Fix mis-recognize that Windows-formatted SD card has valid partitions

For Windows-formatted SD card, it is not partitioned (no MBR), but its PBR has the
same boot signature (0xaa55) as MBR. We would easily mis-recognize this SD card has valid
partitions if we only check partition type. We add check by only accepting 0x00 (inactive)
/0x80 (active) for valid partition status.

Author: ccli8

features/storage/blockdevice/MBRBlockDevice.cpp

@@ -97,6 +97,18 @@ static int partition_absolute(
         memset(table->entries, 0, sizeof(table->entries));
     }
 
+    // For Windows-formatted SD card, it is not partitioned (no MBR), but its PBR has the
+    // same boot signature (0xaa55) as MBR. We would easily mis-recognize this SD card has valid
+    // partitions if we only check partition type. We add check by only accepting 0x00 (inactive)
+    // /0x80 (active) for valid partition status.
+    for (int i = 1; i <= 4; i++) {
+        if (table->entries[i-1].status != 0x00 &&
+            table->entries[i-1].status != 0x80) {
+            memset(table->entries, 0, sizeof(table->entries));
+            break;
+        }
+    }
+
     // Setup new partition
     MBED_ASSERT(part >= 1 && part <= 4);
     table->entries[part-1].status = 0x00; // inactive (not bootable)
@@ -241,6 +253,14 @@ int MBRBlockDevice::init()
         goto fail;
     }
 
+    // Check for valid partition status
+    // Same reason as in partition_absolute regarding Windows-formatted SD card
+    if (table->entries[_part-1].status != 0x00 &&
+        table->entries[_part-1].status != 0x80) {
+        err = BD_ERROR_INVALID_PARTITION;
+        goto fail;
+    }
+
     // Check for valid entry
     // 0x00 = no entry
     // 0x05, 0x0f = extended partitions, currently not supported