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

vxlan: use one headroom snapshot for neighbour replies

vxlan na create() samples LL RESERVED SPACE() to size the reply skb and then samples it again to reserve headroom. A concurrent vxlan changelink() can update needed headroom between the two reads, creating a TOCTOU race. The second value can exceed the allocation and make the Ethernet header write out of bounds.

The race is reproducible on the unpatched kernel. It occurred when vxlan na create() generated a neighbour reply while vxlan changelink() changed the link headroom. KASAN caught a four-byte write two bytes beyond a 704-byte skbuff small head allocation.

Snapshot the headroom once and use that value for both allocation and reservation.