Re: [PATCH] userns,pidns: Verify the userns for new pid namespaces

[Kirill Tkhai <ktkhai@virtuozzo.com>]

On 02.05.2017 13:03, Kirill Tkhai wrote:
> 
> 
> On 29.04.2017 22:25, Eric W. Biederman wrote:
>>
>> It is pointless and confusing to allow a pid namespace hierarchy and
>> the user namespace hierarchy to get out of sync.  The owner of a child
>> pid namespace should be the owner of the parent pid namespace or
>> a descendant of the owner of the parent pid namespace.
>>
>> Otherwise it is possible to construct scenarios where it is legal to
>> do something in a parent pid namespace but in a child pid namespace.
>>
>> It requires use of setns into a pid namespace (but not into a user
>> namespace) to create such a scenario.
>>
>> Add the function in_userns to help in making this determination.
>>
>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
>> ---
>>
>> While review a patch from Kiril Tkhai I realized we were missing this
>> sanity check....
>>
>>  include/linux/user_namespace.h |  8 +++++++-
>>  kernel/pid_namespace.c         |  4 ++++
>>  kernel/user_namespace.c        | 18 ++++++++++++------
>>  3 files changed, 23 insertions(+), 7 deletions(-)
>>
>> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
>> index 32354b4b4b2b..497ed50004db 100644
>> --- a/include/linux/user_namespace.h
>> +++ b/include/linux/user_namespace.h
>> @@ -112,8 +112,9 @@ extern ssize_t proc_projid_map_write(struct file *, const char __user *, size_t,
>>  extern ssize_t proc_setgroups_write(struct file *, const char __user *, size_t, loff_t *);
>>  extern int proc_setgroups_show(struct seq_file *m, void *v);
>>  extern bool userns_may_setgroups(const struct user_namespace *ns);
>> +extern bool in_userns(const struct user_namespace *ancestor,
>> +		       const struct user_namespace *child);
>>  extern bool current_in_userns(const struct user_namespace *target_ns);
>> -
>>  struct ns_common *ns_get_owner(struct ns_common *ns);
>>  #else
>>  
>> @@ -144,6 +145,11 @@ static inline bool userns_may_setgroups(const struct user_namespace *ns)
>>  	return true;
>>  }
>>  
>> +static inline bool in_userns(const struct user_namespace *target_ns)
>> +{
>> +	return true;
>> +}
>> +
>>  static inline bool current_in_userns(const struct user_namespace *target_ns)
>>  {
>>  	return true;
>> diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
>> index de461aa0bf9a..749147f5a613 100644
>> --- a/kernel/pid_namespace.c
>> +++ b/kernel/pid_namespace.c
>> @@ -101,6 +101,10 @@ static struct pid_namespace *create_pid_namespace(struct user_namespace *user_ns
>>  	int i;
>>  	int err;
>>  
>> +	err = -EINVAL;
>> +	if (!in_userns(parent_pid_ns->user_ns, user_ns))
>> +		goto out;
>> +
>>  	err = -ENOSPC;
>>  	if (level > MAX_PID_NS_LEVEL)
>>  		goto out;
>> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
>> index 2f735cbe05e8..7d8658fbabc8 100644
>> --- a/kernel/user_namespace.c
>> +++ b/kernel/user_namespace.c
>> @@ -986,19 +986,25 @@ bool userns_may_setgroups(const struct user_namespace *ns)
>>  }
>>  
>>  /*
>> - * Returns true if @ns is the same namespace as or a descendant of
>> - * @target_ns.
>> + * Returns true if @child is the same namespace or a descendant of
>> + * @ancestor.
>>   */
>> -bool current_in_userns(const struct user_namespace *target_ns)
>> +bool in_userns(const struct user_namespace *ancestor,
>> +	       const struct user_namespace *child)
>>  {
>> -	struct user_namespace *ns;
>> -	for (ns = current_user_ns(); ns; ns = ns->parent) {
>> -		if (ns == target_ns)
>> +	const struct user_namespace *ns;
>> +	for (ns = child; ns; ns = ns->parent) {
>> +		if (ns == ancestor)
>>  			return true;
>>  	}
>>  	return false;
>>  }
> 
> We have user_namespace::level, so it's possible to stop iterations earlier
> and save some cpu cycles:
> 
> 	for (ns = child; ns->level >= ancestor->level; ns = ns->parent)

Just ">" here.

> 		;
> 	return (ns == ancestor);
> 
>>  
>> +bool current_in_userns(const struct user_namespace *target_ns)
>> +{
>> +	return in_userns(target_ns, current_user_ns());
>> +}
>> +
>>  static inline struct user_namespace *to_user_ns(struct ns_common *ns)
>>  {
>>  	return container_of(ns, struct user_namespace, ns);
>>