CVE-2026-98122

In the Linux kernel, the following vulnerability has been resolved:

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 <[email protected]>