[PATCH RFC v1b] firewire: cdev: prevent kernel stack leaking into ioctl arguments

[Stefan Richter]

Found by the UC-KLEE tool:  A user could supply less input to
firewire-cdev ioctls than write- or write/read-type ioctl handlers
expect.  The handlers used data from uninitialized kernel stack then.

This could partially leak back to the user if the kernel subsequently
generated fw_cdev_event_'s (to be read from the firewire-cdev fd)
which notably would contain the _u64 closure field which many of the
ioctl argument structures contain.

The fact that the handlers would act on random garbage input is a
lesser issue since all handlers must check their input anyway.

Remarks:
  - There was never any leak from kernel stack to the ioctl output
    buffer itself.  IOW, it was not possible to read kernel stack by a
    read-type or write/read-type ioctl alone; the leak could at most
    happen in combination with read()ing subsequent event data.
  - The affected character device file interface is specified in
    include/uapi/linux/firewire-cdev.h.  An overview is given in
    Documentation/ABI/stable/firewire-cdev.

This fix simply always null-initializes the entire ioctl argument buffer
regardless of the actual length of expected user input.  That is, a
runtime overhead of memset(..., 40) is added to each firewirew-cdev
ioctl() call.

Reported-by: David Ramos <daramos@xxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Stefan Richter <stefanr@xxxxxxxxxxxxxxxxx>
---
 drivers/firewire/core-cdev.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/drivers/firewire/core-cdev.c
+++ b/drivers/firewire/core-cdev.c
@@ -1625,32 +1625,31 @@ static int (* const ioctl_handlers[])(st
 
 static int dispatch_ioctl(struct client *client,
 			  unsigned int cmd, void __user *arg)
 {
 	union ioctl_arg buffer;
 	int ret;
 
 	if (fw_device_is_shutdown(client->device))
 		return -ENODEV;
 
 	if (_IOC_TYPE(cmd) != '#' ||
 	    _IOC_NR(cmd) >= ARRAY_SIZE(ioctl_handlers) ||
 	    _IOC_SIZE(cmd) > sizeof(buffer))
 		return -ENOTTY;
 
-	if (_IOC_DIR(cmd) == _IOC_READ)
-		memset(&buffer, 0, _IOC_SIZE(cmd));
+	memset(&buffer, 0, sizeof(buffer));
 
 	if (_IOC_DIR(cmd) & _IOC_WRITE)
 		if (copy_from_user(&buffer, arg, _IOC_SIZE(cmd)))
 			return -EFAULT;
 
 	ret = ioctl_handlers[_IOC_NR(cmd)](client, &buffer);
 	if (ret < 0)
 		return ret;
 
 	if (_IOC_DIR(cmd) & _IOC_READ)
 		if (copy_to_user(arg, &buffer, _IOC_SIZE(cmd)))
 			return -EFAULT;
 
 	return ret;
 }


-- 
Stefan Richter
-=====-====- =-== -=-==
http://arcgraph.de/sr/
--
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