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userns: Require CAP_SYS_ADMIN for most uses of setns.

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.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
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1 parent 520d9ea commit 5e4a08476b50fa39210fca82e03325cc46b9c235 @ebiederm ebiederm committed Dec 14, 2012
Showing with 10 additions and 5 deletions.
  1. +2 −1 fs/namespace.c
  2. +2 −1 ipc/namespace.c
  3. +2 −1 kernel/pid_namespace.c
  4. +2 −1 kernel/utsname.c
  5. +2 −1 net/core/net_namespace.c
View
@@ -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)
View
@@ -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 */
View
@@ -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;
/*
View
@@ -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);
View
@@ -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);

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