[PATCH 3.16 139/157] floppy: fix out-of-bounds read in next_valid_format

[Ben Hutchings]

3.16.72-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Denis Efremov <efremov@xxxxxxxxx>

commit 5635f897ed83fd539df78e98ba69ee91592f9bb8 upstream.

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@xxxxxxxxx>
Tested-by: Willy Tarreau <w@xxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.16: Drop changes in compat_setdrvprm(), as compat
 ioctls go via fd_ioctl_locked() after translation in compat_ioctl.c.]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/drivers/block/floppy.c
+++ b/drivers/block/floppy.c
@@ -3383,6 +3383,20 @@ static int fd_getgeo(struct block_device
 	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)
 {
@@ -3509,6 +3523,8 @@ static int fd_locked_ioctl(struct block_
 		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: