linux-kernel - Re: [PATCH] net: make net namespace sysctls belong to container's owner

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Date:	Mon, 08 Aug 2016 16:08:48 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Al Viro <viro@...iv.linux.org.uk>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH] net: make net namespace sysctls belong to container's owner

Dmitry Torokhov <dmitry.torokhov@...il.com> writes:

> If net namespace is attached to a user namespace let's make container's
> root owner of sysctls affecting said network namespace instead of global
> root.
>
> This also allows us to clean up net_ctl_permissions() because we do not
> need to fudge permissions anymore for the container's owner since it now
> owns the objects in question.

Acked-by: "Eric W. Biederman" <ebiederm@...ssion.com>

Overall this seems reasonable.  However I am not a fan of your error
handling.

> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@...il.com>
> ---
>
> This helps when running Android CTS in a container, but I think it makes
> sense regardless.

> +static void net_ctl_set_ownership(struct ctl_table_header *head,
> +				  struct ctl_table *table,
> +				  kuid_t *uid, kgid_t *gid)
> +{
> +	struct net *net = container_of(head->set, struct net, sysctls);
> +
> +	*uid = make_kuid(net->user_ns, 0);
> +	if (!uid_valid(*uid))
> +		*uid = GLOBAL_ROOT_UID;
> +
> +	*gid = make_kgid(net->user_ns, 0);
> +	if (!gid_valid(*gid))
> +		*gid = GLOBAL_ROOT_GID;

This code should eiter be:
	*uid = make_kuid(net->user_ns, 0);
        *gid = make_kgid(net->user_ns, 0);

Or it should be:
	tmp_uid = make_kuid(net->user_ns, 0);
        if (uid_valid(tmp_uid))
        	*uid = tmp_uid;

	tmp_gid = make_kgid(net->user_ns, 0);
        if (gid_valid(tmp_gid))
        	*gid = tmp_gid;

It is just very fragile to assume to know what uid and gid
would be if this code fails.

As of v4.8-rc1 INVALID_UID and INVALID_GID can be set in inode->i_uid
and inode->i_gid without causing horrible vfs confusion (making the
first option viable), but I expect with the mention of Android you want
to backport this so I will ask that you ask to implement the error
handling that doesn't assume you know better than the generic code.

If you don't have a better value to set something to it really should be
left alone.

Eric