Re: [PATCH net-next] tun: fix group permission check

[stsp <stsp2@yandex.ru>]

17.11.2024 18:04, Willem de Bruijn пишет:
> Stas Sergeev wrote:
>> Currently tun checks the group permission even if the user have matched.
>> Besides going against the usual permission semantic, this has a
>> very interesting implication: if the tun group is not among the
>> supplementary groups of the tun user, then effectively no one can
>> access the tun device. CAP_SYS_ADMIN still can, but its the same as
>> not setting the tun ownership.
>>
>> This patch relaxes the group checking so that either the user match
>> or the group match is enough. This avoids the situation when no one
>> can access the device even though the ownership is properly set.
>>
>> Also I simplified the logic by removing the redundant inversions:
>> tun_not_capable() --> !tun_capable()
>>
>> Signed-off-by: Stas Sergeev <stsp2@yandex.ru>
> This behavior goes back through many patches to commit 8c644623fe7e:
>
>      [NET]: Allow group ownership of TUN/TAP devices.
>
>      Introduce a new syscall TUNSETGROUP for group ownership setting of tap
>      devices. The user now is allowed to send packages if either his euid or
>      his egid matches the one specified via tunctl (via -u or -g
>      respecitvely). If both, gid and uid, are set via tunctl, both have to
>      match.
>
> The choice evidently was on purpose. Even if indeed non-standard.

So what would you suggest?
Added Guido Guenther <agx@sigxcpu.org> to CC
for an opinion.
The main problem here is that by
setting user and group properly, you
end up with device inaccessible by
anyone, unless the user belongs to
the tun group. I don't think someone
wants to set up inaccessible devices,
so this property doesn't seem useful.
OTOH if the user does have that group
in his list, then, AFAICT, adding such
group to tun changes nothing: neither
limits nor extends the scope.
If you had group already set and you
set also user, then you limit the scope,
but its the same as just setting user alone.
So I really can't think of any valid usage
scenario of setting both tun user and tun
group.