floppy: fix out-of-bounds read in next_valid_format

This fixes a global out-of-bounds read access in the next_valid_format
function of the floppy driver.

The values from autodetect field of the struct floppy_drive_params are
used as indices for the floppy_type array in the next_valid_format
function 'floppy_type[DP->autodetect[probed_format]].sect'.

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

CAP_SYS_ADMIN is required to call FDSETDRVPRM.

The patch adds the check for values of the autodetect 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

@@ -3380,6 +3380,20 @@ static int fd_getgeo(struct block_device *bdev, struct hd_geometry *geo)
 	return 0;
 }
 
+static bool valid_floppy_drive_params(const short autodetect[8])
+{
+	size_t floppy_type_size = ARRAY_SIZE(floppy_type);
+	size_t i = 0;
+
+	for (i = 0; i < 8; ++i) {
+		if (autodetect[i] < 0 ||
+		    autodetect[i] >= floppy_type_size)
+			return false;
+	}
+
+	return true;
+}
+
 static int fd_locked_ioctl(struct block_device *bdev, fmode_t mode, unsigned int cmd,
 		    unsigned long param)
 {
@@ -3506,6 +3520,8 @@ 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))
+			return -EINVAL;
 		*UDP = inparam.dp;
 		break;
 	case FDGETDRVPRM:
@@ -3703,6 +3719,8 @@ 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))
+		return -EINVAL;
 	mutex_lock(&floppy_mutex);
 	UDP->cmos = v.cmos;
 	UDP->max_dtr = v.max_dtr;