io_uring: propagate array_index_nospec opcode into req->opcode

Commit 1e988c3 ("io_uring: prevent opcode speculation") added
array_index_nospec() to io_init_req(), but applied it only to a local
opcode variable. req->opcode is initialized from sqe->opcode before the
bounds check and remains the raw value.

Keep req->opcode as the canonical opcode in io_init_req(): reject
out-of-range values architecturally, then write the array_index_nospec()
result back to req->opcode before any table lookup. This keeps downstream
users of req->opcode from observing the raw user byte on a mispredicted
path.

No functional change: array_index_nospec() is a no-op for opcodes in
[0, IORING_OP_LAST), and out-of-range opcodes are still rejected at the
bounds check above the assignment.

Fixes: 1e988c3 ("io_uring: prevent opcode speculation")
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Link: https://patch.msgid.link/20260517213010.696135-1-michael.bommarito@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: mjbommar

io_uring/io_uring.c

@@ -1738,10 +1738,9 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
 	const struct io_issue_def *def;
 	unsigned int sqe_flags;
 	int personality;
-	u8 opcode;
 
 	req->ctx = ctx;
-	req->opcode = opcode = READ_ONCE(sqe->opcode);
+	req->opcode = READ_ONCE(sqe->opcode);
 	/* same numerical values with corresponding REQ_F_*, safe to copy */
 	sqe_flags = READ_ONCE(sqe->flags);
 	req->flags = (__force io_req_flags_t) sqe_flags;
@@ -1751,13 +1750,13 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
 	req->cancel_seq_set = false;
 	req->async_data = NULL;
 
-	if (unlikely(opcode >= IORING_OP_LAST)) {
+	if (unlikely(req->opcode >= IORING_OP_LAST)) {
 		req->opcode = 0;
 		return io_init_fail_req(req, -EINVAL);
 	}
-	opcode = array_index_nospec(opcode, IORING_OP_LAST);
+	req->opcode = array_index_nospec(req->opcode, IORING_OP_LAST);
 
-	def = &io_issue_defs[opcode];
+	def = &io_issue_defs[req->opcode];
 	if (def->is_128 && !(ctx->flags & IORING_SETUP_SQE128)) {
 		/*
 		 * A 128b op on a non-128b SQ requires mixed SQE support as