[bpf,1/2] bpf: Fix [::] -> [::1] rewrite in sys_sendmsg

sys_sendmsg has supported unspecified destination IPv6 (wildcard) for
unconnected UDP sockets since 876c7f41. When [::] is passed by user as
destination, sys_sendmsg rewrites it with [::1] to be consistent with
BSD (see "BSD'ism" comment in the code).

This didn't work when cgroup-bpf was enabled though since the rewrite
[::] -> [::1] happened before passing control to cgroup-bpf block where
fl6.daddr was updated with passed by user sockaddr_in6.sin6_addr (that
might or might not be changed by BPF program). That way if user passed
[::] as dst IPv6 it was first rewritten with [::1] by original code from
876c7f41, but then rewritten back with [::] by cgroup-bpf block.

It happened even when BPF_CGROUP_UDP6_SENDMSG program was not present
(CONFIG_CGROUP_BPF=y was enough).

The fix is to apply BSD'ism after cgroup-bpf block so that [::] is
replaced with [::1] no matter where it came from: passed by user to
sys_sendmsg or set by BPF_CGROUP_UDP6_SENDMSG program.

Fixes: 1cedee13d25a ("bpf: Hooks for sys_sendmsg")
Reported-by: Nitin Rawat <nitin.rawat@intel.com>
Signed-off-by: Andrey Ignatov <rdna@fb.com>
---
 net/ipv6/udp.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

Rawat, Nitin <nitin.rawat@intel.com> [Fri, 2019-01-04 00:35 -0800]:
> Hi All,
> Andrey,thanks for the fix .
> I compiled the patch ½ provided by you , there was a compilation issue in your
> patch in the below line .
>  
> Original line : if (ipv6_addr_any(fl6.daddr))
> Expected line:  if (ipv6_addr_any(&fl6.daddr))
>  
> I have made the above changes and test case was working fine for unconnected
> test case .
> Please let me know your view on that.

Oops, I had this locally but missed it in the commit while baking the
final patch somehow.
Good catch, thanks! I'll send v2 with the fix.


> There is also a another way to solve this way by putting the check just inside
> the unconnected check , this will leave connected code path untouched
> Hence I feel would be simpler
> 
> @@ -1343,6 +1343,11 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg,
> size_t len)
>                         fl6.fl6_dport = sin6->sin6_port;
>                         fl6.daddr = sin6->sin6_addr;
>                 }
> +               if (ipv6_addr_any(&fl6.daddr))
> +                {
> +                    fl6.daddr.s6_addr[15] = 0x1; /* :: means loopback
> (BSD'ism) */
> +                }
> +
>         }
>  
>         final_p = fl6_update_dst(&fl6, opt, &final);

Yeah, it can be fixed like this but I intentionally didn't do it this
way since it adds code duplication (copy/paste and checking
ipv6_addr_any / setting [::1] twice).


> I have tested second approach and it is working fine for unconnected case.
> Please let us know your views on the above fix
>  
> Regards,
> Nitin
> -----Original Message-----
> From: Andrey Ignatov [mailto:rdna@fb.com]
> Sent: Friday, January 4, 2019 4:13 AM
> To: netdev@vger.kernel.org
> Cc: Andrey Ignatov <rdna@fb.com>; ast@kernel.org; daniel@iogearbox.net; Rawat,
> Nitin <nitin.rawat@intel.com>; kernel-team@fb.com
> Subject: [PATCH bpf 1/2] bpf: Fix [::] -> [::1] rewrite in sys_sendmsg
>  
> sys_sendmsg has supported unspecified destination IPv6 (wildcard) for
> unconnected UDP sockets since 876c7f41. When [::] is passed by user as
> destination, sys_sendmsg rewrites it with [::1] to be consistent with BSD (see
> "BSD'ism" comment in the code).
>  
> This didn't work when cgroup-bpf was enabled though since the rewrite [::] ->
> [::1] happened before passing control to cgroup-bpf block where fl6.daddr was
> updated with passed by user sockaddr_in6.sin6_addr (that might or might not be
> changed by BPF program). That way if user passed [::] as dst IPv6 it was first
> rewritten with [::1] by original code from 876c7f41, but then rewritten back
> with [::] by cgroup-bpf block.
>  
> It happened even when BPF_CGROUP_UDP6_SENDMSG program was not present
> (CONFIG_CGROUP_BPF=y was enough).
>  
> The fix is to apply BSD'ism after cgroup-bpf block so that [::] is replaced
> with [::1] no matter where it came from: passed by user to sys_sendmsg or set
> by BPF_CGROUP_UDP6_SENDMSG program.
>  
> Fixes: 1cedee13d25a ("bpf: Hooks for sys_sendmsg")
> Reported-by: Nitin Rawat <nitin.rawat@intel.com>
> Signed-off-by: Andrey Ignatov <rdna@fb.com>
> ---
> net/ipv6/udp.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>  
> diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c index 9cbf363172bd..cd563f985e1a
> 100644
> --- a/net/ipv6/udp.c
> +++ b/net/ipv6/udp.c
> @@ -1390,10 +1390,7 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg,
> size_t len)
>          ipc6.opt = opt;
>  
>          fl6.flowi6_proto = sk->sk_protocol;
> -       if (!ipv6_addr_any(daddr))
> -               fl6.daddr = *daddr;
> -       else
> -               fl6.daddr.s6_addr[15] = 0x1; /* :: means loopback (BSD'ism) */
> +       fl6.daddr = *daddr;
>          if (ipv6_addr_any(&fl6.saddr) && !ipv6_addr_any(&np->saddr))
>                  fl6.saddr = np->saddr;
>          fl6.fl6_sport = inet->inet_sport;
> @@ -1421,6 +1418,9 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg,
> size_t len)
>                  }
>          }
>  
> +       if (ipv6_addr_any(fl6.daddr))
> +               fl6.daddr.s6_addr[15] = 0x1; /* :: means loopback (BSD'ism) */
> +
>          final_p = fl6_update_dst(&fl6, opt, &final);
>          if (final_p)
>                  connected = false;
> --
> 2.17.1
>  
>