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

commit 1114c87 upstream.

atomic_write_reply() at drivers/infiniband/sw/rxe/rxe_resp.c
unconditionally dereferences 8 bytes at payload_addr(pkt):

    value = *(u64 *)payload_addr(pkt);

check_rkey() previously accepted an ATOMIC_WRITE 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
atomic_write_reply() 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
rxe_mr_do_atomic_write(). 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 ATOMIC_WRITE as exactly 8 bytes. Anything else is
protocol-invalid. Hoist a strict length check into check_rkey() 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 ATOMIC_WRITE 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.

Cc: stable@vger.kernel.org
Fixes: 034e285 ("RDMA/rxe: Make responder support atomic write on RC service")
Link: https://patch.msgid.link/r/20260418162141.3610201-1-michael.bommarito@gmail.com
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Reviewed-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: mjbommar

drivers/infiniband/sw/rxe/rxe_resp.c

@@ -526,7 +526,19 @@ static enum resp_states check_rkey(struct rxe_qp *qp,
 	}
 
 skip_check_range:
-	if (pkt->mask & (RXE_WRITE_MASK | RXE_ATOMIC_WRITE_MASK)) {
+	if (pkt->mask & RXE_ATOMIC_WRITE_MASK) {
+		/* IBA oA19-28: ATOMIC_WRITE payload is exactly 8 bytes.
+		 * Reject any other length before the responder reads
+		 * sizeof(u64) bytes from payload_addr(pkt); a shorter
+		 * payload would read past the logical end of the packet
+		 * into skb->head tailroom.
+		 */
+		if (resid != sizeof(u64) || pktlen != sizeof(u64) ||
+		    bth_pad(pkt)) {
+			state = RESPST_ERR_LENGTH;
+			goto err;
+		}
+	} else if (pkt->mask & RXE_WRITE_MASK) {
 		if (resid > mtu) {
 			if (pktlen != mtu || bth_pad(pkt)) {
 				state = RESPST_ERR_LENGTH;