CVE-2026-64009xfrm: Check for underflow in xfrm_state_mtu

Official description

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.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/8014f70c4e6e5ab101ae3860a614e65e988372e3
• https://git.kernel.org/stable/c/1021d2877b689a648b27815c854557a917122e93
• https://git.kernel.org/stable/c/2a41b1b31c61c52b972278ce1732a1443f5e89ed
• https://git.kernel.org/stable/c/fccd685b32df5aaf6bad4381eeda216468e283f0
• https://git.kernel.org/stable/c/820e501be8aee4b365d218d83227b314309c5fda
• https://git.kernel.org/stable/c/82ac903e0b519849647657b8c48d21237ada06a2
• https://git.kernel.org/stable/c/3db50ceeacb52806d8fe86fb1dfe944df0b9f789
• https://git.kernel.org/stable/c/742b04d0550b0ec89dcbc99537ec88653bd1ad90