[PATCH net] vxlan: mdb: Fix use-after-free in vxlan_mdb_remote_src_del()

vxlan_mdb_is_valid_source(), which validates MDBE_ATTR_SOURCE and every
MDBE_ATTR_SRC_LIST member, accepts the all-zeros address.

A source list is only accepted on a (*, G) entry, whose source is the
all-zeros address, and for each member of the list an (S, G) entry is
derived from it by substituting the source. Entries are keyed by a plain
memcmp() of struct vxlan_mdb_entry_key, so if MDBE_ATTR_SOURCE is present
and holds the all-zeros address and the source list holds it as well, the
derived (S, G) key is byte-identical to the (*, G) key and resolves to the
same entry. Omitting MDBE_ATTR_SOURCE is not equivalent, as the key is
then left with a zero address family.

vxlan_mdb_remote_src_del() removes the forwarding entry of a source before
freeing the source entry:

	vxlan_mdb_remote_src_fwd_del(vxlan, group, remote, &ent->addr);
	vxlan_mdb_remote_src_entry_del(ent);

With the keys aliased, the first call deletes the remote of the entry that
owns 'ent' instead of a separate (S, G) entry, and frees 'ent'. The second
call then runs on the freed entry, and its hlist_del() reads ->pprev and
->next out of it and writes through them.

Adding the (*, G) entry with NLM_F_REPLACE and no source list marks the
all-zeros source for deletion and reaches this from the sweep at the end
of vxlan_mdb_remote_srcs_replace().

  BUG: KASAN: slab-use-after-free in __vxlan_mdb_add+0x1cd/0xd70
  Read of size 8 at addr ffff888102852500 by task poc/84
   __vxlan_mdb_add+0x1cd/0xd70
   vxlan_mdb_add+0xc0/0x140
   rtnl_mdb_add+0x157/0x2a0
   rtnetlink_rcv_msg+0x207/0x5a0
  Allocated by task 84:
   __kmalloc_cache_noprof+0x153/0x360
   vxlan_mdb_remote_srcs_add+0x2eb/0x440
   __vxlan_mdb_add+0x803/0xd70
  Freed by task 84:
   kfree+0x14c/0x3b0
   vxlan_mdb_remote_del+0x129/0x1a0
   __vxlan_mdb_del+0x4f/0xe0
   vxlan_mdb_remote_src_fwd_del.isra.0+0x162/0x1b0
   __vxlan_mdb_add+0x1c5/0xd70

The MDB operations are netns-scoped, so an unprivileged user can perform
them in a new user and network namespace.

Reject the all-zeros address in vxlan_mdb_is_valid_source(), which covers
both call sites. A (*, G) entry is expressed by omitting the source, so
nothing legitimate is refused.

Discovered by XBOW, triaged by Baul Lee [off-list ref]

Fixes: a3a48de5eade ("vxlan: mdb: Add MDB control path support")
Signed-off-by: Baul Lee <redacted>
---
 drivers/net/vxlan/vxlan_mdb.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/net/vxlan/vxlan_mdb.c b/drivers/net/vxlan/vxlan_mdb.c
index d71e1925ecfd..7986c5b5dc67 100644
--- a/drivers/net/vxlan/vxlan_mdb.c
+++ b/drivers/net/vxlan/vxlan_mdb.c

@@ -354,6 +354,10 @@ static bool vxlan_mdb_is_valid_source(const struct nlattr *attr, __be16 proto,
 			NL_SET_ERR_MSG_MOD(extack, "IPv4 multicast source address is not allowed");
 			return false;
 		}
+		if (nla_get_in_addr(attr) == htonl(INADDR_ANY)) {
+			NL_SET_ERR_MSG_MOD(extack, "IPv4 all-zeros source address is not allowed");
+			return false;
+		}
 		break;
 #if IS_ENABLED(CONFIG_IPV6)
 	case htons(ETH_P_IPV6): {

@@ -368,6 +372,10 @@ static bool vxlan_mdb_is_valid_source(const struct nlattr *attr, __be16 proto,
 			NL_SET_ERR_MSG_MOD(extack, "IPv6 multicast source address is not allowed");
 			return false;
 		}
+		if (ipv6_addr_any(&src)) {
+			NL_SET_ERR_MSG_MOD(extack, "IPv6 all-zeros source address is not allowed");
+			return false;
+		}
 		break;
 	}
 #endif