git: 2ce85919bbba - main - Add net.inet.ip.source_address_validation

The branch main has been updated by glebius:

URL: https://cgit.FreeBSD.org/src/commit/?id=2ce85919bbba9e29ee85508abb4ba0c662c2b080

commit 2ce85919bbba9e29ee85508abb4ba0c662c2b080
Author:     Gleb Smirnoff <glebius@FreeBSD.org>
AuthorDate: 2021-11-12 17:00:33 +0000
Commit:     Gleb Smirnoff <glebius@FreeBSD.org>
CommitDate: 2021-11-12 17:00:33 +0000

    Add net.inet.ip.source_address_validation
    
    Drop packets arriving from the network that have our source IP
    address.  If maliciously crafted they can create evil effects
    like an RST exchange between two of our listening TCP ports.
    Such packets just can't be legitimate.  Enable the tunable
    by default.  Long time due for a modern Internet host.
    
    Reviewed by:            donner, melifaro
    Differential revision:  https://reviews.freebsd.org/D32914
---
 share/man/man4/inet.4  |  8 ++++++++
 sys/netinet/ip_input.c | 16 ++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/share/man/man4/inet.4 b/share/man/man4/inet.4
index 7a8a653dff25..dbab301302b1 100644
--- a/share/man/man4/inet.4
+++ b/share/man/man4/inet.4
@@ -219,6 +219,14 @@ or destination address rewriting
 .Xr pfil 4
 filters may override and bypass this check.
 Disabled by default.
+.It Va ip.source_address_validation
+Boolean: perform source address validation for packets destined for the local
+host.
+Consider this as following Section 3.2 of RFC3704/BCP84, where we treat local
+host as our own infrastructure.
+This has no effect on packets to be forwarded, so don't consider it as
+anti-spoof feature for a router.
+Enabled by default.
 .It Va ip.rfc6864
 Boolean: control IP IDs generation behaviour.
 True value enables RFC6864 support, which specifies that IP ID field of
diff --git a/sys/netinet/ip_input.c b/sys/netinet/ip_input.c
index a678c15caad5..9db1f8c6f2e7 100644
--- a/sys/netinet/ip_input.c
+++ b/sys/netinet/ip_input.c
@@ -124,6 +124,12 @@ SYSCTL_BOOL(_net_inet_ip, OID_AUTO, rfc1122_strong_es,
     CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(ip_strong_es), false,
     "Packet's IP destination address must match address on arrival interface");
 
+VNET_DEFINE_STATIC(bool, ip_sav) = true;
+#define	V_ip_sav	VNET(ip_sav)
+SYSCTL_BOOL(_net_inet_ip, OID_AUTO, source_address_validation,
+    CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(ip_sav), true,
+    "Drop incoming packets with source address that is a local address");
+
 VNET_DEFINE(pfil_head_t, inet_pfil_head);	/* Packet filter hooks */
 
 static struct netisr_handler ip_nh = {
@@ -683,6 +689,16 @@ passin:
 			goto bad;
 		}
 
+		/*
+		 * net.inet.ip.source_address_validation: drop incoming
+		 * packets that pretend to be ours.
+		 */
+		if (V_ip_sav && !(ifp->if_flags & IFF_LOOPBACK) &&
+		    __predict_false(in_localip_fib(ip->ip_src, ifp->if_fib))) {
+			IPSTAT_INC(ips_badaddr);
+			goto bad;
+		}
+
 		counter_u64_add(ia->ia_ifa.ifa_ipackets, 1);
 		counter_u64_add(ia->ia_ifa.ifa_ibytes, m->m_pkthdr.len);
 		goto ours;