isdn: Disable IIOCDBGVAR

It was possible to directly leak the kernel address where the isdn_dev structure pointer was stored. This is a kernel ASLR bypass for anyone with access to the ioctl. The code had been present since the beginning of git history, though this shouldn't ever be needed for normal operation, therefore remove it.

---
netdev doesn't like explict stable markings, so I'll just ask here that it get included in -stable please. :)

---
 drivers/isdn/i4l/isdn_common.c | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

index 7a501dbe7123..6a5b3f00f9ad 100644
@@ -1640,13 +1640,7 @@ isdn_ioctl(struct file *file, uint cmd, ulong arg)
 			} else
 				return -EINVAL;
 		case IIOCDBGVAR:
-			if (arg) {
-				if (copy_to_user(argp, &dev, sizeof(ulong)))
-					return -EFAULT;
-				return 0;
-			} else
-				return -EINVAL;
-			break;
+			return -EINVAL;
 		default:
 			if ((cmd & IIOCDRVCTL) == IIOCDRVCTL)
 				cmd = ((cmd >> _IOC_NRSHIFT) & _IOC_NRMASK) & ISDN_DRVIOCTL_MASK;

2.17.1


-- 
Kees Cook
Pixel Security

From: Kees Cook <[email protected]>
Date: Wed, 15 Aug 2018 12:14:05 -0700

Applied and queued up for -stable, thanks :)