mmc: mxcmmc: fix race conditions for host->req and host->data access

mxcmci_dma_callback() is invoked by DMA drivers in soft-irq
context and can be interrupted by the mxcmci_irq() interrupt
which can finish the mmc request or data transfer and set
host->req or host->data pointers to NULL. Then mxcmci_data_done()
crashes with a null pointer dereferences. Protect all accesses
to host->req and host->data by spin locks.

Also check host->data pointer in mxcmci_watchdog() before
dereferencing it.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Chris Ball <cjb@laptop.org>

drivers/mmc/host/mxcmmc.c

@@ -623,24 +623,40 @@ static void mxcmci_datawork(struct work_struct *work)
 
 static void mxcmci_data_done(struct mxcmci_host *host, unsigned int stat)
 {
-	struct mmc_data *data = host->data;
+	struct mmc_request *req;
 	int data_error;
+	unsigned long flags;
+
+	spin_lock_irqsave(&host->lock, flags);
 
-	if (!data)
+	if (!host->data) {
+		spin_unlock_irqrestore(&host->lock, flags);
 		return;
+	}
+
+	if (!host->req) {
+		spin_unlock_irqrestore(&host->lock, flags);
+		return;
+	}
+
+	req = host->req;
+	if (!req->stop)
+		host->req = NULL; /* we will handle finish req below */
 
 	data_error = mxcmci_finish_data(host, stat);
 
+	spin_unlock_irqrestore(&host->lock, flags);
+
 	mxcmci_read_response(host, stat);
 	host->cmd = NULL;
 
-	if (host->req->stop) {
-		if (mxcmci_start_cmd(host, host->req->stop, 0)) {
-			mxcmci_finish_request(host, host->req);
+	if (req->stop) {
+		if (mxcmci_start_cmd(host, req->stop, 0)) {
+			mxcmci_finish_request(host, req);
 			return;
 		}
 	} else {
-		mxcmci_finish_request(host, host->req);
+		mxcmci_finish_request(host, req);
 	}
 }
 
@@ -931,7 +947,8 @@ static void mxcmci_watchdog(unsigned long data)
 
 	/* Mark transfer as erroneus and inform the upper layers */
 
-	host->data->error = -ETIMEDOUT;
+	if (host->data)
+		host->data->error = -ETIMEDOUT;
 	host->req = NULL;
 	host->cmd = NULL;
 	host->data = NULL;