floppy: fix invalid pointer dereference in drive_name

This fixes the invalid pointer dereference in the drive_name function of
the floppy driver.

The native_format field of the struct floppy_drive_params is used as
floppy_type array index in the drive_name function.  Thus, the field
should be checked the same way as the autodetect field.

To trigger the bug, one could use a value out of range and set the drive
parameters with the FDSETDRVPRM ioctl.  Next, FDGETDRVTYP ioctl should
be used to call the drive_name.  A floppy disk is not required to be
inserted.

CAP_SYS_ADMIN is required to call FDSETDRVPRM.

The patch adds the check for a value of the native_format field to be in
the '0 <= x < ARRAY_SIZE(floppy_type)' range of the floppy_type array
indices.

The bug was found by syzkaller.
Signed-off-by: Denis Efremov <efremov@ispras.ru>
Tested-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/block/floppy.c

@@ -3380,7 +3380,8 @@ static int fd_getgeo(struct block_device *bdev, struct hd_geometry *geo)
 	return 0;
 }
 
-static bool valid_floppy_drive_params(const short autodetect[8])
+static bool valid_floppy_drive_params(const short autodetect[8],
+		int native_format)
 {
 	size_t floppy_type_size = ARRAY_SIZE(floppy_type);
 	size_t i = 0;
@@ -3391,6 +3392,9 @@ static bool valid_floppy_drive_params(const short autodetect[8])
 			return false;
 	}
 
+	if (native_format < 0 || native_format >= floppy_type_size)
+		return false;
+
 	return true;
 }
 
@@ -3520,7 +3524,8 @@ static int fd_locked_ioctl(struct block_device *bdev, fmode_t mode, unsigned int
 		SUPBOUND(size, strlen((const char *)outparam) + 1);
 		break;
 	case FDSETDRVPRM:
-		if (!valid_floppy_drive_params(inparam.dp.autodetect))
+		if (!valid_floppy_drive_params(inparam.dp.autodetect,
+				inparam.dp.native_format))
 			return -EINVAL;
 		*UDP = inparam.dp;
 		break;
@@ -3719,7 +3724,7 @@ static int compat_setdrvprm(int drive,
 		return -EPERM;
 	if (copy_from_user(&v, arg, sizeof(struct compat_floppy_drive_params)))
 		return -EFAULT;
-	if (!valid_floppy_drive_params(v.autodetect))
+	if (!valid_floppy_drive_params(v.autodetect, v.native_format))
 		return -EINVAL;
 	mutex_lock(&floppy_mutex);
 	UDP->cmos = v.cmos;