tun: Check supplemental groups in TUN/TAP driver.

Michael Tokarev wrote:
[]
> 2, and this is the main one: How about supplementary groups?
>
> Here I have a valid usage case: a group of testers running various
> versions of windows using KVM (kernel virtual machine), 1 at a time,
> to test some software.  kvm is set up to use bridge with a tap device
> (there should be a way to connect to the machine).  Anyone on that group
> has to be able to start/stop the virtual machines.
>
> My first attempt - pretty obvious when I saw -g option of tunctl - is
> to add group ownership for the tun device and add a supplementary group
> to each user (their primary group should be different).  But that fails,
> since kernel only checks for egid, not any other group ids.
>
> What's the reasoning to not allow supplementary groups and to only check
> for egid?
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -123,7 +123,7 @@ static int tun_attach(struct tun_struct *tun, struct file *file)
 
 	/* Check permissions */
 	if (((tun->owner != -1 && cred->euid != tun->owner) ||
-	     (tun->group != -1 && cred->egid != tun->group)) &&
+	     (tun->group != -1 && !in_egroup_p(tun->group))) &&
 		!capable(CAP_NET_ADMIN))
 		return -EPERM;