Merge: gre: Fix again IPv6 link-local address generation.

MR: https://gitlab.com/redhat/centos-stream/src/kernel/centos-stream-10/-/merge_requests/893

JIRA: https://issues.redhat.com/browse/RHEL-69103
Upstream Status: linux.git

commit 3e6a024
Author: Guillaume Nault <gnault@redhat.com>
Date:   Sat May 3 00:57:52 2025 +0200

    gre: Fix again IPv6 link-local address generation.

    Use addrconf_addr_gen() to generate IPv6 link-local addresses on GRE
    devices in most cases and fall back to using add_v4_addrs() only in
    case the GRE configuration is incompatible with addrconf_addr_gen().

    GRE used to use addrconf_addr_gen() until commit e5dd729 ("ip/ip6_gre:
    use the same logic as SIT interfaces when computing v6LL address")
    restricted this use to gretap and ip6gretap devices, and created
    add_v4_addrs() (borrowed from SIT) for non-Ethernet GRE ones.

    The original problem came when commit 9af2851 ("addrconf: refuse
    isatap eui64 for INADDR_ANY") made __ipv6_isatap_ifid() fail when its
    addr parameter was 0. The commit says that this would create an invalid
    address, however, I couldn't find any RFC saying that the generated
    interface identifier would be wrong. Anyway, since gre over IPv4
    devices pass their local tunnel address to __ipv6_isatap_ifid(), that
    commit broke their IPv6 link-local address generation when the local
    address was unspecified.

    Then commit e5dd729 ("ip/ip6_gre: use the same logic as SIT
    interfaces when computing v6LL address") tried to fix that case by
    defining add_v4_addrs() and calling it to generate the IPv6 link-local
    address instead of using addrconf_addr_gen() (apart for gretap and
    ip6gretap devices, which would still use the regular
    addrconf_addr_gen(), since they have a MAC address).

    That broke several use cases because add_v4_addrs() isn't properly
    integrated into the rest of IPv6 Neighbor Discovery code. Several of
    these shortcomings have been fixed over time, but add_v4_addrs()
    remains broken on several aspects. In particular, it doesn't send any
    Router Sollicitations, so the SLAAC process doesn't start until the
    interface receives a Router Advertisement. Also, add_v4_addrs() mostly
    ignores the address generation mode of the interface
    (/proc/sys/net/ipv6/conf/*/addr_gen_mode), thus breaking the
    IN6_ADDR_GEN_MODE_RANDOM and IN6_ADDR_GEN_MODE_STABLE_PRIVACY cases.

    Fix the situation by using add_v4_addrs() only in the specific scenario
    where the normal method would fail. That is, for interfaces that have
    all of the following characteristics:

      * run over IPv4,
      * transport IP packets directly, not Ethernet (that is, not gretap
        interfaces),
      * tunnel endpoint is INADDR_ANY (that is, 0),
      * device address generation mode is EUI64.

    In all other cases, revert back to the regular addrconf_addr_gen().

    Also, remove the special case for ip6gre interfaces in add_v4_addrs(),
    since ip6gre devices now always use addrconf_addr_gen() instead.

    Note:
      This patch was originally applied as commit 183185a ("gre: Fix
      IPv6 link-local address generation."). However, it was then reverted
      by commit fc486c2 ("Revert "gre: Fix IPv6 link-local address
      generation."") because it uncovered another bug that ended up
      breaking net/forwarding/ip6gre_custom_multipath_hash.sh. That other
      bug has now been fixed by commit 4d0ab3a ("ipv6: Start path
      selection from the first nexthop"). Therefore we can now revive this
      GRE patch (no changes since original commit 183185a ("gre: Fix
      IPv6 link-local address generation.").

    Fixes: e5dd729 ("ip/ip6_gre: use the same logic as SIT interfaces when computing v6LL address")
    Signed-off-by: Guillaume Nault <gnault@redhat.com>
    Reviewed-by: Ido Schimmel <idosch@nvidia.com>
    Link: https://patch.msgid.link/a88cc5c4811af36007645d610c95102dccb360a6.1746225214.git.gnault@redhat.com
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Guillaume Nault <gnault@redhat.com>

Approved-by: Florian Westphal <fwestpha@redhat.com>
Approved-by: Xin Long <lxin@redhat.com>
Approved-by: CKI KWF Bot <cki-ci-bot+kwf-gitlab-com@redhat.com>

Merged-by: CKI GitLab Kmaint Pipeline Bot <26919896-cki-kmaint-pipeline-bot@users.noreply.gitlab.com>

Author: CKI KWF Bot

net/ipv6/addrconf.c

@@ -3237,16 +3237,13 @@ static void add_v4_addrs(struct inet6_dev *idev)
 	struct in6_addr addr;
 	struct net_device *dev;
 	struct net *net = dev_net(idev->dev);
-	int scope, plen, offset = 0;
+	int scope, plen;
 	u32 pflags = 0;
 
 	ASSERT_RTNL();
 
 	memset(&addr, 0, sizeof(struct in6_addr));
-	/* in case of IP6GRE the dev_addr is an IPv6 and therefore we use only the last 4 bytes */
-	if (idev->dev->addr_len == sizeof(struct in6_addr))
-		offset = sizeof(struct in6_addr) - 4;
-	memcpy(&addr.s6_addr32[3], idev->dev->dev_addr + offset, 4);
+	memcpy(&addr.s6_addr32[3], idev->dev->dev_addr, 4);
 
 	if (!(idev->dev->flags & IFF_POINTOPOINT) && idev->dev->type == ARPHRD_SIT) {
 		scope = IPV6_ADDR_COMPATv4;
@@ -3557,7 +3554,13 @@ static void addrconf_gre_config(struct net_device *dev)
 		return;
 	}
 
-	if (dev->type == ARPHRD_ETHER) {
+	/* Generate the IPv6 link-local address using addrconf_addr_gen(),
+	 * unless we have an IPv4 GRE device not bound to an IP address and
+	 * which is in EUI64 mode (as __ipv6_isatap_ifid() would fail in this
+	 * case). Such devices fall back to add_v4_addrs() instead.
+	 */
+	if (!(dev->type == ARPHRD_IPGRE && *(__be32 *)dev->dev_addr == 0 &&
+	      idev->cnf.addr_gen_mode == IN6_ADDR_GEN_MODE_EUI64)) {
 		addrconf_addr_gen(idev, true);
 		return;
 	}