stsp wrote:

27.01.2025 12:10, Ondrej Mosnacek пишет:

quoted

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;