sd: Avoid sending medium write commands if device is write protected

The SYNCHRONIZE_CACHE command is a medium write command and hence can
fail when the device is write protected. Avoid sending such commands by
making sure that write-cache-enable is disabled even though the device
claim to support it.
Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>
Signed-off-by: Dolev Raviv <draviv@codeaurora.org>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Venkatesh Srinivas <venkateshs@google.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/scsi/sd.c

@@ -185,7 +185,7 @@ cache_type_store(struct device *dev, struct device_attribute *attr,
 	if (ct < 0)
 		return -EINVAL;
 	rcd = ct & 0x01 ? 1 : 0;
-	wce = ct & 0x02 ? 1 : 0;
+	wce = (ct & 0x02) && !sdkp->write_prot ? 1 : 0;
 
 	if (sdkp->cache_override) {
 		sdkp->WCE = wce;
@@ -2490,6 +2490,10 @@ sd_read_cache_type(struct scsi_disk *sdkp, unsigned char *buffer)
 			sdkp->DPOFUA = 0;
 		}
 
+		/* No cache flush allowed for write protected devices */
+		if (sdkp->WCE && sdkp->write_prot)
+			sdkp->WCE = 0;
+
 		if (sdkp->first_scan || old_wce != sdkp->WCE ||
 		    old_rcd != sdkp->RCD || old_dpofua != sdkp->DPOFUA)
 			sd_printk(KERN_NOTICE, sdkp,