io_uring: fix not locked access to fixed buf table

We can look inside the fixed buffer table only while holding
->uring_lock, however in some cases we don't do the right async prep for
IORING_OP_{WRITE,READ}_FIXED ending up with NULL req->imu forcing making
an io-wq worker to try to resolve the fixed buffer without proper
locking.

Move req->imu setup into early req init paths, i.e. io_prep_rw(), which
is called unconditionally for rw requests and under uring_lock.

Fixes: 634d00df ("io_uring: add full-fledged dynamic buffers support")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>

fs/io_uring.c

@@ -3636,6 +3636,20 @@ static int io_prep_rw(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 	int ret;
 
 	kiocb->ki_pos = READ_ONCE(sqe->off);
+	/* used for fixed read/write too - just read unconditionally */
+	req->buf_index = READ_ONCE(sqe->buf_index);
+
+	if (req->opcode == IORING_OP_READ_FIXED ||
+	    req->opcode == IORING_OP_WRITE_FIXED) {
+		struct io_ring_ctx *ctx = req->ctx;
+		u16 index;
+
+		if (unlikely(req->buf_index >= ctx->nr_user_bufs))
+			return -EFAULT;
+		index = array_index_nospec(req->buf_index, ctx->nr_user_bufs);
+		req->imu = ctx->user_bufs[index];
+		io_req_set_rsrc_node(req, ctx, 0);
+	}
 
 	ioprio = READ_ONCE(sqe->ioprio);
 	if (ioprio) {
@@ -3648,12 +3662,9 @@ static int io_prep_rw(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 		kiocb->ki_ioprio = get_current_ioprio();
 	}
 
-	req->imu = NULL;
 	req->rw.addr = READ_ONCE(sqe->addr);
 	req->rw.len = READ_ONCE(sqe->len);
 	req->rw.flags = READ_ONCE(sqe->rw_flags);
-	/* used for fixed read/write too - just read unconditionally */
-	req->buf_index = READ_ONCE(sqe->buf_index);
 	return 0;
 }
 
@@ -3785,20 +3796,9 @@ static int __io_import_fixed(struct io_kiocb *req, int rw, struct iov_iter *iter
 static int io_import_fixed(struct io_kiocb *req, int rw, struct iov_iter *iter,
 			   unsigned int issue_flags)
 {
-	struct io_mapped_ubuf *imu = req->imu;
-	u16 index, buf_index = req->buf_index;
-
-	if (likely(!imu)) {
-		struct io_ring_ctx *ctx = req->ctx;
-
-		if (unlikely(buf_index >= ctx->nr_user_bufs))
-			return -EFAULT;
-		io_req_set_rsrc_node(req, ctx, issue_flags);
-		index = array_index_nospec(buf_index, ctx->nr_user_bufs);
-		imu = READ_ONCE(ctx->user_bufs[index]);
-		req->imu = imu;
-	}
-	return __io_import_fixed(req, rw, iter, imu);
+	if (WARN_ON_ONCE(!req->imu))
+		return -EFAULT;
+	return __io_import_fixed(req, rw, iter, req->imu);
 }
 
 static int io_buffer_add_list(struct io_ring_ctx *ctx,