dropping capabilities in user namespace

[Aditya Kali]

Hi all,

I am trying to understand the behavior of how we can drop capabilities
inside user namespace. i.e., I want to start a process inside user
namespace with its effective and permitted capability sets cleared.

A typical way in which a root (uid=0) process can drop its privileges is:

prctl(PR_SET_KEEPCAPS, 0, 0, 0, 0);
setresuid(uid, uid, uid);  // At this point, permitted and effective
capabilities are cleared
exec()

But this sequence of operation inside a user namespace does not work
as expected:

Assume /proc/pid/uid_map has entry: uid uid 1

attach_user_ns(pid);  // OR create_user_ns() & write_uid_map()
prctl(PR_SET_KEEPCAPS, 0, 0, 0, 0);
setresuid(uid, uid, uid);  // Fails to reset capabilities
exec()

The exec()ed process starts with correct uid set, but still with all
the capabilities.

The differentiating factor here seems to be the 'root_uid' value in
security/commoncap.c:cap_emulate_setxuid():

static inline void cap_emulate_setxuid(struct cred *new, const struct cred *old)
{
  kuid_t root_uid = make_kuid(old->user_ns, 0);

  if ((uid_eq(old->uid, root_uid) ||
      uid_eq(old->euid, root_uid) ||
      uid_eq(old->suid, root_uid)) &&
     (!uid_eq(new->uid, root_uid) &&
      !uid_eq(new->euid, root_uid) &&
      !uid_eq(new->suid, root_uid)) &&
     !issecure(SECURE_KEEP_CAPS)) {
    cap_clear(new->cap_permitted);
    cap_clear(new->cap_effective);
  }
  ...

There are couple of problems here:
(1) In above example when there is no mapping for uid 0 inside
old->user_ns, make_kuid() returns INVALID_UID. Since we go on to
compare root_uid without first checking if its even valid, we never
satisfy the 'if' condition and never clear the caps. This looks like a
bug.

(2) Even if there is some mapping for uid 0 inside old->user_ns (say
"0 1111 1"), since old->uid = 0, and root_uid=1111 (or some non-zero
uid), the 'if' condition again remains unsatisfied.

It looks like currently the only case where global root (uid=0)
process can drop its capabilities inside a user namespace is by having
"0 0 <length>" mapping in the uid_map file. It seems wrong to expose
global root in user namespace just to drop privileges! So I feel we
need to fix the condition checks everywhere we are using make_kuid()
in security/commoncap.c.
Can the security experts please advice how this is supposed to work?

(FYI: Commit 18815a18085364d8514c0d0c4c986776cb74272c "userns: Convert
capabilities related permsion checks" introduced the make_uid() change
in cap_emulate_setxuid() & other places).

Thanks,
-- 
Aditya
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