userns: Kill task_user_ns

The task_user_ns function hides the fact that it is getting the user
namespace from struct cred on the task.  struct cred may go away as
soon as the rcu lock is released.  This leads to a race where we
can dereference a stale user namespace pointer.

To make it obvious a struct cred is involved kill task_user_ns.

To kill the race modify the users of task_user_ns to only
reference the user namespace while the rcu lock is held.

Cc: Kees Cook <keescook@chromium.org>
Cc: James Morris <james.l.morris@oracle.com>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

1 files changed, 9 insertions, 3 deletions

diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
index b4c29848b49d..0e72239aeb05 100644
--- a/security/yama/yama_lsm.c
+++ b/security/yama/yama_lsm.c
@@ -262,14 +262,18 @@ int yama_ptrace_access_check(struct task_struct *child,
                         /* No additional restrictions. */
                         break;
                 case YAMA_SCOPE_RELATIONAL:
+                        rcu_read_lock();
                         if (!task_is_descendant(current, child) &&
                             !ptracer_exception_found(current, child) &&
-                            !ns_capable(task_user_ns(child), CAP_SYS_PTRACE))
+                            !ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
                                 rc = -EPERM;
+                        rcu_read_unlock();
                         break;
                 case YAMA_SCOPE_CAPABILITY:
-                        if (!ns_capable(task_user_ns(child), CAP_SYS_PTRACE))
+                        rcu_read_lock();
+                        if (!ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
                                 rc = -EPERM;
+                        rcu_read_unlock();
                         break;
                 case YAMA_SCOPE_NO_ATTACH:
                 default:
@@ -307,8 +311,10 @@ int yama_ptrace_traceme(struct task_struct *parent)
         /* Only disallow PTRACE_TRACEME on more aggressive settings. */
         switch (ptrace_scope) {
         case YAMA_SCOPE_CAPABILITY:
-                if (!ns_capable(task_user_ns(parent), CAP_SYS_PTRACE))
+                rcu_read_lock();
+                if (!ns_capable(__task_cred(parent)->user_ns, CAP_SYS_PTRACE))
                         rc = -EPERM;
+                rcu_read_unlock();
                 break;
         case YAMA_SCOPE_NO_ATTACH:
                 rc = -EPERM;