drivers/staging/exfat - by default, prohibit mount of fat/vfat

Concerns have been raised about the exfat driver accidentally mounting
fat/vfat file systems.  Add an extra configure option to help prevent that.
Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Link: https://lore.kernel.org/r/245727.1567183359@turing-policeSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/exfat/Kconfig

@@ -4,6 +4,14 @@ config EXFAT_FS
 	help
 	  This adds support for the exFAT file system.
 
+config EXFAT_DONT_MOUNT_VFAT
+	bool "Prohibit mounting of fat/vfat filesysems by exFAT"
+	default y
+	help
+	  By default, the exFAT driver will only mount exFAT filesystems, and refuse
+	  to mount fat/vfat filesystems.  Set this to 'n' to allow the exFAT driver
+	  to mount these filesystems.
+
 config EXFAT_DISCARD
 	bool "enable discard support"
 	depends on EXFAT_FS

drivers/staging/exfat/exfat_super.c

@@ -486,10 +486,16 @@ static int ffsMountVol(struct super_block *sb)
 			break;
 
 	if (i < 53) {
+#ifdef CONFIG_EXFAT_DONT_MOUNT_VFAT
+		ret = -EINVAL;
+		printk(KERN_INFO "EXFAT: Attempted to mount VFAT filesystem\n");
+		goto out;
+#else
 		if (GET16(p_pbr->bpb + 11)) /* num_fat_sectors */
 			ret = fat16_mount(sb, p_pbr);
 		else
 			ret = fat32_mount(sb, p_pbr);
+#endif
 	} else {
 		ret = exfat_mount(sb, p_pbr);
 	}