Fix ifa refcount leak in ifa_ifwithnet()

In 4f6c66cc9c75c8, ifa_ifwithnet() was changed to no longer
ifa_ref() the returned ifaddr, and instead the caller was required
to stay in the net_epoch for as long as they wanted the ifaddr
to remain valid.  However, this missed the case where an AF_LINK
lookup would call ifaddr_byindex(), which still does ifa_ref()
the ifaddr.  This would cause a refcount leak.

Fix this by inlining the relevant parts of ifaddr_byindex() here,
with the ifa_ref() call removed.  This also avoids an unnecessary
entry and exit from the net_epoch for this case.

I've audited all in-tree consumers of ifa_ifwithnet() that could
possibly perform an AF_LINK lookup and confirmed that none of them
will expect the ifaddr to have a reference that they need to
release.

MFC after: 2 months
Sponsored by: Dell Inc
Differential Revision:	https://reviews.freebsd.org/D28705
Reviewed by: melifaro

1 files changed, 9 insertions, 3 deletions

diff --git a/sys/net/if.c b/sys/net/if.c
index 84436c0074c8..8700485ff6a5 100644
--- a/sys/net/if.c
+++ b/sys/net/if.c
@@ -1953,6 +1953,7 @@ ifa_ifwithnet(const struct sockaddr *addr, int ignore_ptp, int fibnum)
 	struct ifaddr *ifa_maybe = NULL;
 	u_int af = addr->sa_family;
 	const char *addr_data = addr->sa_data, *cplim;
+	const struct sockaddr_dl *sdl;
 
 	NET_EPOCH_ASSERT();
 	/*
@@ -1960,9 +1961,14 @@ ifa_ifwithnet(const struct sockaddr *addr, int ignore_ptp, int fibnum)
 	 * so do that if we can.
 	 */
 	if (af == AF_LINK) {
-	    const struct sockaddr_dl *sdl = (const struct sockaddr_dl *)addr;
-	    if (sdl->sdl_index && sdl->sdl_index <= V_if_index)
-		return (ifaddr_byindex(sdl->sdl_index));
+		sdl = (const struct sockaddr_dl *)addr;
+		if (sdl->sdl_index && sdl->sdl_index <= V_if_index) {
+			ifp = ifnet_byindex(sdl->sdl_index);
+			if (ifp == NULL)
+				return (NULL);
+
+			return (ifp->if_addr);
+		}
 	}
 
 	/*