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Subject[PATCH 3/3] user namespaces: reset task's credentials on CLONE_NEWUSER
Currently, creating a new user namespace does not reset
the task's uid or gid. Since generally that is done as
root because it requires CAP_SYS_ADMIN, and since the
first uid in the new namespace is 0, one usually doesn't
notice. However, if one does

capset cap_sys_admin=ep ns_exec
su - hallyn
ns_exec -U /bin/sh
id

then one will see hallyn's userid, and all preexisting
groups.

With this patch, cloning a new user namespace will set
the task's uid and gid to 0, and reset the group_info to
the empty set assigned to init.

Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
---
kernel/user_namespace.c | 14 ++++++++++++++
1 files changed, 14 insertions(+), 0 deletions(-)
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index d59f193..16e6296 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -10,6 +10,9 @@
#include <linux/slab.h>
#include <linux/user_namespace.h>

+/* defined in kernel/sys.c */
+extern struct group_info init_groups;
+
/*
* Clone a new ns copying an original user ns, setting refcount to 1
* @old_ns: namespace to clone
@@ -47,6 +50,17 @@ int create_new_userns(int flags, struct task_struct *tsk)
put_user_ns(ns);

task_switch_uid(tsk, ns->root_user);
+ tsk->uid = tsk->euid = tsk->suid = tsk->fsuid = 0;
+ tsk->gid = tsk->egid = tsk->sgid = tsk->fsgid = 0;
+
+ /* this can't be safe for unshare, can it? it's safe
+ * for fork, though. I'm tempted to limit clone_newuser to
+ * fork only */
+ task_lock(tsk);
+ put_group_info(tsk->group_info);
+ tsk->group_info = &init_groups;
+ get_group_info(tsk->group_info);
+ task_unlock(tsk);

return 0;
}
--
1.5.4.3


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