Re: [PATCH resend 2/2] userns: control capabilities of some user namespaces

[Mahesh Bandewar (àààà ààààààà)]

On Fri, Nov 10, 2017 at 2:25 AM, Serge E. Hallyn <serge@xxxxxxxxxx> wrote:
> Quoting Mahesh Bandewar (mahesh@xxxxxxxxxxxx):
>> From: Mahesh Bandewar <maheshb@xxxxxxxxxx>
>>
>> With this new notion of "controlled" user-namespaces, the controlled
>> user-namespaces are marked at the time of their creation while the
>> capabilities of processes that belong to them are controlled using the
>> global mask.
>>
>> Init-user-ns is always uncontrolled and a process that has SYS_ADMIN
>> that belongs to uncontrolled user-ns can create another (child) user-
>> namespace that is uncontrolled. Any other process (that either does
>> not have SYS_ADMIN or belongs to a controlled user-ns) can only
>> create a user-ns that is controlled.
>>
>> global-capability-whitelist (controlled_userns_caps_whitelist) is used
>> at the capability check-time and keeps the semantics for the processes
>> that belong to uncontrolled user-ns as it is. Processes that belong to
>> controlled user-ns however are subjected to different checks-
>>
>>    (a) if the capability in question is controlled and process belongs
>>        to controlled user-ns, then it's always denied.
>>    (b) if the capability in question is NOT controlled then fall back
>>        to the traditional check.
>>
>> Signed-off-by: Mahesh Bandewar <maheshb@xxxxxxxxxx>
>> ---
>>  include/linux/capability.h     |  1 +
>>  include/linux/user_namespace.h | 20 ++++++++++++++++++++
>>  kernel/capability.c            |  5 +++++
>>  kernel/user_namespace.c        |  3 +++
>>  security/commoncap.c           |  8 ++++++++
>>  5 files changed, 37 insertions(+)
>>
>> diff --git a/include/linux/capability.h b/include/linux/capability.h
>> index 6c0b9677c03f..b8c6cac18658 100644
>> --- a/include/linux/capability.h
>> +++ b/include/linux/capability.h
>> @@ -250,6 +250,7 @@ extern bool ptracer_capable(struct task_struct *tsk, struct user_namespace *ns);
>>  extern int get_vfs_caps_from_disk(const struct dentry *dentry, struct cpu_vfs_cap_data *cpu_caps);
>>  int proc_douserns_caps_whitelist(struct ctl_table *table, int write,
>>                                void __user *buff, size_t *lenp, loff_t *ppos);
>> +bool is_capability_controlled(int cap);
>>
>>  extern int cap_convert_nscap(struct dentry *dentry, void **ivalue, size_t size);
>>
>> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
>> index c18e01252346..e890fe81b47e 100644
>> --- a/include/linux/user_namespace.h
>> +++ b/include/linux/user_namespace.h
>> @@ -22,6 +22,7 @@ struct uid_gid_map {        /* 64 bytes -- 1 cache line */
>>  };
>>
>>  #define USERNS_SETGROUPS_ALLOWED 1UL
>> +#define USERNS_CONTROLLED     2UL
>>
>>  #define USERNS_INIT_FLAGS USERNS_SETGROUPS_ALLOWED
>>
>> @@ -102,6 +103,16 @@ static inline void put_user_ns(struct user_namespace *ns)
>>               __put_user_ns(ns);
>>  }
>>
>> +static inline bool is_user_ns_controlled(const struct user_namespace *ns)
>> +{
>> +     return ns->flags & USERNS_CONTROLLED;
>> +}
>> +
>> +static inline void mark_user_ns_controlled(struct user_namespace *ns)
>> +{
>> +     ns->flags |= USERNS_CONTROLLED;
>> +}
>> +
>>  struct seq_operations;
>>  extern const struct seq_operations proc_uid_seq_operations;
>>  extern const struct seq_operations proc_gid_seq_operations;
>> @@ -160,6 +171,15 @@ static inline struct ns_common *ns_get_owner(struct ns_common *ns)
>>  {
>>       return ERR_PTR(-EPERM);
>>  }
>> +
>> +static inline bool is_user_ns_controlled(const struct user_namespace *ns)
>> +{
>> +     return false;
>> +}
>> +
>> +static inline void mark_user_ns_controlled(struct user_namespace *ns)
>> +{
>> +}
>>  #endif
>>
>>  #endif /* _LINUX_USER_H */
>> diff --git a/kernel/capability.c b/kernel/capability.c
>> index 62dbe3350c1b..40a38cc4ff43 100644
>> --- a/kernel/capability.c
>> +++ b/kernel/capability.c
>> @@ -510,6 +510,11 @@ bool ptracer_capable(struct task_struct *tsk, struct user_namespace *ns)
>>  }
>>
>>  /* Controlled-userns capabilities routines */
>> +bool is_capability_controlled(int cap)
>> +{
>> +     return !cap_raised(controlled_userns_caps_whitelist, cap);
>> +}
>> +
>>  #ifdef CONFIG_SYSCTL
>>  int proc_douserns_caps_whitelist(struct ctl_table *table, int write,
>>                                void __user *buff, size_t *lenp, loff_t *ppos)
>> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
>> index c490f1e4313b..f393ea5108f0 100644
>> --- a/kernel/user_namespace.c
>> +++ b/kernel/user_namespace.c
>> @@ -53,6 +53,9 @@ static void set_cred_user_ns(struct cred *cred, struct user_namespace *user_ns)
>>       cred->cap_effective = CAP_FULL_SET;
>>       cred->cap_ambient = CAP_EMPTY_SET;
>>       cred->cap_bset = CAP_FULL_SET;
>> +     if (!ns_capable(user_ns->parent, CAP_SYS_ADMIN) ||
>> +         is_user_ns_controlled(user_ns->parent))
>> +             mark_user_ns_controlled(user_ns);
>
> Hm, why do this here, rather than at create_user_ns()? It
> shouldn't be recalculated when someone does setns() should it?
>
You are absolutely right! It doesn't make sense to recalculate for
every setns() call. It's a side effect of couple of iterations /
approaches that I tried before finalizing this one. I'll move this
block to create_user_ns() after the set_cred_user_ns() call so that
this wont be triggered in setns() path.

Thanks,
--mahesh..