gre: Fix 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 e5dd729460ca
("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 9af28511be10 ("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 e5dd729460ca ("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.

Fixes: e5dd729460ca ("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/559c32ce5c9976b269e6337ac9abb6a96abe5096.1741375285.git.gnault@redhat.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index ac8cc10765360fe5a8c61d4654308b7559a38c09..8b6258819dadecc9770b177e3e5181775356b384 100644 (file)
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -3209,16 +3209,13 @@ 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;
@@ -3529,7 +3526,13 @@ 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;
        }