net: ipv4: fix l3slave check for index returned in IP_PKTINFO

rt_iif is only set to the actual egress device for the output path. The
recent change to consider the l3slave flag when returning IP_PKTINFO
works for local traffic (the correct device index is returned), but it
broke the more typical use case of packets received from a remote host
always returning the VRF index rather than the original ingress device.
Update the fixup to consider l3slave and rt_iif actually getting set.

Fixes: 1dfa76390bf05 ("net: ipv4: add check for l3slave for index returned in IP_PKTINFO")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 2 deletions

diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index e558e4f9597b..a599aa83fdad 100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -1207,7 +1207,6 @@ e_inval:
 void ipv4_pktinfo_prepare(const struct sock *sk, struct sk_buff *skb)
 {
         struct in_pktinfo *pktinfo = PKTINFO_SKB_CB(skb);
-        bool l3slave = ipv4_l3mdev_skb(IPCB(skb)->flags);
         bool prepare = (inet_sk(sk)->cmsg_flags & IP_CMSG_PKTINFO) ||
                        ipv6_sk_rxinfo(sk);
 
@@ -1221,8 +1220,13 @@ void ipv4_pktinfo_prepare(const struct sock *sk, struct sk_buff *skb)
                  * (e.g., process binds socket to eth0 for Tx which is
                  * redirected to loopback in the rtable/dst).
                  */
-                if (pktinfo->ipi_ifindex == LOOPBACK_IFINDEX || l3slave)
+                struct rtable *rt = skb_rtable(skb);
+                bool l3slave = ipv4_l3mdev_skb(IPCB(skb)->flags);
+
+                if (pktinfo->ipi_ifindex == LOOPBACK_IFINDEX)
                         pktinfo->ipi_ifindex = inet_iif(skb);
+                else if (l3slave && rt && rt->rt_iif)
+                        pktinfo->ipi_ifindex = rt->rt_iif;
 
                 pktinfo->ipi_spec_dst.s_addr = fib_compute_spec_dst(skb);
         } else {