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

[Kees Cook]

On Mon, Jan 30, 2012 at 8:17 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
> ---
>  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.

-Kees

-- 
Kees Cook
ChromeOS Security
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