Description of problem: There is a problem with the ioctl subsystem for drm, though it is most explicitly exposed by the intel GEM driver. Under driver-defined ioctls, drm does not sanitize the ioctl command, allowing the caller to specify how much memory should be kmalloc'd and copied back to the caller, regardless of what the driver ioctl actually does (it doesn't even need to succeed).

drivers/gpu/drm/drmdrv.c

long drmioctl(struct file filp, unsigned int cmd, unsigned long arg) ... unsigned int nr = DRMIOCTLNR(cmd); ... if ((nr >= DRMCOMMANDBASE) && (nr < DRMCOMMANDEND) && (nr < DRMCOMMANDBASE + dev->driver->numioctls)) ioctl = &dev->driver->ioctls[nr - DRMCOMMANDBASE]; ... if (cmd & (IOCIN | IOCOUT)) { if (IOCSIZE(cmd) <= sizeof(stackkdata)) { kdata = stackkdata; } else { kdata = kmalloc(IOCSIZE(cmd), GFPKERNEL); ... } } ... retcode = func(dev, kdata, filepriv); ... if (cmd & IOCOUT) { if (copytouser((void user )arg, kdata, IOCSIZE(cmd)) != 0) retcode = -EFAULT; }

"cmd" is caller-controlled, and can do whatever it likes for IOCSIZE(cmd), IOCIN and IOCOUT, resulting in leakage of previously freed kernel heap memory contents up to 16K in size.