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

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Date:	Fri, 20 Apr 2012 15:58:30 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"Serge E. Hallyn" <serge@...lyn.com>
Cc:	linux-kernel@...r.kernel.org,
	Linux Containers <containers@...ts.linux-foundation.org>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	linux-security-module@...r.kernel.org,
	Al Viro <viro@...IV.linux.org.uk>,
	linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 16/43] userns: Simplify the user_namespace by making userns->creator a kuid.

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

> Quoting Eric W. Beiderman (ebiederm@...ssion.com):
>> From: Eric W. Biederman <ebiederm@...ssion.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@...ssion.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"


Eric
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