Re: [PATCH 16/43] userns: Simplify the user_namespace by making userns->creator a kuid.

Serge E. Hallyn
2012-04-24T20:21:49Z

Quoting Eric W. Biederman (ebiederm@xmission.com):
> "Serge E. Hallyn" <serge@hallyn.com> writes:
>
> > Quoting Eric W. Biederman (ebiederm@xmission.com):
> >> "Serge E. Hallyn" <serge@hallyn.com> writes:
> >>
> >> > Quoting Eric W. Beiderman (ebiederm@xmission.com):
> >> >> From: Eric W. Biederman <ebiederm@xmission.com>
> >> >>
> >> >> - Transform userns->creator from a user_struct reference to a simple
> >> >> kuid_t, kgid_t pair.
> >> >>
> >> >> In cap_capable this allows the check to see if we are the creator of
> >> >> a namespace to become the classic suser style euid permission check.
> >> >>
> >> >> This allows us to remove the need for a struct cred in the mapping
> >> >> functions and still be able to dispaly the user namespace creators
> >> >> uid and gid as 0.
> >> >>
> >> >> - Remove the now unnecessary delayed_work in free_user_ns.
> >> >>
> >> >> All that is left for free_user_ns to do is to call kmem_cache_free
> >> >> and put_user_ns. Those functions can be called in any context
> >> >> so call them directly from free_user_ns removing the need for delayed work.
> >> >>
> >> >> Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
> >> >> ---
> >> >> include/linux/user_namespace.h | 4 ++--
> >> >> kernel/user.c | 7 ++++---
> >> >> kernel/user_namespace.c | 39 ++++++++++++++++++---------------------
> >> >> security/commoncap.c | 5 +++--
> >> >> 4 files changed, 27 insertions(+), 28 deletions(-)
> >> >>
> >> >> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> >> >> index d767508..8a391bd 100644
> >> >> --- a/include/linux/user_namespace.h
> >> >> +++ b/include/linux/user_namespace.h
> >> >> @@ -9,8 +9,8 @@
> >> >> struct user_namespace {
> >> >> struct kref kref;
> >> >> struct user_namespace *parent;
> >> >> - struct user_struct *creator;
> >> >> - struct work_struct destroyer;
> >> >> + kuid_t owner;
> >> >> + kgid_t group;
> >> >> };
> >> >>
> >> >> extern struct user_namespace init_user_ns;
> >> >> diff --git a/kernel/user.c b/kernel/user.c
> >> >> index 025077e..cff3856 100644
> >> >> --- a/kernel/user.c
> >> >> +++ b/kernel/user.c
> >> >> @@ -25,7 +25,8 @@ struct user_namespace init_user_ns = {
> >> >> .kref = {
> >> >> .refcount = ATOMIC_INIT(3),
> >> >> },
> >> >> - .creator = &root_user,
> >> >> + .owner = GLOBAL_ROOT_UID,
> >> >> + .group = GLOBAL_ROOT_GID,
> >> >> };
> >> >> EXPORT_SYMBOL_GPL(init_user_ns);
> >> >>
> >> >> @@ -54,9 +55,9 @@ struct hlist_head uidhash_table[UIDHASH_SZ];
> >> >> */
> >> >> static DEFINE_SPINLOCK(uidhash_lock);
> >> >>
> >> >> -/* root_user.__count is 2, 1 for init task cred, 1 for init_user_ns->user_ns */
> >> >> +/* root_user.__count is 1, for init task cred */
> >> >> struct user_struct root_user = {
> >> >> - .__count = ATOMIC_INIT(2),
> >> >> + .__count = ATOMIC_INIT(1),
> >> >> .processes = ATOMIC_INIT(1),
> >> >> .files = ATOMIC_INIT(0),
> >> >> .sigpending = ATOMIC_INIT(0),
> >> >> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> >> >> index 898e973..f69741a 100644
> >> >> --- a/kernel/user_namespace.c
> >> >> +++ b/kernel/user_namespace.c
> >> >> @@ -27,6 +27,16 @@ int create_user_ns(struct cred *new)
> >> >> {
> >> >> struct user_namespace *ns, *parent_ns = new->user_ns;
> >> >> struct user_struct *root_user;
> >> >> + kuid_t owner = make_kuid(new->user_ns, new->euid);
> >> >> + kgid_t group = make_kgid(new->user_ns, new->egid);
> >> >> +
> >> >> + /* The creator needs a mapping in the parent user namespace
> >> >> + * or else we won't be able to reasonably tell userspace who
> >> >> + * created a user_namespace.
> >> >> + */
> >> >> + if (!kuid_has_mapping(parent_ns, owner) ||
> >> >> + !kgid_has_mapping(parent_ns, group))
> >> >> + return -EPERM;
> >> >>
> >> >> ns = kmem_cache_alloc(user_ns_cachep, GFP_KERNEL);
> >> >> if (!ns)
> >> >> @@ -43,7 +53,9 @@ int create_user_ns(struct cred *new)
> >> >>
> >> >> /* set the new root user in the credentials under preparation */
> >> >> ns->parent = parent_ns;
> >> >
> >> > I think in the past the creator cred pinned the ns->parent. Do you now
> >> > need to explicitly pin ns->parent (and release it in free_user_ns())?
> >>
> >> Yes we do have to explicitly reference count the parent namespace.
> >> But that happened in the patch 7:
> >> "userns: Add an explicit reference to the parent user namespace"
>
> Make that patch 8 not patch 7:
> "userns: Add an explicit reference to the parent user namespace"
> Perhaps the patch number reference pointed you to look at the wrong code.

D'oh, yup. That explains it better.

And so parent_userns keeps the refcount from the cred 'new' after
new->ns = ns; That works, thanks.

> > Perhaps that suffices, but I'm not convinced. The struct cred is
> > pinning it's own ns, but if t1 does clone(CLONE_NEWUSER) to produce
> > t2, which does the same to procduce t3, and then t2 exits, I'm not
> > seeing what will pin t2's userns.
>
> t3's userns hold's a reference to the departed t2's userns.
> t2's userns hold's a reference to t1's userns.
>
> free_user_ns does put that userns reference.
>
> It is all there and explict. Usernamespaces refer directly to each

Actually can we make it just one tinge more explicit, and put a comment
above the 'new->user_ns = ns'? There's currently the comment

/* Leave the reference to our user_ns with the new cred */

But that's about the initial refcount on the new ns. Perhaps change that to:

/*
* Leave the reference to our new user_ns with the new cred,
* and leave the reference on the old ns to pin new->parent_ns
*/

> other. That was all needed to get struct user out of the usernamespace
> game.
>
> Eric

Thanks, Eric. So then

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>

which, given the other nits are addressed, should cover the whole
set with my acks.

thanks,
-serge