firewire: cdev: fix information leak

A userspace client got to see uninitialized stack-allocated memory if it
specified an _IOC_READ type of ioctl and an argument size larger than
expected by firewire-core's ioctl handlers (but not larger than the
core's union ioctl_arg).

Fix this by clearing the requested buffer size to zero, but only at _IOR
ioctls.  This way, there is almost no runtime penalty to legitimate
ioctls.  The only legitimate _IOR is FW_CDEV_IOC_GET_CYCLE_TIMER with 12
or 16 bytes to memset.

[Another way to fix this would be strict checking of argument size (and
possibly direction) vs. command number.  However, we then need a lookup
table, and we need to allow for slight size deviations in case of 32bit
userland on 64bit kernel.]

Reported-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

diff --git a/drivers/firewire/core-cdev.c b/drivers/firewire/core-cdev.c
index 5eba9e0..0d3df09 100644 (file)
--- a/drivers/firewire/core-cdev.c
+++ b/drivers/firewire/core-cdev.c
@@ -1356,24 +1356,24 @@ static int dispatch_ioctl(struct client *client,
                return -ENODEV;
 
        if (_IOC_TYPE(cmd) != '#' ||
-           _IOC_NR(cmd) >= ARRAY_SIZE(ioctl_handlers))
+           _IOC_NR(cmd) >= ARRAY_SIZE(ioctl_handlers) ||
+           _IOC_SIZE(cmd) > sizeof(buffer))
                return -EINVAL;
 
-       if (_IOC_DIR(cmd) & _IOC_WRITE) {
-               if (_IOC_SIZE(cmd) > sizeof(buffer) ||
-                   copy_from_user(&buffer, arg, _IOC_SIZE(cmd)))
+       if (_IOC_DIR(cmd) == _IOC_READ)
+               memset(&buffer, 0, _IOC_SIZE(cmd));
+
+       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 (_IOC_SIZE(cmd) > sizeof(buffer) ||
-                   copy_to_user(arg, &buffer, _IOC_SIZE(cmd)))
+       if (_IOC_DIR(cmd) & _IOC_READ)
+               if (copy_to_user(arg, &buffer, _IOC_SIZE(cmd)))
                        return -EFAULT;
-       }
 
        return ret;
 }