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linux-next/kernel/nsproxy.c
Christian Brauner 303cc571d1
nsproxy: attach to namespaces via pidfds 
For quite a while we have been thinking about using pidfds to attach to
namespaces. This patchset has existed for about a year already but we've
wanted to wait to see how the general api would be received and adopted.
Now that more and more programs in userspace have started using pidfds
for process management it's time to send this one out.

This patch makes it possible to use pidfds to attach to the namespaces
of another process, i.e. they can be passed as the first argument to the
setns() syscall. When only a single namespace type is specified the
semantics are equivalent to passing an nsfd. That means
setns(nsfd, CLONE_NEWNET) equals setns(pidfd, CLONE_NEWNET). However,
when a pidfd is passed, multiple namespace flags can be specified in the
second setns() argument and setns() will attach the caller to all the
specified namespaces all at once or to none of them. Specifying 0 is not
valid together with a pidfd.

Here are just two obvious examples:
setns(pidfd, CLONE_NEWPID | CLONE_NEWNS | CLONE_NEWNET);
setns(pidfd, CLONE_NEWUSER);
Allowing to also attach subsets of namespaces supports various use-cases
where callers setns to a subset of namespaces to retain privilege, perform
an action and then re-attach another subset of namespaces.

If the need arises, as Eric suggested, we can extend this patchset to
assume even more context than just attaching all namespaces. His suggestion
specifically was about assuming the process' root directory when
setns(pidfd, 0) or setns(pidfd, SETNS_PIDFD) is specified. For now, just
keep it flexible in terms of supporting subsets of namespaces but let's
wait until we have users asking for even more context to be assumed. At
that point we can add an extension.

The obvious example where this is useful is a standard container
manager interacting with a running container: pushing and pulling files
or directories, injecting mounts, attaching/execing any kind of process,
managing network devices all these operations require attaching to all
or at least multiple namespaces at the same time. Given that nowadays
most containers are spawned with all namespaces enabled we're currently
looking at at least 14 syscalls, 7 to open the /proc/<pid>/ns/<ns>
nsfds, another 7 to actually perform the namespace switch. With time
namespaces we're looking at about 16 syscalls.
(We could amortize the first 7 or 8 syscalls for opening the nsfds by
 stashing them in each container's monitor process but that would mean
 we need to send around those file descriptors through unix sockets
 everytime we want to interact with the container or keep on-disk
 state. Even in scenarios where a caller wants to join a particular
 namespace in a particular order callers still profit from batching
 other namespaces. That mostly applies to the user namespace but
 all container runtimes I found join the user namespace first no matter
 if it privileges or deprivileges the container similar to how unshare
 behaves.)
With pidfds this becomes a single syscall no matter how many namespaces
are supposed to be attached to.

A decently designed, large-scale container manager usually isn't the
parent of any of the containers it spawns so the containers don't die
when it crashes or needs to update or reinitialize. This means that
for the manager to interact with containers through pids is inherently
racy especially on systems where the maximum pid number is not
significicantly bumped. This is even more problematic since we often spawn
and manage thousands or ten-thousands of containers. Interacting with a
container through a pid thus can become risky quite quickly. Especially
since we allow for an administrator to enable advanced features such as
syscall interception where we're performing syscalls in lieu of the
container. In all of those cases we use pidfds if they are available and
we pass them around as stable references. Using them to setns() to the
target process' namespaces is as reliable as using nsfds. Either the
target process is already dead and we get ESRCH or we manage to attach
to its namespaces but we can't accidently attach to another process'
namespaces. So pidfds lend themselves to be used with this api.
The other main advantage is that with this change the pidfd becomes the
only relevant token for most container interactions and it's the only
token we need to create and send around.

Apart from significiantly reducing the number of syscalls from double
digit to single digit which is a decent reason post-spectre/meltdown
this also allows to switch to a set of namespaces atomically, i.e.
either attaching to all the specified namespaces succeeds or we fail. If
we fail we haven't changed a single namespace. There are currently three
namespaces that can fail (other than for ENOMEM which really is not
very interesting since we then have other problems anyway) for
non-trivial reasons, user, mount, and pid namespaces. We can fail to
attach to a pid namespace if it is not our current active pid namespace
or a descendant of it. We can fail to attach to a user namespace because
we are multi-threaded or because our current mount namespace shares
filesystem state with other tasks, or because we're trying to setns()
to the same user namespace, i.e. the target task has the same user
namespace as we do. We can fail to attach to a mount namespace because
it shares filesystem state with other tasks or because we fail to lookup
the new root for the new mount namespace. In most non-pathological
scenarios these issues can be somewhat mitigated. But there are cases where
we're half-attached to some namespace and failing to attach to another one.
I've talked about some of these problem during the hallway track (something
only the pre-COVID-19 generation will remember) of Plumbers in Los Angeles
in 2018(?). Even if all these issues could be avoided with super careful
userspace coding it would be nicer to have this done in-kernel. Pidfds seem
to lend themselves nicely for this.

The other neat thing about this is that setns() becomes an actual
counterpart to the namespace bits of unshare().

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: Serge Hallyn <serge@hallyn.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Serge Hallyn <serge@hallyn.com>
Cc: Jann Horn <jannh@google.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Link: https://lore.kernel.org/r/20200505140432.181565-3-christian.brauner@ubuntu.com
2020-05-13 11:41:22 +02:00

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// SPDX-License-Identifier: GPL-2.0-only
/*
* Copyright (C) 2006 IBM Corporation
*
* Author: Serge Hallyn <serue@us.ibm.com>
*
* Jun 2006 - namespaces support
* OpenVZ, SWsoft Inc.
* Pavel Emelianov <xemul@openvz.org>
*/
#include <linux/slab.h>
#include <linux/export.h>
#include <linux/nsproxy.h>
#include <linux/init_task.h>
#include <linux/mnt_namespace.h>
#include <linux/utsname.h>
#include <linux/pid_namespace.h>
#include <net/net_namespace.h>
#include <linux/ipc_namespace.h>
#include <linux/time_namespace.h>
#include <linux/fs_struct.h>
#include <linux/proc_fs.h>
#include <linux/proc_ns.h>
#include <linux/file.h>
#include <linux/syscalls.h>
#include <linux/cgroup.h>
#include <linux/perf_event.h>
static struct kmem_cache *nsproxy_cachep;
struct nsproxy init_nsproxy = {
.count = ATOMIC_INIT(1),
.uts_ns = &init_uts_ns,
#if defined(CONFIG_POSIX_MQUEUE) || defined(CONFIG_SYSVIPC)
.ipc_ns = &init_ipc_ns,
#endif
.mnt_ns = NULL,
.pid_ns_for_children = &init_pid_ns,
#ifdef CONFIG_NET
.net_ns = &init_net,
#endif
#ifdef CONFIG_CGROUPS
.cgroup_ns = &init_cgroup_ns,
#endif
#ifdef CONFIG_TIME_NS
.time_ns = &init_time_ns,
.time_ns_for_children = &init_time_ns,
#endif
};
static inline struct nsproxy *create_nsproxy(void)
{
struct nsproxy *nsproxy;
nsproxy = kmem_cache_alloc(nsproxy_cachep, GFP_KERNEL);
if (nsproxy)
atomic_set(&nsproxy->count, 1);
return nsproxy;
}
/*
* Create new nsproxy and all of its the associated namespaces.
* Return the newly created nsproxy. Do not attach this to the task,
* leave it to the caller to do proper locking and attach it to task.
*/
static struct nsproxy *create_new_namespaces(unsigned long flags,
struct task_struct *tsk, struct user_namespace *user_ns,
struct fs_struct *new_fs)
{
struct nsproxy *new_nsp;
int err;
new_nsp = create_nsproxy();
if (!new_nsp)
return ERR_PTR(-ENOMEM);
new_nsp->mnt_ns = copy_mnt_ns(flags, tsk->nsproxy->mnt_ns, user_ns, new_fs);
if (IS_ERR(new_nsp->mnt_ns)) {
err = PTR_ERR(new_nsp->mnt_ns);
goto out_ns;
}
new_nsp->uts_ns = copy_utsname(flags, user_ns, tsk->nsproxy->uts_ns);
if (IS_ERR(new_nsp->uts_ns)) {
err = PTR_ERR(new_nsp->uts_ns);
goto out_uts;
}
new_nsp->ipc_ns = copy_ipcs(flags, user_ns, tsk->nsproxy->ipc_ns);
if (IS_ERR(new_nsp->ipc_ns)) {
err = PTR_ERR(new_nsp->ipc_ns);
goto out_ipc;
}
new_nsp->pid_ns_for_children =
copy_pid_ns(flags, user_ns, tsk->nsproxy->pid_ns_for_children);
if (IS_ERR(new_nsp->pid_ns_for_children)) {
err = PTR_ERR(new_nsp->pid_ns_for_children);
goto out_pid;
}
new_nsp->cgroup_ns = copy_cgroup_ns(flags, user_ns,
tsk->nsproxy->cgroup_ns);
if (IS_ERR(new_nsp->cgroup_ns)) {
err = PTR_ERR(new_nsp->cgroup_ns);
goto out_cgroup;
}
new_nsp->net_ns = copy_net_ns(flags, user_ns, tsk->nsproxy->net_ns);
if (IS_ERR(new_nsp->net_ns)) {
err = PTR_ERR(new_nsp->net_ns);
goto out_net;
}
new_nsp->time_ns_for_children = copy_time_ns(flags, user_ns,
tsk->nsproxy->time_ns_for_children);
if (IS_ERR(new_nsp->time_ns_for_children)) {
err = PTR_ERR(new_nsp->time_ns_for_children);
goto out_time;
}
new_nsp->time_ns = get_time_ns(tsk->nsproxy->time_ns);
return new_nsp;
out_time:
put_net(new_nsp->net_ns);
out_net:
put_cgroup_ns(new_nsp->cgroup_ns);
out_cgroup:
if (new_nsp->pid_ns_for_children)
put_pid_ns(new_nsp->pid_ns_for_children);
out_pid:
if (new_nsp->ipc_ns)
put_ipc_ns(new_nsp->ipc_ns);
out_ipc:
if (new_nsp->uts_ns)
put_uts_ns(new_nsp->uts_ns);
out_uts:
if (new_nsp->mnt_ns)
put_mnt_ns(new_nsp->mnt_ns);
out_ns:
kmem_cache_free(nsproxy_cachep, new_nsp);
return ERR_PTR(err);
}
/*
* called from clone. This now handles copy for nsproxy and all
* namespaces therein.
*/
int copy_namespaces(unsigned long flags, struct task_struct *tsk)
{
struct nsproxy *old_ns = tsk->nsproxy;
struct user_namespace *user_ns = task_cred_xxx(tsk, user_ns);
struct nsproxy *new_ns;
int ret;
if (likely(!(flags & (CLONE_NEWNS | CLONE_NEWUTS | CLONE_NEWIPC |
CLONE_NEWPID | CLONE_NEWNET |
CLONE_NEWCGROUP | CLONE_NEWTIME)))) {
if (likely(old_ns->time_ns_for_children == old_ns->time_ns)) {
get_nsproxy(old_ns);
return 0;
}
} else if (!ns_capable(user_ns, CAP_SYS_ADMIN))
return -EPERM;
/*
* CLONE_NEWIPC must detach from the undolist: after switching
* to a new ipc namespace, the semaphore arrays from the old
* namespace are unreachable. In clone parlance, CLONE_SYSVSEM
* means share undolist with parent, so we must forbid using
* it along with CLONE_NEWIPC.
*/
if ((flags & (CLONE_NEWIPC | CLONE_SYSVSEM)) ==
(CLONE_NEWIPC | CLONE_SYSVSEM))
return -EINVAL;
new_ns = create_new_namespaces(flags, tsk, user_ns, tsk->fs);
if (IS_ERR(new_ns))
return PTR_ERR(new_ns);
ret = timens_on_fork(new_ns, tsk);
if (ret) {
free_nsproxy(new_ns);
return ret;
}
tsk->nsproxy = new_ns;
return 0;
}
void free_nsproxy(struct nsproxy *ns)
{
if (ns->mnt_ns)
put_mnt_ns(ns->mnt_ns);
if (ns->uts_ns)
put_uts_ns(ns->uts_ns);
if (ns->ipc_ns)
put_ipc_ns(ns->ipc_ns);
if (ns->pid_ns_for_children)
put_pid_ns(ns->pid_ns_for_children);
if (ns->time_ns)
put_time_ns(ns->time_ns);
if (ns->time_ns_for_children)
put_time_ns(ns->time_ns_for_children);
put_cgroup_ns(ns->cgroup_ns);
put_net(ns->net_ns);
kmem_cache_free(nsproxy_cachep, ns);
}
/*
* Called from unshare. Unshare all the namespaces part of nsproxy.
* On success, returns the new nsproxy.
*/
int unshare_nsproxy_namespaces(unsigned long unshare_flags,
struct nsproxy **new_nsp, struct cred *new_cred, struct fs_struct *new_fs)
{
struct user_namespace *user_ns;
int err = 0;
if (!(unshare_flags & (CLONE_NEWNS | CLONE_NEWUTS | CLONE_NEWIPC |
CLONE_NEWNET | CLONE_NEWPID | CLONE_NEWCGROUP |
CLONE_NEWTIME)))
return 0;
user_ns = new_cred ? new_cred->user_ns : current_user_ns();
if (!ns_capable(user_ns, CAP_SYS_ADMIN))
return -EPERM;
*new_nsp = create_new_namespaces(unshare_flags, current, user_ns,
new_fs ? new_fs : current->fs);
if (IS_ERR(*new_nsp)) {
err = PTR_ERR(*new_nsp);
goto out;
}
out:
return err;
}
void switch_task_namespaces(struct task_struct *p, struct nsproxy *new)
{
struct nsproxy *ns;
might_sleep();
task_lock(p);
ns = p->nsproxy;
p->nsproxy = new;
task_unlock(p);
if (ns && atomic_dec_and_test(&ns->count))
free_nsproxy(ns);
}
void exit_task_namespaces(struct task_struct *p)
{
switch_task_namespaces(p, NULL);
}
static int check_setns_flags(unsigned long flags)
{
if (!flags || (flags & ~(CLONE_NEWNS | CLONE_NEWUTS | CLONE_NEWIPC |
CLONE_NEWNET | CLONE_NEWUSER | CLONE_NEWPID |
CLONE_NEWCGROUP)))
return -EINVAL;
#ifndef CONFIG_USER_NS
if (flags & CLONE_NEWUSER)
return -EINVAL;
#endif
#ifndef CONFIG_PID_NS
if (flags & CLONE_NEWPID)
return -EINVAL;
#endif
#ifndef CONFIG_UTS_NS
if (flags & CLONE_NEWUTS)
return -EINVAL;
#endif
#ifndef CONFIG_IPC_NS
if (flags & CLONE_NEWIPC)
return -EINVAL;
#endif
#ifndef CONFIG_CGROUPS
if (flags & CLONE_NEWCGROUP)
return -EINVAL;
#endif
#ifndef CONFIG_NET_NS
if (flags & CLONE_NEWNET)
return -EINVAL;
#endif
return 0;
}
static void put_nsset(struct nsset *nsset)
{
unsigned flags = nsset->flags;
if (flags & CLONE_NEWUSER)
put_cred(nsset_cred(nsset));
/*
* We only created a temporary copy if we attached to more than just
* the mount namespace.
*/
if (nsset->fs && (flags & CLONE_NEWNS) && (flags & ~CLONE_NEWNS))
free_fs_struct(nsset->fs);
if (nsset->nsproxy)
free_nsproxy(nsset->nsproxy);
}
static int prepare_nsset(unsigned flags, struct nsset *nsset)
{
struct task_struct *me = current;
nsset->nsproxy = create_new_namespaces(0, me, current_user_ns(), me->fs);
if (IS_ERR(nsset->nsproxy))
return PTR_ERR(nsset->nsproxy);
if (flags & CLONE_NEWUSER)
nsset->cred = prepare_creds();
else
nsset->cred = current_cred();
if (!nsset->cred)
goto out;
/* Only create a temporary copy of fs_struct if we really need to. */
if (flags == CLONE_NEWNS) {
nsset->fs = me->fs;
} else if (flags & CLONE_NEWNS) {
nsset->fs = copy_fs_struct(me->fs);
if (!nsset->fs)
goto out;
}
nsset->flags = flags;
return 0;
out:
put_nsset(nsset);
return -ENOMEM;
}
static inline int validate_ns(struct nsset *nsset, struct ns_common *ns)
{
return ns->ops->install(nsset, ns);
}
/*
* This is the inverse operation to unshare().
* Ordering is equivalent to the standard ordering used everywhere else
* during unshare and process creation. The switch to the new set of
* namespaces occurs at the point of no return after installation of
* all requested namespaces was successful in commit_nsset().
*/
static int validate_nsset(struct nsset *nsset, struct pid *pid)
{
int ret = 0;
unsigned flags = nsset->flags;
struct user_namespace *user_ns = NULL;
struct pid_namespace *pid_ns = NULL;
struct nsproxy *nsp;
struct task_struct *tsk;
/* Take a "snapshot" of the target task's namespaces. */
rcu_read_lock();
tsk = pid_task(pid, PIDTYPE_PID);
if (!tsk) {
rcu_read_unlock();
return -ESRCH;
}
if (!ptrace_may_access(tsk, PTRACE_MODE_READ_REALCREDS)) {
rcu_read_unlock();
return -EPERM;
}
task_lock(tsk);
nsp = tsk->nsproxy;
if (nsp)
get_nsproxy(nsp);
task_unlock(tsk);
if (!nsp) {
rcu_read_unlock();
return -ESRCH;
}
#ifdef CONFIG_PID_NS
if (flags & CLONE_NEWPID) {
pid_ns = task_active_pid_ns(tsk);
if (unlikely(!pid_ns)) {
rcu_read_unlock();
ret = -ESRCH;
goto out;
}
get_pid_ns(pid_ns);
}
#endif
#ifdef CONFIG_USER_NS
if (flags & CLONE_NEWUSER)
user_ns = get_user_ns(__task_cred(tsk)->user_ns);
#endif
rcu_read_unlock();
/*
* Install requested namespaces. The caller will have
* verified earlier that the requested namespaces are
* supported on this kernel. We don't report errors here
* if a namespace is requested that isn't supported.
*/
#ifdef CONFIG_USER_NS
if (flags & CLONE_NEWUSER) {
ret = validate_ns(nsset, &user_ns->ns);
if (ret)
goto out;
}
#endif
if (flags & CLONE_NEWNS) {
ret = validate_ns(nsset, from_mnt_ns(nsp->mnt_ns));
if (ret)
goto out;
}
#ifdef CONFIG_UTS_NS
if (flags & CLONE_NEWUTS) {
ret = validate_ns(nsset, &nsp->uts_ns->ns);
if (ret)
goto out;
}
#endif
#ifdef CONFIG_IPC_NS
if (flags & CLONE_NEWIPC) {
ret = validate_ns(nsset, &nsp->ipc_ns->ns);
if (ret)
goto out;
}
#endif
#ifdef CONFIG_PID_NS
if (flags & CLONE_NEWPID) {
ret = validate_ns(nsset, &pid_ns->ns);
if (ret)
goto out;
}
#endif
#ifdef CONFIG_CGROUPS
if (flags & CLONE_NEWCGROUP) {
ret = validate_ns(nsset, &nsp->cgroup_ns->ns);
if (ret)
goto out;
}
#endif
#ifdef CONFIG_NET_NS
if (flags & CLONE_NEWNET) {
ret = validate_ns(nsset, &nsp->net_ns->ns);
if (ret)
goto out;
}
#endif
out:
if (pid_ns)
put_pid_ns(pid_ns);
if (nsp)
put_nsproxy(nsp);
put_user_ns(user_ns);
return ret;
}
/*
* This is the point of no return. There are just a few namespaces
* that do some actual work here and it's sufficiently minimal that
* a separate ns_common operation seems unnecessary for now.
* Unshare is doing the same thing. If we'll end up needing to do
* more in a given namespace or a helper here is ultimately not
* exported anymore a simple commit handler for each namespace
* should be added to ns_common.
*/
static void commit_nsset(struct nsset *nsset)
{
unsigned flags = nsset->flags;
struct task_struct *me = current;
#ifdef CONFIG_USER_NS
if (flags & CLONE_NEWUSER) {
/* transfer ownership */
commit_creds(nsset_cred(nsset));
nsset->cred = NULL;
}
#endif
/* We only need to commit if we have used a temporary fs_struct. */
if ((flags & CLONE_NEWNS) && (flags & ~CLONE_NEWNS)) {
set_fs_root(me->fs, &nsset->fs->root);
set_fs_pwd(me->fs, &nsset->fs->pwd);
}
#ifdef CONFIG_IPC_NS
if (flags & CLONE_NEWIPC)
exit_sem(me);
#endif
/* transfer ownership */
switch_task_namespaces(me, nsset->nsproxy);
nsset->nsproxy = NULL;
}
SYSCALL_DEFINE2(setns, int, fd, int, flags)
{
struct file *file;
struct ns_common *ns = NULL;
struct nsset nsset = {};
int err = 0;
file = fget(fd);
if (!file)
return -EBADF;
if (proc_ns_file(file)) {
ns = get_proc_ns(file_inode(file));
if (flags && (ns->ops->type != flags))
err = -EINVAL;
flags = ns->ops->type;
} else if (!IS_ERR(pidfd_pid(file))) {
err = check_setns_flags(flags);
} else {
err = -EBADF;
}
if (err)
goto out;
err = prepare_nsset(flags, &nsset);
if (err)
goto out;
if (proc_ns_file(file))
err = validate_ns(&nsset, ns);
else
err = validate_nsset(&nsset, file->private_data);
if (!err) {
commit_nsset(&nsset);
perf_event_namespaces(current);
}
put_nsset(&nsset);
out:
fput(file);
return err;
}
int __init nsproxy_cache_init(void)
{
nsproxy_cachep = KMEM_CACHE(nsproxy, SLAB_PANIC);
return 0;
}
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