On Fri, Nov 10, 2017 at 2:25 AM, Serge E. Hallyn [off-list ref] wrote:

Quoting Mahesh Bandewar (mahesh@bandewar.net):

quoted

From: Mahesh Bandewar <redacted>

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 <redacted>
---
 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..