Re: [PATCH v3 3/4] Allow unprivileged CLONE_NEWUTS and CLONE_NEWIPC with no_new_privs

[Andy Lutomirski <luto@amacapital.net>]

On Wed, Feb 1, 2012 at 11:02 AM, Kees Cook <keescook@chromium.org> wrote:
> On Mon, Jan 30, 2012 at 8:17 AM, Andy Lutomirski <luto@amacapital.net> wrote:
>> They are normally disallowed because they could be used to subvert
>> setuid programs.  But if setuid is disabled, then they are safe.
>>
>> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
>> ---
>>  kernel/nsproxy.c |    8 +++++++-
>>  1 files changed, 7 insertions(+), 1 deletions(-)
>>
>> diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
>> index b576f7f..47cf873 100644
>> --- a/kernel/nsproxy.c
>> +++ b/kernel/nsproxy.c
>> @@ -191,7 +191,13 @@ int unshare_nsproxy_namespaces(unsigned long unshare_flags,
>>                               CLONE_NEWNET)))
>>                return 0;
>>
>> -       if (!capable(CAP_SYS_ADMIN))
>> +       /* We require either no_new_privs or CAP_SYS_ADMIN for all modes */
>> +       if (!current->no_new_privs && !capable(CAP_SYS_ADMIN))
>> +               return -EPERM;
>> +
>> +       /* NEWNS and NEWNET always require CAP_SYS_ADMIN. */
>> +       if ((unshare_flags & (CLONE_NEWNS | CLONE_NEWNET)) &&
>> +           !capable(CAP_SYS_ADMIN))
>>                return -EPERM;
>>
>>        *new_nsp = create_new_namespaces(unshare_flags, current,
>
> While I think it's unlikely that the list handled by
> unshare_nsproxy_namespaces() is going to change, I'd still prefer that
> the logic of this test be reversed so that the nnp-allowed flags are
> listed instead of the CAP_SYS_ADMIN-required ones so that it will
> default to disallowing new flags. It's a little less readable, but
> maybe something like this (untested):
>
> unsigned long handled_mask = (CLONE_NEWNS | CLONE_NEWUTS | CLONE_NEWIPC |
>                                 CLONE_NEWNET);
> unsigned long npp_mask = (CLONE_NEWUTS | CLONE_NEWIPC);
>
> if (!(unshare_flags & handled_mask))
>        return 0;
>
> if (  !(current->no_new_privs &&
>        !(unshare_flags & (handled_mask ^ npp_mask))) &&
>      !capable(CAP_SYS_ADMIN))
>        return -EPERM;
> ...
>
> This also has the side-effect of removing the double-check of
> capable() in some cases.

Agreed -- will fix.

This patch is also missing the corresponding change for clone.  I'll add that.

I'm tempted to add CLONE_NEWPID as well (it's _useful_), but there's
an unresolved issue with SCM_CREDENTIALS.

--Andy