CVE-2026-64009

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

xfrm: Check for underflow in xfrm_state_mtu

Leo Lin reported OOB write issue in esp component:

xfrm_state_mtu() returns u32 but performs its arithmetic in unsigned

modulo-2^32 space using an attacker-influenced "header_len + authsize +

net_adj" subtracted from a small "mtu" argument. A nobody user can

install an IPv4 ESP tunnel SA with a large authentication key

(XFRMA_ALG_AUTH_TRUNC, e.g. hmac(sha512), 64-byte key, 64-byte trunc),

configure a small interface MTU (68 bytes), and set XFRMA_TFCPAD to a

large value. When a single UDP datagram is then sent through the

tunnel, xfrm_state_mtu() underflows to a near-2^32 value, and

esp_output() consumes it as a signed int via:

padto = min(x->tfcpad, xfrm_state_mtu(x, mtu_cached))

esp.tfclen = padto - skb->len (assigned to int)

esp.tfclen ends up negative (e.g. -207). It is sign-extended to size_t

when passed to memset() inside esp_output_fill_trailer(), producing a

~16 EB write of zeroes at skb_tail_pointer(skb). KASAN logs it as

"Write of size 18446744073709551537 at addr ffff888...".

Check for underflow and return 1. This causes the sendmsg attempt to

fail with ENETUNREACH.