Commit 6a21cc50 authored by Tycho Andersen's avatar Tycho Andersen Committed by Kees Cook
Browse files

seccomp: add a return code to trap to userspace

This patch introduces a means for syscalls matched in seccomp to notify
some other task that a particular filter has been triggered.

The motivation for this is primarily for use with containers. For example,
if a container does an init_module(), we obviously don't want to load this
untrusted code, which may be compiled for the wrong version of the kernel
anyway. Instead, we could parse the module image, figure out which module
the container is trying to load and load it on the host.

As another example, containers cannot mount() in general since various
filesystems assume a trusted image. However, if an orchestrator knows that
e.g. a particular block device has not been exposed to a container for
writing, it want to allow the container to mount that block device (that
is, handle the mount for it).

This patch adds functionality that is already possible via at least two
other means that I know about, both of which involve ptrace(): first, one
could ptrace at...
parent a5662e4d
......@@ -79,6 +79,7 @@ Code Seq#(hex) Include File Comments
0x1b all InfiniBand Subsystem <http://infiniband.sourceforge.net/>
0x20 all drivers/cdrom/cm206.h
0x22 all scsi/sg.h
'!' 00-1F uapi/linux/seccomp.h
'#' 00-3F IEEE 1394 Subsystem Block for the entire subsystem
'$' 00-0F linux/perf_counter.h, linux/perf_event.h
'%' 00-0F include/uapi/linux/stm.h
......
......@@ -122,6 +122,11 @@ In precedence order, they are:
Results in the lower 16-bits of the return value being passed
to userland as the errno without executing the system call.
``SECCOMP_RET_USER_NOTIF``:
Results in a ``struct seccomp_notif`` message sent on the userspace
notification fd, if it is attached, or ``-ENOSYS`` if it is not. See below
on discussion of how to handle user notifications.
``SECCOMP_RET_TRACE``:
When returned, this value will cause the kernel to attempt to
notify a ``ptrace()``-based tracer prior to executing the system
......@@ -183,6 +188,85 @@ The ``samples/seccomp/`` directory contains both an x86-specific example
and a more generic example of a higher level macro interface for BPF
program generation.
Userspace Notification
======================
The ``SECCOMP_RET_USER_NOTIF`` return code lets seccomp filters pass a
particular syscall to userspace to be handled. This may be useful for
applications like container managers, which wish to intercept particular
syscalls (``mount()``, ``finit_module()``, etc.) and change their behavior.
To acquire a notification FD, use the ``SECCOMP_FILTER_FLAG_NEW_LISTENER``
argument to the ``seccomp()`` syscall:
.. code-block:: c
fd = seccomp(SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_NEW_LISTENER, &prog);
which (on success) will return a listener fd for the filter, which can then be
passed around via ``SCM_RIGHTS`` or similar. Note that filter fds correspond to
a particular filter, and not a particular task. So if this task then forks,
notifications from both tasks will appear on the same filter fd. Reads and
writes to/from a filter fd are also synchronized, so a filter fd can safely
have many readers.
The interface for a seccomp notification fd consists of two structures:
.. code-block:: c
struct seccomp_notif_sizes {
__u16 seccomp_notif;
__u16 seccomp_notif_resp;
__u16 seccomp_data;
};
struct seccomp_notif {
__u64 id;
__u32 pid;
__u32 flags;
struct seccomp_data data;
};
struct seccomp_notif_resp {
__u64 id;
__s64 val;
__s32 error;
__u32 flags;
};
The ``struct seccomp_notif_sizes`` structure can be used to determine the size
of the various structures used in seccomp notifications. The size of ``struct
seccomp_data`` may change in the future, so code should use:
.. code-block:: c
struct seccomp_notif_sizes sizes;
seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes);
to determine the size of the various structures to allocate. See
samples/seccomp/user-trap.c for an example.
Users can read via ``ioctl(SECCOMP_IOCTL_NOTIF_RECV)`` (or ``poll()``) on a
seccomp notification fd to receive a ``struct seccomp_notif``, which contains
five members: the input length of the structure, a unique-per-filter ``id``,
the ``pid`` of the task which triggered this request (which may be 0 if the
task is in a pid ns not visible from the listener's pid namespace), a ``flags``
member which for now only has ``SECCOMP_NOTIF_FLAG_SIGNALED``, representing
whether or not the notification is a result of a non-fatal signal, and the
``data`` passed to seccomp. Userspace can then make a decision based on this
information about what to do, and ``ioctl(SECCOMP_IOCTL_NOTIF_SEND)`` a
response, indicating what should be returned to userspace. The ``id`` member of
``struct seccomp_notif_resp`` should be the same ``id`` as in ``struct
seccomp_notif``.
It is worth noting that ``struct seccomp_data`` contains the values of register
arguments to the syscall, but does not contain pointers to memory. The task's
memory is accessible to suitably privileged traces via ``ptrace()`` or
``/proc/pid/mem``. However, care should be taken to avoid the TOCTOU mentioned
above in this document: all arguments being read from the tracee's memory
should be read into the tracer's memory before any policy decisions are made.
This allows for an atomic decision on syscall arguments.
Sysctls
=======
......
......@@ -4,9 +4,10 @@
#include <uapi/linux/seccomp.h>
#define SECCOMP_FILTER_FLAG_MASK (SECCOMP_FILTER_FLAG_TSYNC | \
SECCOMP_FILTER_FLAG_LOG | \
SECCOMP_FILTER_FLAG_SPEC_ALLOW)
#define SECCOMP_FILTER_FLAG_MASK (SECCOMP_FILTER_FLAG_TSYNC | \
SECCOMP_FILTER_FLAG_LOG | \
SECCOMP_FILTER_FLAG_SPEC_ALLOW | \
SECCOMP_FILTER_FLAG_NEW_LISTENER)
#ifdef CONFIG_SECCOMP
......
......@@ -15,11 +15,13 @@
#define SECCOMP_SET_MODE_STRICT 0
#define SECCOMP_SET_MODE_FILTER 1
#define SECCOMP_GET_ACTION_AVAIL 2
#define SECCOMP_GET_NOTIF_SIZES 3
/* Valid flags for SECCOMP_SET_MODE_FILTER */
#define SECCOMP_FILTER_FLAG_TSYNC (1UL << 0)
#define SECCOMP_FILTER_FLAG_LOG (1UL << 1)
#define SECCOMP_FILTER_FLAG_SPEC_ALLOW (1UL << 2)
#define SECCOMP_FILTER_FLAG_TSYNC (1UL << 0)
#define SECCOMP_FILTER_FLAG_LOG (1UL << 1)
#define SECCOMP_FILTER_FLAG_SPEC_ALLOW (1UL << 2)
#define SECCOMP_FILTER_FLAG_NEW_LISTENER (1UL << 3)
/*
* All BPF programs must return a 32-bit value.
......@@ -35,6 +37,7 @@
#define SECCOMP_RET_KILL SECCOMP_RET_KILL_THREAD
#define SECCOMP_RET_TRAP 0x00030000U /* disallow and force a SIGSYS */
#define SECCOMP_RET_ERRNO 0x00050000U /* returns an errno */
#define SECCOMP_RET_USER_NOTIF 0x7fc00000U /* notifies userspace */
#define SECCOMP_RET_TRACE 0x7ff00000U /* pass to a tracer or disallow */
#define SECCOMP_RET_LOG 0x7ffc0000U /* allow after logging */
#define SECCOMP_RET_ALLOW 0x7fff0000U /* allow */
......@@ -60,4 +63,35 @@ struct seccomp_data {
__u64 args[6];
};
struct seccomp_notif_sizes {
__u16 seccomp_notif;
__u16 seccomp_notif_resp;
__u16 seccomp_data;
};
struct seccomp_notif {
__u64 id;
__u32 pid;
__u32 flags;
struct seccomp_data data;
};
struct seccomp_notif_resp {
__u64 id;
__s64 val;
__s32 error;
__u32 flags;
};
#define SECCOMP_IOC_MAGIC '!'
#define SECCOMP_IO(nr) _IO(SECCOMP_IOC_MAGIC, nr)
#define SECCOMP_IOR(nr, type) _IOR(SECCOMP_IOC_MAGIC, nr, type)
#define SECCOMP_IOW(nr, type) _IOW(SECCOMP_IOC_MAGIC, nr, type)
#define SECCOMP_IOWR(nr, type) _IOWR(SECCOMP_IOC_MAGIC, nr, type)
/* Flags for seccomp notification fd ioctl. */
#define SECCOMP_IOCTL_NOTIF_RECV SECCOMP_IOWR(0, struct seccomp_notif)
#define SECCOMP_IOCTL_NOTIF_SEND SECCOMP_IOWR(1, \
struct seccomp_notif_resp)
#define SECCOMP_IOCTL_NOTIF_ID_VALID SECCOMP_IOR(2, __u64)
#endif /* _UAPI_LINUX_SECCOMP_H */
......@@ -33,12 +33,74 @@
#endif
#ifdef CONFIG_SECCOMP_FILTER
#include <linux/file.h>
#include <linux/filter.h>
#include <linux/pid.h>
#include <linux/ptrace.h>
#include <linux/security.h>
#include <linux/tracehook.h>
#include <linux/uaccess.h>
#include <linux/anon_inodes.h>
enum notify_state {
SECCOMP_NOTIFY_INIT,
SECCOMP_NOTIFY_SENT,
SECCOMP_NOTIFY_REPLIED,
};
struct seccomp_knotif {
/* The struct pid of the task whose filter triggered the notification */
struct task_struct *task;
/* The "cookie" for this request; this is unique for this filter. */
u64 id;
/*
* The seccomp data. This pointer is valid the entire time this
* notification is active, since it comes from __seccomp_filter which
* eclipses the entire lifecycle here.
*/
const struct seccomp_data *data;
/*
* Notification states. When SECCOMP_RET_USER_NOTIF is returned, a
* struct seccomp_knotif is created and starts out in INIT. Once the
* handler reads the notification off of an FD, it transitions to SENT.
* If a signal is received the state transitions back to INIT and
* another message is sent. When the userspace handler replies, state
* transitions to REPLIED.
*/
enum notify_state state;
/* The return values, only valid when in SECCOMP_NOTIFY_REPLIED */
int error;
long val;
/* Signals when this has entered SECCOMP_NOTIFY_REPLIED */
struct completion ready;
struct list_head list;
};
/**
* struct notification - container for seccomp userspace notifications. Since
* most seccomp filters will not have notification listeners attached and this
* structure is fairly large, we store the notification-specific stuff in a
* separate structure.
*
* @request: A semaphore that users of this notification can wait on for
* changes. Actual reads and writes are still controlled with
* filter->notify_lock.
* @next_id: The id of the next request.
* @notifications: A list of struct seccomp_knotif elements.
* @wqh: A wait queue for poll.
*/
struct notification {
struct semaphore request;
u64 next_id;
struct list_head notifications;
wait_queue_head_t wqh;
};
/**
* struct seccomp_filter - container for seccomp BPF programs
......@@ -50,6 +112,8 @@
* @log: true if all actions except for SECCOMP_RET_ALLOW should be logged
* @prev: points to a previously installed, or inherited, filter
* @prog: the BPF program to evaluate
* @notif: the struct that holds all notification related information
* @notify_lock: A lock for all notification-related accesses.
*
* seccomp_filter objects are organized in a tree linked via the @prev
* pointer. For any task, it appears to be a singly-linked list starting
......@@ -66,6 +130,8 @@ struct seccomp_filter {
bool log;
struct seccomp_filter *prev;
struct bpf_prog *prog;
struct notification *notif;
struct mutex notify_lock;
};
/* Limit any path through the tree to 256KB worth of instructions. */
......@@ -386,6 +452,7 @@ static struct seccomp_filter *seccomp_prepare_filter(struct sock_fprog *fprog)
if (!sfilter)
return ERR_PTR(-ENOMEM);
mutex_init(&sfilter->notify_lock);
ret = bpf_prog_create_from_user(&sfilter->prog, fprog,
seccomp_check_filter, save_orig);
if (ret < 0) {
......@@ -479,7 +546,6 @@ static long seccomp_attach_filter(unsigned int flags,
static void __get_seccomp_filter(struct seccomp_filter *filter)
{
/* Reference count is bounded by the number of total processes. */
refcount_inc(&filter->usage);
}
......@@ -550,11 +616,13 @@ static void seccomp_send_sigsys(int syscall, int reason)
#define SECCOMP_LOG_TRACE (1 << 4)
#define SECCOMP_LOG_LOG (1 << 5)
#define SECCOMP_LOG_ALLOW (1 << 6)
#define SECCOMP_LOG_USER_NOTIF (1 << 7)
static u32 seccomp_actions_logged = SECCOMP_LOG_KILL_PROCESS |
SECCOMP_LOG_KILL_THREAD |
SECCOMP_LOG_TRAP |
SECCOMP_LOG_ERRNO |
SECCOMP_LOG_USER_NOTIF |
SECCOMP_LOG_TRACE |
SECCOMP_LOG_LOG;
......@@ -575,6 +643,9 @@ static inline void seccomp_log(unsigned long syscall, long signr, u32 action,
case SECCOMP_RET_TRACE:
log = requested && seccomp_actions_logged & SECCOMP_LOG_TRACE;
break;
case SECCOMP_RET_USER_NOTIF:
log = requested && seccomp_actions_logged & SECCOMP_LOG_USER_NOTIF;
break;
case SECCOMP_RET_LOG:
log = seccomp_actions_logged & SECCOMP_LOG_LOG;
break;
......@@ -646,6 +717,68 @@ void secure_computing_strict(int this_syscall)
#else
#ifdef CONFIG_SECCOMP_FILTER
static u64 seccomp_next_notify_id(struct seccomp_filter *filter)
{
/*
* Note: overflow is ok here, the id just needs to be unique per
* filter.
*/
lockdep_assert_held(&filter->notify_lock);
return filter->notif->next_id++;
}
static void seccomp_do_user_notification(int this_syscall,
struct seccomp_filter *match,
const struct seccomp_data *sd)
{
int err;
long ret = 0;
struct seccomp_knotif n = {};
mutex_lock(&match->notify_lock);
err = -ENOSYS;
if (!match->notif)
goto out;
n.task = current;
n.state = SECCOMP_NOTIFY_INIT;
n.data = sd;
n.id = seccomp_next_notify_id(match);
init_completion(&n.ready);
list_add(&n.list, &match->notif->notifications);
up(&match->notif->request);
wake_up_poll(&match->notif->wqh, EPOLLIN | EPOLLRDNORM);
mutex_unlock(&match->notify_lock);
/*
* This is where we wait for a reply from userspace.
*/
err = wait_for_completion_interruptible(&n.ready);
mutex_lock(&match->notify_lock);
if (err == 0) {
ret = n.val;
err = n.error;
}
/*
* Note that it's possible the listener died in between the time when
* we were notified of a respons (or a signal) and when we were able to
* re-acquire the lock, so only delete from the list if the
* notification actually exists.
*
* Also note that this test is only valid because there's no way to
* *reattach* to a notifier right now. If one is added, we'll need to
* keep track of the notif itself and make sure they match here.
*/
if (match->notif)
list_del(&n.list);
out:
mutex_unlock(&match->notify_lock);
syscall_set_return_value(current, task_pt_regs(current),
err, ret);
}
static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
const bool recheck_after_trace)
{
......@@ -728,6 +861,10 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
return 0;
case SECCOMP_RET_USER_NOTIF:
seccomp_do_user_notification(this_syscall, match, sd);
goto skip;
case SECCOMP_RET_LOG:
seccomp_log(this_syscall, 0, action, true);
return 0;
......@@ -834,6 +971,263 @@ static long seccomp_set_mode_strict(void)
}
#ifdef CONFIG_SECCOMP_FILTER
static int seccomp_notify_release(struct inode *inode, struct file *file)
{
struct seccomp_filter *filter = file->private_data;
struct seccomp_knotif *knotif;
mutex_lock(&filter->notify_lock);
/*
* If this file is being closed because e.g. the task who owned it
* died, let's wake everyone up who was waiting on us.
*/
list_for_each_entry(knotif, &filter->notif->notifications, list) {
if (knotif->state == SECCOMP_NOTIFY_REPLIED)
continue;
knotif->state = SECCOMP_NOTIFY_REPLIED;
knotif->error = -ENOSYS;
knotif->val = 0;
complete(&knotif->ready);
}
kfree(filter->notif);
filter->notif = NULL;
mutex_unlock(&filter->notify_lock);
__put_seccomp_filter(filter);
return 0;
}
static long seccomp_notify_recv(struct seccomp_filter *filter,
void __user *buf)
{
struct seccomp_knotif *knotif = NULL, *cur;
struct seccomp_notif unotif;
ssize_t ret;
memset(&unotif, 0, sizeof(unotif));
ret = down_interruptible(&filter->notif->request);
if (ret < 0)
return ret;
mutex_lock(&filter->notify_lock);
list_for_each_entry(cur, &filter->notif->notifications, list) {
if (cur->state == SECCOMP_NOTIFY_INIT) {
knotif = cur;
break;
}
}
/*
* If we didn't find a notification, it could be that the task was
* interrupted by a fatal signal between the time we were woken and
* when we were able to acquire the rw lock.
*/
if (!knotif) {
ret = -ENOENT;
goto out;
}
unotif.id = knotif->id;
unotif.pid = task_pid_vnr(knotif->task);
unotif.data = *(knotif->data);
knotif->state = SECCOMP_NOTIFY_SENT;
wake_up_poll(&filter->notif->wqh, EPOLLOUT | EPOLLWRNORM);
ret = 0;
out:
mutex_unlock(&filter->notify_lock);
if (ret == 0 && copy_to_user(buf, &unotif, sizeof(unotif))) {
ret = -EFAULT;
/*
* Userspace screwed up. To make sure that we keep this
* notification alive, let's reset it back to INIT. It
* may have died when we released the lock, so we need to make
* sure it's still around.
*/
knotif = NULL;
mutex_lock(&filter->notify_lock);
list_for_each_entry(cur, &filter->notif->notifications, list) {
if (cur->id == unotif.id) {
knotif = cur;
break;
}
}
if (knotif) {
knotif->state = SECCOMP_NOTIFY_INIT;
up(&filter->notif->request);
}
mutex_unlock(&filter->notify_lock);
}
return ret;
}
static long seccomp_notify_send(struct seccomp_filter *filter,
void __user *buf)
{
struct seccomp_notif_resp resp = {};
struct seccomp_knotif *knotif = NULL, *cur;
long ret;
if (copy_from_user(&resp, buf, sizeof(resp)))
return -EFAULT;
if (resp.flags)
return -EINVAL;
ret = mutex_lock_interruptible(&filter->notify_lock);
if (ret < 0)
return ret;
list_for_each_entry(cur, &filter->notif->notifications, list) {
if (cur->id == resp.id) {
knotif = cur;
break;
}
}
if (!knotif) {
ret = -ENOENT;
goto out;
}
/* Allow exactly one reply. */
if (knotif->state != SECCOMP_NOTIFY_SENT) {
ret = -EINPROGRESS;
goto out;
}
ret = 0;
knotif->state = SECCOMP_NOTIFY_REPLIED;
knotif->error = resp.error;
knotif->val = resp.val;
complete(&knotif->ready);
out:
mutex_unlock(&filter->notify_lock);
return ret;
}
static long seccomp_notify_id_valid(struct seccomp_filter *filter,
void __user *buf)
{
struct seccomp_knotif *knotif = NULL;
u64 id;
long ret;
if (copy_from_user(&id, buf, sizeof(id)))
return -EFAULT;
ret = mutex_lock_interruptible(&filter->notify_lock);
if (ret < 0)
return ret;
ret = -ENOENT;
list_for_each_entry(knotif, &filter->notif->notifications, list) {
if (knotif->id == id) {
if (knotif->state == SECCOMP_NOTIFY_SENT)
ret = 0;
goto out;
}
}
out:
mutex_unlock(&filter->notify_lock);
return ret;
}
static long seccomp_notify_ioctl(struct file *file, unsigned int cmd,
unsigned long arg)
{
struct seccomp_filter *filter = file->private_data;
void __user *buf = (void __user *)arg;
switch (cmd) {
case SECCOMP_IOCTL_NOTIF_RECV:
return seccomp_notify_recv(filter, buf);
case SECCOMP_IOCTL_NOTIF_SEND:
return seccomp_notify_send(filter, buf);
case SECCOMP_IOCTL_NOTIF_ID_VALID:
return seccomp_notify_id_valid(filter, buf);
default:
return -EINVAL;
}
}
static __poll_t seccomp_notify_poll(struct file *file,
struct poll_table_struct *poll_tab)
{
struct seccomp_filter *filter = file->private_data;
__poll_t ret = 0;
struct seccomp_knotif *cur;
poll_wait(file, &filter->notif->wqh, poll_tab);
ret = mutex_lock_interruptible(&filter->notify_lock);
if (ret < 0)
return EPOLLERR;
list_for_each_entry(cur, &filter->notif->notifications, list) {
if (cur->state == SECCOMP_NOTIFY_INIT)
ret |= EPOLLIN | EPOLLRDNORM;
if (cur->state == SECCOMP_NOTIFY_SENT)
ret |= EPOLLOUT | EPOLLWRNORM;
if ((ret & EPOLLIN) && (ret & EPOLLOUT))
break;
}
mutex_unlock(&filter->notify_lock);
return ret;
}
static const struct file_operations seccomp_notify_ops = {
.poll = seccomp_notify_poll,
.release = seccomp_notify_release,
.unlocked_ioctl = seccomp_notify_ioctl,
};
static struct file *init_listener(struct seccomp_filter *filter)
{
struct file *ret = ERR_PTR(-EBUSY);
struct seccomp_filter *cur;
for (cur = current->seccomp.filter; cur; cur = cur->prev) {
if (cur->notif)
goto out;
}
ret = ERR_PTR(-ENOMEM);
filter->notif = kzalloc(sizeof(*(filter->notif)), GFP_KERNEL);
if (!filter->notif)
goto out;
sema_init(&filter->notif->request, 0);