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 baul.lee@xbow.com