macvtap: fix uninitialized access on TUNSETIFF

flags field in ifreq is only 16 bit wide, but
we read it as a 32 bit value.
If userspace doesn't zero-initialize unused fields,
this will lead to failures.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 3 deletions

diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
index 60f7ee5fafbe..80fdb82352fa 100644
--- a/drivers/net/macvtap.c
+++ b/drivers/net/macvtap.c
@@ -999,7 +999,7 @@ static long macvtap_ioctl(struct file *file, unsigned int cmd,
         void __user *argp = (void __user *)arg;
         struct ifreq __user *ifr = argp;
         unsigned int __user *up = argp;
-        unsigned int u;
+        unsigned short u;
         int __user *sp = argp;
         int s;
         int ret;
@@ -1014,7 +1014,7 @@ static long macvtap_ioctl(struct file *file, unsigned int cmd,
                 if ((u & ~MACVTAP_FEATURES) != (IFF_NO_PI | IFF_TAP))
                         ret = -EINVAL;
                 else
-                        q->flags = u;
+                        q->flags = (q->flags & ~MACVTAP_FEATURES) | u;
 
                 return ret;
 
@@ -1027,8 +1027,9 @@ static long macvtap_ioctl(struct file *file, unsigned int cmd,
                 }
 
                 ret = 0;
+                u = q->flags;
                 if (copy_to_user(&ifr->ifr_name, vlan->dev->name, IFNAMSIZ) ||
-                    put_user(q->flags, &ifr->ifr_flags))
+                    put_user(u, &ifr->ifr_flags))
                         ret = -EFAULT;
                 macvtap_put_vlan(vlan);
                 rtnl_unlock();