[PATCH 2/4] userns: Require CAP_SYS_ADMIN for most uses of setns.

[ebiederm@xmission.com (Eric W. Biederman)]

Andy Lutomirski <luto@amacapital.net> found a nasty little bug in
the permissions of setns.  With unprivileged user namespaces it
became possible to create new namespaces without privilege.

However the setns calls were relaxed to only require CAP_SYS_ADMIN in
the user nameapce of the targed namespace.

Which made the following nasty sequence possible.

pid = clone(CLONE_NEWUSER | CLONE_NEWNS);
if (pid == 0) { /* child */
	system("mount --bind /home/me/passwd /etc/passwd");
}
else if (pid != 0) { /* parent */
	char path[PATH_MAX];
	snprintf(path, sizeof(path), "/proc/%u/ns/mnt");
	fd = open(path, O_RDONLY);
	setns(fd, 0);
	system("su -");
}

Prevent this possibility by requiring CAP_SYS_ADMIN
in the current user namespace when joing all but the user namespace.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
 fs/namespace.c           |    3 ++-
 ipc/namespace.c          |    3 ++-
 kernel/pid_namespace.c   |    3 ++-
 kernel/utsname.c         |    3 ++-
 net/core/net_namespace.c |    3 ++-
 5 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index c1bbe86..398a50f 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2781,7 +2781,8 @@ static int mntns_install(struct nsproxy *nsproxy, void *ns)
 	struct path root;
 
 	if (!ns_capable(mnt_ns->user_ns, CAP_SYS_ADMIN) ||
-	    !nsown_capable(CAP_SYS_CHROOT))
+	    !nsown_capable(CAP_SYS_CHROOT) ||
+	    !nsown_capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
 	if (fs->users != 1)
diff --git a/ipc/namespace.c b/ipc/namespace.c
index cf3386a..7c1fa45 100644
--- a/ipc/namespace.c
+++ b/ipc/namespace.c
@@ -170,7 +170,8 @@ static void ipcns_put(void *ns)
 static int ipcns_install(struct nsproxy *nsproxy, void *new)
 {
 	struct ipc_namespace *ns = new;
-	if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN))
+	if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN) ||
+	    !nsown_capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
 	/* Ditch state from the old ipc namespace */
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index 560da0d..fdbd0cd 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -325,7 +325,8 @@ static int pidns_install(struct nsproxy *nsproxy, void *ns)
 	struct pid_namespace *active = task_active_pid_ns(current);
 	struct pid_namespace *ancestor, *new = ns;
 
-	if (!ns_capable(new->user_ns, CAP_SYS_ADMIN))
+	if (!ns_capable(new->user_ns, CAP_SYS_ADMIN) ||
+	    !nsown_capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
 	/*
diff --git a/kernel/utsname.c b/kernel/utsname.c
index f6336d5..08b197e 100644
--- a/kernel/utsname.c
+++ b/kernel/utsname.c
@@ -113,7 +113,8 @@ static int utsns_install(struct nsproxy *nsproxy, void *new)
 {
 	struct uts_namespace *ns = new;
 
-	if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN))
+	if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN) ||
+	    !nsown_capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
 	get_uts_ns(ns);
diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index 2e9a313..8acce01 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -649,7 +649,8 @@ static int netns_install(struct nsproxy *nsproxy, void *ns)
 {
 	struct net *net = ns;
 
-	if (!ns_capable(net->user_ns, CAP_SYS_ADMIN))
+	if (!ns_capable(net->user_ns, CAP_SYS_ADMIN) ||
+	    !nsown_capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
 	put_net(nsproxy->net_ns);
-- 
1.7.5.4