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SubjectRe: For review: seccomp_user_notif(2) manual page
On Wed, Sep 30, 2020 at 1:07 PM Michael Kerrisk (man-pages)
<mtk.manpages@gmail.com> wrote:
> I knew it would be a big ask, but below is kind of the manual page
> I was hoping you might write [1] for the seccomp user-space notification
> mechanism. Since you didn't (and because 5.9 adds various new pieces
> such as SECCOMP_ADDFD_FLAG_SETFD and SECCOMP_IOCTL_NOTIF_ADDFD
> that also will need documenting [2]), I did :-). But of course I may
> have made mistakes...
[...]
> NAME
> seccomp_user_notif - Seccomp user-space notification mechanism
>
> SYNOPSIS
> #include <linux/seccomp.h>
> #include <linux/filter.h>
> #include <linux/audit.h>
>
> int seccomp(unsigned int operation, unsigned int flags, void *args);

Should the ioctl() calls be listed here, similar to e.g. the SYNOPSIS
of the ioctl_* manpages?

> DESCRIPTION
> This page describes the user-space notification mechanism pro‐
> vided by the Secure Computing (seccomp) facility. As well as the
> use of the SECCOMP_FILTER_FLAG_NEW_LISTENER flag, the SEC‐
> COMP_RET_USER_NOTIF action value, and the SECCOMP_GET_NOTIF_SIZES
> operation described in seccomp(2), this mechanism involves the
> use of a number of related ioctl(2) operations (described below).
>
> Overview
> In conventional usage of a seccomp filter, the decision about how
> to treat a particular system call is made by the filter itself.
> The user-space notification mechanism allows the handling of the
> system call to instead be handed off to a user-space process.
> The advantages of doing this are that, by contrast with the sec‐
> comp filter, which is running on a virtual machine inside the
> kernel, the user-space process has access to information that is
> unavailable to the seccomp filter and it can perform actions that
> can't be performed from the seccomp filter.
>
> In the discussion that follows, the process that has installed
> the seccomp filter is referred to as the target, and the process

Technically, this definition of "target" is a bit inaccurate because:

- seccomp filters are inherited
- seccomp filters apply to threads, not processes
- seccomp filters can be semi-remotely installed via TSYNC

(I assume that in manpages, we should try to go for the "a task is a
thread and a thread group is a process" definition, right?)

Perhaps "the threads on which the seccomp filter is installed are
referred to as the target", or something like that would be better?

> that is notified by the user-space notification mechanism is
> referred to as the supervisor. An overview of the steps per‐
> formed by these two processes is as follows:
>
> 1. The target process establishes a seccomp filter in the usual
> manner, but with two differences:
>
> · The seccomp(2) flags argument includes the flag SECCOMP_FIL‐
> TER_FLAG_NEW_LISTENER. Consequently, the return value of
> the (successful) seccomp(2) call is a new "listening" file
> descriptor that can be used to receive notifications.
>
> · In cases where it is appropriate, the seccomp filter returns
> the action value SECCOMP_RET_USER_NOTIF. This return value
> will trigger a notification event.
>
> 2. In order that the supervisor process can obtain notifications
> using the listening file descriptor, (a duplicate of) that
> file descriptor must be passed from the target process to the
> supervisor process. One way in which this could be done is by
> passing the file descriptor over a UNIX domain socket connec‐
> tion between the two processes (using the SCM_RIGHTS ancillary
> message type described in unix(7)). Another possibility is
> that the supervisor might inherit the file descriptor via
> fork(2).

With the caveat that if the supervisor inherits the file descriptor
via fork(), that (more or less) implies that the supervisor is subject
to the same filter (although it could bypass the filter using a helper
thread that responds SECCOMP_USER_NOTIF_FLAG_CONTINUE, but I don't
expect any clean software to do that).

> 3. The supervisor process will receive notification events on the
> listening file descriptor. These events are returned as
> structures of type seccomp_notif. Because this structure and
> its size may evolve over kernel versions, the supervisor must
> first determine the size of this structure using the sec‐
> comp(2) SECCOMP_GET_NOTIF_SIZES operation, which returns a
> structure of type seccomp_notif_sizes. The supervisor allo‐
> cates a buffer of size seccomp_notif_sizes.seccomp_notif bytes
> to receive notification events. In addition,the supervisor
> allocates another buffer of size seccomp_notif_sizes.sec‐
> comp_notif_resp bytes for the response (a struct sec‐
> comp_notif_resp structure) that it will provide to the kernel
> (and thus the target process).
>
> 4. The target process then performs its workload, which includes
> system calls that will be controlled by the seccomp filter.
> Whenever one of these system calls causes the filter to return
> the SECCOMP_RET_USER_NOTIF action value, the kernel does not
> execute the system call; instead, execution of the target
> process is temporarily blocked inside the kernel and a notifi‐

where "blocked" refers to the interruptible, restartable kind - if the
child receives a signal with an SA_RESTART signal handler in the
meantime, it'll leave the syscall, go through the signal handler, then
restart the syscall again and send the same request to the supervisor
again. so the supervisor may see duplicate syscalls.

What's really gross here is that signal(7) promises that some syscalls
like epoll_wait(2) never restart, but seccomp doesn't know about that;
if userspace installs a filter that uses SECCOMP_RET_USER_NOTIF for a
non-restartable syscall, the result is that UAPI gets broken a little
bit. Luckily normal users of seccomp probably won't use
SECCOMP_RET_USER_NOTIF for restartable syscalls, but if someone does
want to do that, we might have to add some "suppress syscall
restarting" flag into the seccomp action value, or something like
that... yuck.

> cation event is generated on the listening file descriptor.
>
> 5. The supervisor process can now repeatedly monitor the listen‐
> ing file descriptor for SECCOMP_RET_USER_NOTIF-triggered
> events. To do this, the supervisor uses the SEC‐
> COMP_IOCTL_NOTIF_RECV ioctl(2) operation to read information
> about a notification event; this operation blocks until an

(interruptably - but I guess that maybe doesn't have to be said
explicitly here?)

> event is available.

Maybe we should note here that you can use the multi-fd-polling APIs
(select/poll/epoll) instead, and that if the notification goes away
before you call SECCOMP_IOCTL_NOTIF_RECV, the ioctl will return
-ENOENT instead of blocking, and therefore as long as nobody else
reads from the same fd, you can assume that after the fd reports as
readable, you can call SECCOMP_IOCTL_NOTIF_RECV once without blocking.

Exceeeeept that this part looks broken:

if (mutex_lock_interruptible(&filter->notify_lock) < 0)
return EPOLLERR;

which I think means that we can have a race where a signal arrives
while poll() is trying to add itself to the waitqueue of the seccomp
fd, and then we'll get a spurious error condition reported on the fd.
That's a kernel bug, I'd say.

> The operation returns a seccomp_notif
> structure containing information about the system call that is
> being attempted by the target process.
>
> 6. The seccomp_notif structure returned by the SEC‐
> COMP_IOCTL_NOTIF_RECV operation includes the same information
> (a seccomp_data structure) that was passed to the seccomp fil‐
> ter. This information allows the supervisor to discover the
> system call number and the arguments for the target process's
> system call. In addition, the notification event contains the
> PID of the target process.

That's a PIDTYPE_PID, which the manpages call a "thread ID".

> The information in the notification can be used to discover
> the values of pointer arguments for the target process's sys‐
> tem call. (This is something that can't be done from within a
> seccomp filter.) To do this (and assuming it has suitable
> permissions), the supervisor opens the corresponding
> /proc/[pid]/mem file,

... which means that here we might have to get into the weeds of how
actually /proc has invisible directories for every TID, even though
only the ones for PIDs are visible, and therefore you can just open
/proc/[tid]/mem and it'll work fine?

> seeks to the memory location that corre‐
> sponds to one of the pointer arguments whose value is supplied
> in the notification event, and reads bytes from that location.
> (The supervisor must be careful to avoid a race condition that
> can occur when doing this; see the description of the SEC‐
> COMP_IOCTL_NOTIF_ID_VALID ioctl(2) operation below.) In addi‐
> tion, the supervisor can access other system information that
> is visible in user space but which is not accessible from a
> seccomp filter.
>
> ┌─────────────────────────────────────────────────────┐
> │FIXME │
> ├─────────────────────────────────────────────────────┤
> │Suppose we are reading a pathname from /proc/PID/mem │
> │for a system call such as mkdir(). The pathname can │
> │be an arbitrary length. How do we know how much (how │
> │many pages) to read from /proc/PID/mem? │
> └─────────────────────────────────────────────────────┘

It can't be an arbitrary length. While pathnames *returned* from the
kernel in some places can have different limits, strings supplied as
path arguments *to* the kernel AFAIK always have an upper limit of
PATH_MAX, else you get -ENAMETOOLONG. See getname_flags().

> 7. Having obtained information as per the previous step, the
> supervisor may then choose to perform an action in response to
> the target process's system call (which, as noted above, is
> not executed when the seccomp filter returns the SEC‐
> COMP_RET_USER_NOTIF action value).

(unless SECCOMP_USER_NOTIF_FLAG_CONTINUE is used)

> One example use case here relates to containers. The target
> process may be located inside a container where it does not
> have sufficient capabilities to mount a filesystem in the con‐
> tainer's mount namespace. However, the supervisor may be a
> more privileged process that that does have sufficient capa‐

nit: s/that that/that/

> bilities to perform the mount operation.
>
> 8. The supervisor then sends a response to the notification. The
> information in this response is used by the kernel to con‐
> struct a return value for the target process's system call and
> provide a value that will be assigned to the errno variable of
> the target process.
>
> The response is sent using the SECCOMP_IOCTL_NOTIF_RECV
> ioctl(2) operation, which is used to transmit a sec‐
> comp_notif_resp structure to the kernel. This structure
> includes a cookie value that the supervisor obtained in the
> seccomp_notif structure returned by the SEC‐
> COMP_IOCTL_NOTIF_RECV operation. This cookie value allows the
> kernel to associate the response with the target process.

(unless if the target thread entered a signal handler or was killed in
the meantime)

> 9. Once the notification has been sent, the system call in the
> target process unblocks, returning the information that was
> provided by the supervisor in the notification response.
>
> As a variation on the last two steps, the supervisor can send a
> response that tells the kernel that it should execute the target
> process's system call; see the discussion of SEC‐
> COMP_USER_NOTIF_FLAG_CONTINUE, below.
>
> ioctl(2) operations
> The following ioctl(2) operations are provided to support seccomp
> user-space notification. For each of these operations, the first
> (file descriptor) argument of ioctl(2) is the listening file
> descriptor returned by a call to seccomp(2) with the SECCOMP_FIL‐
> TER_FLAG_NEW_LISTENER flag.
>
> SECCOMP_IOCTL_NOTIF_RECV
> This operation is used to obtain a user-space notification
> event. If no such event is currently pending, the opera‐
> tion blocks until an event occurs.

Not necessarily; for every time a process entered a signal handler or
was killed while a notification was pending, a call to
SECCOMP_IOCTL_NOTIF_RECV will return -ENOENT.

> The third ioctl(2)
> argument is a pointer to a structure of the following form
> which contains information about the event. This struc‐
> ture must be zeroed out before the call.
>
> struct seccomp_notif {
> __u64 id; /* Cookie */
> __u32 pid; /* PID of target process */

(TID, not PID)

> __u32 flags; /* Currently unused (0) */
> struct seccomp_data data; /* See seccomp(2) */
> };
>
> The fields in this structure are as follows:
>
> id This is a cookie for the notification. Each such
> cookie is guaranteed to be unique for the corre‐
> sponding seccomp filter. In other words, this
> cookie is unique for each notification event from
> the target process.

That sentence about "target process" looks wrong to me. The cookies
are unique across notifications from the filter, but there can be
multiple filters per thread, and multiple threads per filter.

> The cookie value has the fol‐
> lowing uses:
>
> · It can be used with the SEC‐
> COMP_IOCTL_NOTIF_ID_VALID ioctl(2) operation to
> verify that the target process is still alive.
>
> · When returning a notification response to the
> kernel, the supervisor must include the cookie
> value in the seccomp_notif_resp structure that is
> specified as the argument of the SEC‐
> COMP_IOCTL_NOTIF_SEND operation.
>
> pid This is the PID of the target process that trig‐
> gered the notification event.
>
> ┌─────────────────────────────────────────────────────┐
> │FIXME │
> ├─────────────────────────────────────────────────────┤
> │This is a thread ID, rather than a PID, right? │
> └─────────────────────────────────────────────────────┘

Yeah.

>
> flags This is a bit mask of flags providing further
> information on the event. In the current implemen‐
> tation, this field is always zero.
>
> data This is a seccomp_data structure containing infor‐
> mation about the system call that triggered the
> notification. This is the same structure that is
> passed to the seccomp filter. See seccomp(2) for
> details of this structure.
>
> On success, this operation returns 0; on failure, -1 is
> returned, and errno is set to indicate the cause of the
> error. This operation can fail with the following errors:
>
> EINVAL (since Linux 5.5)
> The seccomp_notif structure that was passed to the
> call contained nonzero fields.
>
> ENOENT The target process was killed by a signal as the
> notification information was being generated.

Not just killed, interruption with a signal handler has the same effect.

> ┌─────────────────────────────────────────────────────┐
> │FIXME │
> ├─────────────────────────────────────────────────────┤
> │From my experiments, it appears that if a SEC‐ │
> │COMP_IOCTL_NOTIF_RECV is done after the target │
> │process terminates, then the ioctl() simply blocks │
> │(rather than returning an error to indicate that the │
> │target process no longer exists). │
> │ │
> │I found that surprising, and it required some con‐ │
> │tortions in the example program. It was not possi‐ │
> │ble to code my SIGCHLD handler (which reaps the zom‐ │
> │bie when the worker/target process terminates) to │
> │simply set a flag checked in the main handleNotifi‐ │
> │cations() loop, since this created an unavoidable │
> │race where the child might terminate just after I │
> │had checked the flag, but before I blocked (for‐ │
> │ever!) in the SECCOMP_IOCTL_NOTIF_RECV operation. │
> │Instead, I had to code the signal handler to simply │
> │call _exit(2) in order to terminate the parent │
> │process (the supervisor). │
> │ │
> │Is this expected behavior? It seems to me rather │
> │desirable that SECCOMP_IOCTL_NOTIF_RECV should give │
> │an error if the target process has terminated. │
> └─────────────────────────────────────────────────────┘

You could poll() the fd first. But yeah, it'd probably be a good idea
to change that.

> SECCOMP_IOCTL_NOTIF_ID_VALID
[...]
> In the above scenario, the risk is that the supervisor may
> try to access the memory of a process other than the tar‐
> get. This race can be avoided by following the call to
> open with a SECCOMP_IOCTL_NOTIF_ID_VALID operation to ver‐
> ify that the process that generated the notification is
> still alive. (Note that if the target process subse‐
> quently terminates, its PID won't be reused because there

That's wrong, the PID can be reused, but the /proc/$pid directory is
internally not associated with the numeric PID, but, conceptually
speaking, with a specific incarnation of the PID, or something like
that. (Actually, it is associated with the "struct pid", which is not
reused, instead of the numeric PID.)

> remains an open reference to the /proc[pid]/mem file; in
> this case, a subsequent read(2) from the file will return
> 0, indicating end of file.)
>
> On success (i.e., the notification ID is still valid),
> this operation returns 0 On failure (i.e., the notifica‐

nit: s/returns 0/returns 0./

> tion ID is no longer valid), -1 is returned, and errno is
> set to ENOENT.
>
> SECCOMP_IOCTL_NOTIF_SEND
[...]
> Two kinds of response are possible:
>
> · A response to the kernel telling it to execute the tar‐
> get process's system call. In this case, the flags
> field includes SECCOMP_USER_NOTIF_FLAG_CONTINUE and the
> error and val fields must be zero.
>
> This kind of response can be useful in cases where the
> supervisor needs to do deeper analysis of the target's
> system call than is possible from a seccomp filter
> (e.g., examining the values of pointer arguments), and,
> having verified that the system call is acceptable, the
> supervisor wants to allow it to proceed.

"allow" sounds as if this is an access control thing, but this
mechanism should usually not be used for access control (unless the
"seccomp" syscall is blocked). Maybe reword as "having decided that
the system call does not require emulation by the supervisor, the
supervisor wants it to execute normally", or something like that?

[...]
> On success, this operation returns 0; on failure, -1 is
> returned, and errno is set to indicate the cause of the
> error. This operation can fail with the following errors:
>
> EINPROGRESS
> A response to this notification has already been
> sent.
>
> EINVAL An invalid value was specified in the flags field.
>
> EINVAL The flags field contained SEC‐
> COMP_USER_NOTIF_FLAG_CONTINUE, and the error or val
> field was not zero.
>
> ENOENT The blocked system call in the target process has
> been interrupted by a signal handler.

(you could also get this if a response has already been sent, instead
of EINPROGRESS - the only difference is whether the target thread has
picked up the response yet)

> NOTES
> The file descriptor returned when seccomp(2) is employed with the
> SECCOMP_FILTER_FLAG_NEW_LISTENER flag can be monitored using
> poll(2), epoll(7), and select(2). When a notification is pend‐
> ing, these interfaces indicate that the file descriptor is read‐
> able.

We should probably also point out somewhere that, as
include/uapi/linux/seccomp.h says:

* Similar precautions should be applied when stacking SECCOMP_RET_USER_NOTIF
* or SECCOMP_RET_TRACE. For SECCOMP_RET_USER_NOTIF filters acting on the
* same syscall, the most recently added filter takes precedence. This means
* that the new SECCOMP_RET_USER_NOTIF filter can override any
* SECCOMP_IOCTL_NOTIF_SEND from earlier filters, essentially allowing all
* such filtered syscalls to be executed by sending the response
* SECCOMP_USER_NOTIF_FLAG_CONTINUE. Note that SECCOMP_RET_TRACE can equally
* be overriden by SECCOMP_USER_NOTIF_FLAG_CONTINUE.

In other words, from a security perspective, you must assume that the
target process can bypass any SECCOMP_RET_USER_NOTIF (or
SECCOMP_RET_TRACE) filters unless it is completely prohibited from
calling seccomp(). This should also be noted over in the main
seccomp(2) manpage, especially the SECCOMP_RET_TRACE part.


> EXAMPLES
[...]
> This program can used to demonstrate various aspects of the

nit: "can be used to demonstrate", or alternatively just "demonstrates"

> behavior of the seccomp user-space notification mechanism. To
> help aid such demonstrations, the program logs various messages
> to show the operation of the target process (lines prefixed "T:")
> and the supervisor (indented lines prefixed "S:").
[...]
> Program source
[...]
> #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
> } while (0)

Don't we have err() for this?

> /* Send the file descriptor 'fd' over the connected UNIX domain socket
> 'sockfd'. Returns 0 on success, or -1 on error. */
>
> static int
> sendfd(int sockfd, int fd)
> {
> struct msghdr msgh;
> struct iovec iov;
> int data;
> struct cmsghdr *cmsgp;
>
> /* Allocate a char array of suitable size to hold the ancillary data.
> However, since this buffer is in reality a 'struct cmsghdr', use a
> union to ensure that it is suitable aligned. */

nit: suitably

> union {
> char buf[CMSG_SPACE(sizeof(int))];
> /* Space large enough to hold an 'int' */
> struct cmsghdr align;
> } controlMsg;
>
> /* The 'msg_name' field can be used to specify the address of the
> destination socket when sending a datagram. However, we do not
> need to use this field because 'sockfd' is a connected socket. */
>
> msgh.msg_name = NULL;
> msgh.msg_namelen = 0;
>
> /* On Linux, we must transmit at least one byte of real data in
> order to send ancillary data. We transmit an arbitrary integer
> whose value is ignored by recvfd(). */
>
> msgh.msg_iov = &iov;
> msgh.msg_iovlen = 1;
> iov.iov_base = &data;
> iov.iov_len = sizeof(int);
> data = 12345;
>
> /* Set 'msghdr' fields that describe ancillary data */
>
> msgh.msg_control = controlMsg.buf;
> msgh.msg_controllen = sizeof(controlMsg.buf);
>
> /* Set up ancillary data describing file descriptor to send */
>
> cmsgp = CMSG_FIRSTHDR(&msgh);
> cmsgp->cmsg_level = SOL_SOCKET;
> cmsgp->cmsg_type = SCM_RIGHTS;
> cmsgp->cmsg_len = CMSG_LEN(sizeof(int));
> memcpy(CMSG_DATA(cmsgp), &fd, sizeof(int));
>
> /* Send real plus ancillary data */
>
> if (sendmsg(sockfd, &msgh, 0) == -1)
> return -1;
>
> return 0;
> }

Instead of using unix domain sockets to send the fd to the parent, I
think you could also use clone3() with flags==CLONE_FILES|SIGCHLD,
dup2() the seccomp fd to an fd that was reserved in the parent, call
unshare(CLONE_FILES) in the child after setting up the seccomp fd, and
wake up the parent with something like pthread_cond_signal()? I'm not
sure whether that'd look better or worse in the end though, so maybe
just ignore this comment.

[...]
> /* Access the memory of the target process in order to discover the
> pathname that was given to mkdir() */
>
> static void
> getTargetPathname(struct seccomp_notif *req, int notifyFd,
> char *path, size_t len)
> {
> char procMemPath[PATH_MAX];
> snprintf(procMemPath, sizeof(procMemPath), "/proc/%d/mem", req->pid);
>
> int procMemFd = open(procMemPath, O_RDONLY);

Should example code like this maybe use O_CLOEXEC unless the fd in
question actually has to be inheritable? I know it doesn't actually
matter here, but if this code was used in a multi-threaded context, it
might.

> if (procMemFd == -1)
> errExit("Supervisor: open");
>
> /* Check that the process whose info we are accessing is still alive.
> If the SECCOMP_IOCTL_NOTIF_ID_VALID operation (performed
> in checkNotificationIdIsValid()) succeeds, we know that the
> /proc/PID/mem file descriptor that we opened corresponds to the
> process for which we received a notification. If that process
> subsequently terminates, then read() on that file descriptor
> will return 0 (EOF). */
>
> checkNotificationIdIsValid(notifyFd, req->id);
>
> /* Seek to the location containing the pathname argument (i.e., the
> first argument) of the mkdir(2) call and read that pathname */
>
> if (lseek(procMemFd, req->data.args[0], SEEK_SET) == -1)
> errExit("Supervisor: lseek");
>
> ssize_t s = read(procMemFd, path, PATH_MAX);
> if (s == -1)
> errExit("read");

Why not pread() instead of lseek()+read()?

> if (s == 0) {
> fprintf(stderr, "\tS: read() of /proc/PID/mem "
> "returned 0 (EOF)\n");
> exit(EXIT_FAILURE);
> }
>
> if (close(procMemFd) == -1)
> errExit("close-/proc/PID/mem");

We should probably make sure here that the value we read is actually
NUL-terminated?

> }
>
> /* Handle notifications that arrive via the SECCOMP_RET_USER_NOTIF file
> descriptor, 'notifyFd'. */
>
> static void
> handleNotifications(int notifyFd)
> {
> struct seccomp_notif_sizes sizes;
> char path[PATH_MAX];
> /* For simplicity, we assume that the pathname given to mkdir()
> is no more than PATH_MAX bytes; but this might not be true. */

No, it has to be true, otherwise the kernel would fail the syscall if
it was executing normally.

> /* Discover the sizes of the structures that are used to receive
> notifications and send notification responses, and allocate
> buffers of those sizes. */
>
> if (seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes) == -1)
> errExit("\tS: seccomp-SECCOMP_GET_NOTIF_SIZES");
>
> struct seccomp_notif *req = malloc(sizes.seccomp_notif);
> if (req == NULL)
> errExit("\tS: malloc");
>
> struct seccomp_notif_resp *resp = malloc(sizes.seccomp_notif_resp);

This should probably do something like max(sizes.seccomp_notif_resp,
sizeof(struct seccomp_notif_resp)) in case the program was built
against new UAPI headers that make struct seccomp_notif_resp big, but
is running under an old kernel where that struct is still smaller?

> if (resp == NULL)
> errExit("\tS: malloc");
[...]
> } else {
>
> /* If mkdir() failed in the supervisor, pass the error
> back to the target */
>
> resp->error = -errno;
> printf("\tS: failure! (errno = %d; %s)\n", errno,
> strerror(errno));
> }
> } else if (strncmp(path, "./", strlen("./")) == 0) {

nit: indent messed up

> resp->error = resp->val = 0;
> resp->flags = SECCOMP_USER_NOTIF_FLAG_CONTINUE;
> printf("\tS: target can execute system call\n");
[...]

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