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

Eric W. Biederman
2012-04-26T09:05:47Z

"Serge E. Hallyn" <serge@hallyn.com> writes:

> 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
> */

I have added the following comment. Hopefully that makes it clearer. */

/* Leave the new->user_ns reference with the new user namespace. */
>
>> 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.

Hmm. I don't have a record of you looking at my patch 23.
"userns: Convert setting and getting uid and gid system calls to use kuid and kgid"

Eric