Re: [PATCH v2 04/28] fsuidgid: add fsid mapping helpers

Jann Horn
2020-02-14T19:12:44Z

On Fri, Feb 14, 2020 at 7:37 PM Christian Brauner
<christian.brauner@ubuntu.com> wrote:
> This adds a set of helpers to translate between kfsuid/kfsgid and their
> userspace fsuid/fsgid counter parts relative to a given user namespace.
>
> - kuid_t make_kfsuid(struct user_namespace *from, uid_t fsuid)
> Maps a user-namespace fsuid pair into a kfsuid.
> If no fsuid mappings have been written it behaves identical to calling
> make_kuid(). This ensures backwards compatibility for workloads unaware
> or not in need of fsid mappings.
[...]
> +#ifdef CONFIG_USER_NS_FSID
> +/**
> + * make_kfsuid - Map a user-namespace fsuid pair into a kuid.
> + * @ns: User namespace that the fsuid is in
> + * @fsuid: User identifier
> + *
> + * Maps a user-namespace fsuid pair into a kernel internal kfsuid,
> + * and returns that kfsuid.
> + *
> + * When there is no mapping defined for the user-namespace kfsuid
> + * pair INVALID_UID is returned. Callers are expected to test
> + * for and handle INVALID_UID being returned. INVALID_UID
> + * may be tested for using uid_valid().
> + */
> +kuid_t make_kfsuid(struct user_namespace *ns, uid_t fsuid)
> +{
> + unsigned extents = ns->fsuid_map.nr_extents;
> + smp_rmb();
> +
> + /* Map the fsuid to a global kernel fsuid */
> + if (extents == 0)
> + return KUIDT_INIT(map_id_down(&ns->uid_map, fsuid));
> +
> + return KUIDT_INIT(map_id_down(&ns->fsuid_map, fsuid));
> +}
> +EXPORT_SYMBOL(make_kfsuid);

What effect is this fallback going to have for nested namespaces?

Let's say we have an outer namespace N1 with this uid_map:

0 100000 65535

and with this fsuid_map:

0 300000 65535

Now from in there, a process that is not aware of the existence of
fsuid mappings creates a new user namespace N2 with the following
uid_map:

0 1000 1

At this point, if a process in N2 does chown("foo", 0, 0), is that
going to make "foo" owned by kuid 101000, which isn't even mapped in
N1?

> @@ -1215,11 +1376,13 @@ static bool new_idmap_permitted(const struct file *file,
> uid_eq(ns->owner, cred->euid)) {
> u32 id = new_map->extent[0].lower_first;
> if (cap_setid == CAP_SETUID) {
> - kuid_t uid = make_kuid(ns->parent, id);
> + kuid_t uid = map_fsid ? make_kfsuid(ns->parent, id) :
> + make_kuid(ns->parent, id);
> if (uid_eq(uid, cred->euid))
> return true;

Let's say we have an outer user namespace N1 with this uid_map:

0 1000 3000

and this fsuid_map:

0 2000 3000

and in that namespace, a process is running as UID 1000 (which means
kernel-euid=2000, kernel-fsuid=3000). Now this process unshares its
user namespace and from this nested user namespace N2, tries to write
the following fsuid_map:

0 1000 1

This should work, since the only ID it maps is the one the process had
in N1; but the code is AFAICS going to run as follows:

if ((new_map->nr_extents == 1) && (new_map->extent[0].count == 1) &&
uid_eq(ns->owner, cred->euid)) { // branch taken
u32 id = new_map->extent[0].lower_first;
if (cap_setid == CAP_SETUID) { // branch taken
// uid = make_kfsuid(ns->parent, 1000) = 3000
kuid_t uid = map_fsid ? make_kfsuid(ns->parent, id) :
make_kuid(ns->parent, id);
// uid_eq(3000, 2000)
if (uid_eq(uid, cred->euid)) // not taken
return true;
} else [...]
}

Instead, I think what would succeed is this, which shouldn't be allowed:

0 0 1

which AFAICS will evaluate as follows:

if ((new_map->nr_extents == 1) && (new_map->extent[0].count == 1) &&
uid_eq(ns->owner, cred->euid)) { // branch taken
u32 id = new_map->extent[0].lower_first;
if (cap_setid == CAP_SETUID) { // branch taken
// uid = make_kfsuid(ns->parent, 0) = 2000
kuid_t uid = map_fsid ? make_kfsuid(ns->parent, id) :
make_kuid(ns->parent, id);
// uid_eq(2000, 2000)
if (uid_eq(uid, cred->euid)) // taken
return true;
} else [...]
}

> } else if (cap_setid == CAP_SETGID) {
> - kgid_t gid = make_kgid(ns->parent, id);
> + kgid_t gid = map_fsid ? make_kfsgid(ns->parent, id) :
> + make_kgid(ns->parent, id);
> if (!(ns->flags & USERNS_SETGROUPS_ALLOWED) &&
> gid_eq(gid, cred->egid))
> return true;
> --
> 2.25.0
>