Possible mistake in commit 3ca459eaba1b ("tun: fix group permission check")

[Willem de Bruijn]

stsp wrote:
> 27.01.2025 12:10, Ondrej Mosnacek пишет:
> > Hello,
> >
> > It looks like the commit in $SUBJ may have introduced an unintended
> > change in behavior. According to the commit message, the intent was to
> > require just one of {user, group} to match instead of both, which
> > sounds reasonable, but the commit also changes the behavior for when
> > neither of tun->owner and tun->group is set. Before the commit the
> > access was always allowed, while after the commit CAP_NET_ADMIN is
> > required in this case.
> >
> > I'm asking because the tun_tap subtest of selinux-testuite [1] started
> > to fail after this commit (it assumed CAP_NET_ADMIN was not needed),
> > so I'm trying to figure out if we need to change the test or if it
> > needs to be fixed in the kernel.
> >
> > Thanks,
> >
> > [1] https://github.com/SELinuxProject/selinux-testsuite/
> >
> Hi, IMHO having the persistent
> TAP device inaccessible by anyone
> but the CAP_NET_ADMIN is rather
> useless, so the compatibility should
> be restored on the kernel side.
> I'd raise the questions about adding
> the CAP_NET_ADMIN checks into
> TUNSETOWNER and/or TUNSETPERSIST,
> but this particular change to TUNSETIFF,
> at least on my side, was unintentional.
> 
> Sorry about that. :(

Thanks for the report Ondrej.

Agreed that we need to reinstate this. I suggest this explicit
extra branch after the more likely cases:

        @@ -585,6 +585,9 @@ static inline bool tun_capable(struct tun_struct *tun)
        		return 1;
        	if (gid_valid(tun->group) && in_egroup_p(tun->group))
        		return 1;
        +       if (!uid_valid(tun->owner) && !gid_valid(tun->group))
        +               return 1;
        +
        	return 0;
         }

The intent clearly has always been to allow access if owner and group
are not explicitly set.

It's easy to see when group support was added in commit 8c644623fe7e
("[NET]: Allow group ownership of TUN/TAP devices."), and the even
simpler check before that:

                /* Check permissions */
-               if (tun->owner != -1 &&
-                   current->euid != tun->owner && !capable(CAP_NET_ADMIN))
+               if (((tun->owner != -1 &&
+                     current->euid != tun->owner) ||
+                    (tun->group != -1 &&
+                     current->egid != tun->group)) &&
+                    !capable(CAP_NET_ADMIN))
                        return -EPERM;