(sk_egress) && sk_egress == sk)
>> + return true;
>> +
>> + net_info_ratelimited("No reverse socket lookup match for local addr %pI6:%d remote addr
>> %pI6:%d\n",
>> + &saddr, ntohs(sport), &fl6->daddr,
>> + ntohs(fl6->fl6_dport));
>> + }
>> +
>> + return false;
>> +}
>> +
>> int ip6_datagram_send_ctl(struct net *net, struct sock *sk,
>> struct msghdr *msg, struct flowi6 *fl6,
>> struct ipcm6_cookie *ipc6)
>> @@ -844,7 +867,63 @@ int ip6_datagram_send_ctl(struct net *net, struct sock *sk,
>>
>> break;
>> }
>> + case IPV6_ORIGDSTADDR:
>> + {
>> + struct sockaddr_in6 *sockaddr_in;
>> + struct net_device *dev = NULL;
>> +
>> + if (cmsg->cmsg_len < CMSG_LEN(sizeof(struct sockaddr_in6))) {
>> + err = -EINVAL;
>> + goto exit_f;
>> + }
>> +
>> + sockaddr_in = (struct sockaddr_in6 *)CMSG_DATA(cmsg);
>> +
>> + addr_type = __ipv6_addr_type(&sockaddr_in->sin6_addr);
>> +
>> + if (addr_type & IPV6_ADDR_LINKLOCAL)
>> + return -EINVAL;
>> +
>> + /* If we're egressing with a different source address
>> + * and/or port, we perform a reverse socket lookup. The
>> + * rationale behind this is that we can allow return
>> + * UDP traffic that has ingressed through sk_lookup to
>> + * also egress correctly. In case the reverse lookup
>> + * fails, we continue with the normal path.
>> + *
>> + * The lookup is performed if either source address
>> + * and/or port changed, and neither is "0".
>> + */
>> + if (reverse_sk_lookup(fl6, sk, &sockaddr_in->sin6_addr,
>> + sockaddr_in->sin6_port)) {
>> + /* Override the source port and address to use
>> + * with the one we got in cmsg and bail early.
>> + */
>> + fl6->saddr = sockaddr_in->sin6_addr;
>> + fl6->fl6_sport = sockaddr_in->sin6_port;
>> + break;
>> + }
>>
>> + if (addr_type != IPV6_ADDR_ANY) {
>> + int strict = __ipv6_addr_src_scope(addr_type) <= IPV6_ADDR_SCOPE_LINKLOCAL;
>> +
>> + if (!ipv6_can_nonlocal_bind(net, inet_sk(sk)) &&
>> + !ipv6_chk_addr_and_flags(net,
>> + &sockaddr_in->sin6_addr,
>> + dev, !strict, 0,
>> + IFA_F_TENTATIVE) &&
>> + !ipv6_chk_acast_addr_src(net, dev,
>> + &sockaddr_in->sin6_addr))
>> + err = -EINVAL;
>> + else
>> + fl6->saddr = sockaddr_in->sin6_addr;
>> + }
>> +
>> + if (err)
>> + goto exit_f;
>> +
>> + break;
>> + }
>> case IPV6_FLOWINFO:
>> if (cmsg->cmsg_len < CMSG_LEN(4)) {
>> err = -EINVAL;
>> diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
>> index 6602a2e9cdb5..6121cbb71ad3 100644
>> --- a/net/ipv6/udp.c
>> +++ b/net/ipv6/udp.c
>> @@ -1476,6 +1476,12 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
>>
>> fl6->flowi6_uid = sk->sk_uid;
>>
>> + /* We use fl6's daddr and fl6_sport in the reverse sk_lookup done
>> + * within ip6_datagram_send_ctl() now.
>> + */
>> + fl6->daddr = *daddr;
>> + fl6->fl6_sport = inet->inet_sport;
>> +
>> if (msg->msg_controllen) {
>> opt = &opt_space;
>> memset(opt, 0, sizeof(struct ipv6_txoptions));
>> @@ -1511,10 +1517,8 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
>>
>> fl6->flowi6_proto = sk->sk_protocol;
>> fl6->flowi6_mark = ipc6.sockc.mark;
>> - fl6->daddr = *daddr;
>> if (ipv6_addr_any(&fl6->saddr) && !ipv6_addr_any(&np->saddr))
>> fl6->saddr = np->saddr;
>> - fl6->fl6_sport = inet->inet_sport;
>>
>> if (cgroup_bpf_enabled(CGROUP_UDP6_SENDMSG) && !connected) {
>> err = BPF_CGROUP_RUN_PROG_UDP6_SENDMSG_LOCK(sk,
>>
>> --
>> 2.34.1
>>
>
> -
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