CVE-2026-46114: RDMA/rxe: Reject non-8-byte ATOMIC_WRITE payloads

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

RDMA/rxe: Reject non-8-byte ATOMICWRITE payloads

atomicwritereply() at drivers/infiniband/sw/rxe/rxeresp.c unconditionally dereferences 8 bytes at payloadaddr(pkt):

value = (u64 )payloadaddr(pkt);

checkrkey() previously accepted an ATOMICWRITE request with pktlen == resid == 0 because the length validation only compared pktlen against resid. A remote initiator that sets the RETH length to 0 therefore reaches atomicwritereply() with a zero-byte logical payload, and the responder reads sizeof(u64) bytes from past the logical end of the packet into skb->head tailroom, then writes those 8 bytes into the attacker's MR via rxemrdoatomicwrite(). That is a remote disclosure of 4 bytes of kernel tailroom per probe (the other 4 bytes are the packet's own trailing ICRC).

IBA oA19-28 defines ATOMICWRITE as exactly 8 bytes. Anything else is protocol-invalid. Hoist a strict length check into checkrkey() so the responder never reaches the unchecked dereference, and keep the existing WRITE-family length logic for the normal RDMA WRITE path.

Reproduced on mainline with an unmodified rxe driver: a sustained zero-length ATOMICWRITE probe repeatedly leaks adjacent skb head-buffer bytes into the attacker's MR, including recognisable kernel strings and partial kernel-direct-map pointer words. With this patch applied the responder rejects the PDU and the MR stays all-zero.