For review: seccomp_user_notif(2) manual page [v2]

Michael Kerrisk (man-pages)
2020-10-26T10:04:07Z

Hi all (and especially Tycho and Sargun),

Following review comments on the first draft (thanks to Jann, Kees,
Christian and Tycho), I've made a lot of changes to this page.
I've also added a few FIXMEs relating to outstanding API issues.
I'd like a second pass review of the page before I release it.
But also, this mail serves as a way of noting the outstanding API
issues.

Tycho: I still have an outstanding question for you at [2].

Sargun: can you please prepare something on SECCOMP_ADDFD_FLAG_SETFD
and SECCOMP_IOCTL_NOTIF_ADDFD to be added to this page?

I've shown the rendered version of the page below. The page source
currently sits in a branch at
https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/log/?h=seccomp_user_notif

At this point, I'm mainly interested in feedback about the FIXMEs,
some of which relate to the text of the page itself, while the
others relate to the various outstanding API issues. The first
FIXME provides a small opportunity for some bikeshedding :-);

Thanks,

Michael

[1] https://lore.kernel.org/linux-man/45f07f17-18b6-d187-0914-6f341fe90857@gmail.com/
[2] https://lore.kernel.org/linux-man/8f20d586-9609-ef83-c85a-272e37e684d8@gmail.com/

=====

SECCOMP_USER_NOTIF(2) Linux Programmer's Manual SECCOMP_USER_NOTIF(2)

NAME
seccomp_user_notif - Seccomp user-space notification mechanism

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Might "seccomp_unotify(2)" be a better name for this │
│page? It's slightly shorter to type, and perhaps │
│reads better when spoken. │
└─────────────────────────────────────────────────────┘

SYNOPSIS
#include <linux/seccomp.h>
#include <linux/filter.h>
#include <linux/audit.h>

int seccomp(unsigned int operation, unsigned int flags, void *args);

#include <sys/ioctl.h>

int ioctl(int fd, SECCOMP_IOCTL_NOTIF_RECV,
struct seccomp_notif *req);
int ioctl(int fd, SECCOMP_IOCTL_NOTIF_SEND,
struct seccomp_notif_resp *resp);
int ioctl(int fd, SECCOMP_IOCTL_NOTIF_ID_VALID, __u64 *id);

DESCRIPTION
This page describes the user-space notification mechanism
provided by the Secure Computing (seccomp) facility. As well as
the use of the SECCOMP_FILTER_FLAG_NEW_LISTENER flag, the
SECCOMP_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 system call is made by the filter itself. By
contrast, the user-space notification mechanism allows the
seccomp filter to delegate the handling of the system call to
another user-space process. Note that this mechanism is
explicitly not intended as a method implementing security policy;
see NOTES.

In the discussion that follows, the thread(s) on which the
seccomp filter is installed is (are) referred to as the target,
and the process that is notified by the user-space notification
mechanism is referred to as the supervisor.

A suitably privileged supervisor can use the user-space
notification mechanism to perform actions on behalf of the
target. The advantage of the user-space notification mechanism
is that the supervisor will usually be able to retrieve
information about the target and the performed system call that
the seccomp filter itself cannot. (A seccomp filter is limited
in the information it can obtain and the actions that it can
perform because it is running on a virtual machine inside the
kernel.)

An overview of the steps performed by the target and the
supervisor is as follows:

1. The target establishes a seccomp filter in the usual manner,
but with two differences:

· The seccomp(2) flags argument includes the flag
SECCOMP_FILTER_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. Only one "listening" seccomp filter can be
installed for a thread.

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Is the last sentence above correct? │
│ │
│Kees Cook (25 Oct 2020) notes: │
│ │
│I like this limitation, but I expect that it'll need │
│to change in the future. Even with LSMs, we see the │
│need for arbitrary stacking, and the idea of there │
│being only 1 supervisor will eventually break down. │
│Right now there is only 1 because only container │
│managers are using this feature. But if some daemon │
│starts using it to isolate some thread, suddenly it │
│might break if a container manager is trying to │
│listen to it too, etc. I expect it won't be needed │
│soon, but I do think it'll change. │
│ │
└─────────────────────────────────────────────────────┘

· 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 can obtain notifications using
the listening file descriptor, (a duplicate of) that file
descriptor must be passed from the target to the supervisor.
One way in which this could be done is by passing the file
descriptor over a UNIX domain socket connection between the
target and the supervisor (using the SCM_RIGHTS ancillary
message type described in unix(7)).

3. The supervisor 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
seccomp(2) SECCOMP_GET_NOTIF_SIZES operation, which returns a
structure of type seccomp_notif_sizes. The supervisor
allocates 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.seccomp_notif_resp bytes for the response
(a struct seccomp_notif_resp structure) that it will provide
to the kernel (and thus the target).

4. The target 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 (yet)
execute the system call; instead, execution of the target is
temporarily blocked inside the kernel (in a sleep state that
is interruptible by signals) and a notification event is
generated on the listening file descriptor.

5. The supervisor can now repeatedly monitor the listening file
descriptor for SECCOMP_RET_USER_NOTIF-triggered events. To do
this, the supervisor uses the SECCOMP_IOCTL_NOTIF_RECV
ioctl(2) operation to read information about a notification
event; this operation blocks until an event is available. The
operation returns a seccomp_notif structure containing
information about the system call that is being attempted by
the target.

6. The seccomp_notif structure returned by the
SECCOMP_IOCTL_NOTIF_RECV operation includes the same
information (a seccomp_data structure) that was passed to the
seccomp filter. This information allows the supervisor to
discover the system call number and the arguments for the
target's system call. In addition, the notification event
contains the ID of the thread that triggered the notification
and a unique cookie value that is used in subsequent
SECCOMP_IOCTL_NOTIF_ID_VALID and SECCOMP_IOCTL_NOTIF_SEND
operations.

The information in the notification can be used to discover
the values of pointer arguments for the target's system call.
(This is something that can't be done from within a seccomp
filter.) One way in which the supervisor can do this is to
open the corresponding /proc/[tid]/mem file (see proc(5)) and
read bytes from the location that corresponds to one of the
pointer arguments whose value is supplied in the notification
event. (The supervisor must be careful to avoid a race
condition that can occur when doing this; see the description
of the SECCOMP_IOCTL_NOTIF_ID_VALID ioctl(2) operation below.)
In addition, the supervisor can access other system
information that is visible in user space but which is not
accessible from a seccomp filter.

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

One example use case here relates to containers. The target
may be located inside a container where it does not have
sufficient capabilities to mount a filesystem in the
container's mount namespace. However, the supervisor may be a
more privileged process that does have sufficient capabilities
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
construct a return value for the target's system call and
provide a value that will be assigned to the errno variable of
the target.

The response is sent using the SECCOMP_IOCTL_NOTIF_SEND
ioctl(2) operation, which is used to transmit a
seccomp_notif_resp structure to the kernel. This structure
includes a cookie value that the supervisor obtained in the
seccomp_notif structure returned by the
SECCOMP_IOCTL_NOTIF_RECV operation. This cookie value allows
the kernel to associate the response with the target. This
structure must include the cookie value that the supervisor
obtained in the seccomp_notif structure returned by the
SECCOMP_IOCTL_NOTIF_RECV operation; the cookie allows the
kernel to associate the response with the target.

9. Once the notification has been sent, the system call in the
target thread 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
thread's system call; see the discussion of
SECCOMP_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_FILTER_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
operation blocks until an event occurs. The third
ioctl(2) argument is a pointer to a structure of the
following form which contains information about the event.
This structure must be zeroed out before the call.

struct seccomp_notif {
__u64 id; /* Cookie */
__u32 pid; /* TID of target thread */
__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
corresponding seccomp filter.

· It can be used with the
SECCOMP_IOCTL_NOTIF_ID_VALID ioctl(2) operation
to verify that the target 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
SECCOMP_IOCTL_NOTIF_SEND operation.

pid This is the thread ID of the target thread that
triggered the notification event.

flags This is a bit mask of flags providing further
information on the event. In the current
implementation, this field is always zero.

data This is a seccomp_data structure containing
information 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 thread was killed by a signal as the
notification information was being generated, or
the target's (blocked) system call was interrupted
by a signal handler.

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│From my experiments, it appears that if a │
│SECCOMP_IOCTL_NOTIF_RECV is done after the target │
│thread terminates, then the ioctl() simply blocks │
│(rather than returning an error to indicate that the │
│target no longer exists). │
│ │
│I found that surprising, and it required some │
│contortions in the example program. It was not │
│possible to code my SIGCHLD handler (which reaps the │
│zombie when the worker/target terminates) to simply │
│set a flag checked in the main handleNotifications() │
│loop, since this created an unavoidable race where │
│the child might terminate just after I had checked │
│the flag, but before I blocked (forever!) 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 has terminated. │
│ │
│Jann posted a patch to rectify this, but there was │
│no response (Lore link: https://bit.ly/3jvUBxk) to │
│his question about fixing this issue. (I've tried │
│building with the patch, but encountered an issue │
│with the target process entering D state after a │
│signal.) │
│ │
│For now, this behavior is documented in BUGS. │
│ │
│Kees Cook commented: Let's change [this] ASAP! │
└─────────────────────────────────────────────────────┘

SECCOMP_IOCTL_NOTIF_ID_VALID
This operation can be used to check that a notification ID
returned by an earlier SECCOMP_IOCTL_NOTIF_RECV operation
is still valid (i.e., that the target still exists and its
system call is still blocked waiting for a response).

The third ioctl(2) argument is a pointer to the cookie
(id) returned by the SECCOMP_IOCTL_NOTIF_RECV operation.

This operation is necessary to avoid race conditions that
can occur when the pid returned by the
SECCOMP_IOCTL_NOTIF_RECV operation terminates, and that
process ID is reused by another process. An example of
this kind of race is the following

1. A notification is generated on the listening file
descriptor. The returned seccomp_notif contains the
TID of the target thread (in the pid field of the
structure).

2. The target terminates.

3. Another thread or process is created on the system that
by chance reuses the TID that was freed when the target
terminated.

4. The supervisor open(2)s the /proc/[tid]/mem file for
the TID obtained in step 1, with the intention of (say)
inspecting the memory location(s) that containing the
argument(s) of the system call that triggered the
notification in step 1.

In the above scenario, the risk is that the supervisor may
try to access the memory of a process other than the
target. This race can be avoided by following the call to
open(2) with a SECCOMP_IOCTL_NOTIF_ID_VALID operation to
verify that the process that generated the notification is
still alive. (Note that if the target terminates after
the latter step, a subsequent read(2) from the file
descriptor may 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
notification ID is no longer valid), -1 is returned, and
errno is set to ENOENT.

SECCOMP_IOCTL_NOTIF_SEND
This operation is used to send a notification response
back to the kernel. The third ioctl(2) argument of this
structure is a pointer to a structure of the following
form:

struct seccomp_notif_resp {
__u64 id; /* Cookie value */
__s64 val; /* Success return value */
__s32 error; /* 0 (success) or negative
error number */
__u32 flags; /* See below */
};

The fields of this structure are as follows:

id This is the cookie value that was obtained using
the SECCOMP_IOCTL_NOTIF_RECV operation. This
cookie value allows the kernel to correctly
associate this response with the system call that
triggered the user-space notification.

val This is the value that will be used for a spoofed
success return for the target's system call; see
below.

error This is the value that will be used as the error
number (errno) for a spoofed error return for the
target's system call; see below.

flags This is a bit mask that includes zero or more of
the following flags:

SECCOMP_USER_NOTIF_FLAG_CONTINUE (since Linux 5.5)
Tell the kernel to execute the target's
system call.

Two kinds of response are possible:

· A response to the kernel telling it to execute the
target'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 decided that the system call does not require
emulation by the supervisor, the supervisor wants the
system call to be executed normally in the target.

The SECCOMP_USER_NOTIF_FLAG_CONTINUE flag should be used
with caution; see NOTES.

· A spoofed return value for the target's system call. In
this case, the kernel does not execute the target's
system call, instead causing the system call to return a
spoofed value as specified by fields of the
seccomp_notif_resp structure. The supervisor should set
the fields of this structure as follows:

+ flags does not contain
SECCOMP_USER_NOTIF_FLAG_CONTINUE.

+ error is set either to 0 for a spoofed "success"
return or to a negative error number for a spoofed
"failure" return. In the former case, the kernel
causes the target's system call to return the value
specified in the val field. In the later case, the
kernel causes the target's system call to return -1,
and errno is assigned the negated error value.

+ val is set to a value that will be used as the return
value for a spoofed "success" return for the target's
system call. The value in this field is ignored if
the error field contains a nonzero value.

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Kees Cook suggested: │
│ │
│Strictly speaking, this is architecture specific, │
│but all architectures do it this way. Should seccomp │
│enforce val == 0 when err != 0 ? │
└─────────────────────────────────────────────────────┘

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
SECCOMP_USER_NOTIF_FLAG_CONTINUE, and the error or
val field was not zero.

ENOENT The blocked system call in the target has been
interrupted by a signal handler or the target has
terminated.

NOTES
select()/poll()/epoll semantics
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). These interfaces indicate that
the file descriptor is ready as follows:

· When a notification is pending, these interfaces indicate that
the file descriptor is readable. Following such an indication,
a subsequent SECCOMP_IOCTL_NOTIF_RECV ioctl(2) will not block,
returning either information about a notification or else
failing with the error EINTR if the target has been killed by a
signal or its system call has been interrupted by a signal
handler.

· After the notification has been received (i.e., by the
SECCOMP_IOCTL_NOTIF_RECV ioctl(2) operation), these interfaces
indicate that the file descriptor is writable, meaning that a
notification response can be sent using the
SECCOMP_IOCTL_NOTIF_SEND ioctl(2) operation.

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│But (how) is the writable/(E)POLLOUT useful? │
└─────────────────────────────────────────────────────┘

· After the last thread using the filter has terminated and been
reaped using waitpid(2) (or similar), the file descriptor
indicates an end-of-file condition (readable in select(2);
POLLHUP/EPOLLHUP in poll(2)/ epoll_wait(2)).

Design goals; use of SECCOMP_USER_NOTIF_FLAG_CONTINUE
The intent of the user-space notification feature is to allow
system calls to be performed on behalf of the target. The
target's system call should either be handled by the supervisor
or allowed to continue normally in the kernel (where standard
security policies will be applied).

Note well: this mechanism must not be used to make security
policy decisions about the system call, which would be inherently
race-prone for reasons described next.

The SECCOMP_USER_NOTIF_FLAG_CONTINUE flag must be used with
caution. If set by the supervisor, the target's system call will
continue. However, there is a time-of-check, time-of-use race
here, since an attacker could exploit the interval of time where
the target is blocked waiting on the "continue" response to do
things such as rewriting the system call arguments.

Note furthermore that a user-space notifier can be bypassed if
the existing filters allow the use of seccomp(2) or prctl(2) to
install a filter that returns an action value with a higher
precedence than SECCOMP_RET_USER_NOTIF (see seccomp(2)).

It should thus be absolutely clear that the seccomp user-space
notification mechanism can not be used to implement a security
policy! It should only ever be used in scenarios where a more
privileged process supervises the system calls of a lesser
privileged target to get around kernel-enforced security
restrictions when the supervisor deems this safe. In other
words, in order to continue a system call, the supervisor should
be sure that another security mechanism or the kernel itself will
sufficiently block the system call if its arguments are rewritten
to something unsafe.

Interaction with SA_RESTART signal handlers
Consider the following scenario:

· The target process has used sigaction(2) to install a signal
handler with the SA_RESTART flag.

· The target has made a system call that triggered a seccomp
user-space notification and the target is currently blocked
until the supervisor sends a notification response.

· A signal is delivered to the target and the signal handler is
executed.

· When (if) the supervisor attempts to send a notification
response, the SECCOMP_IOCTL_NOTIF_SEND ioctl(2)) operation will
fail with the ENOENT error.

In this scenario, the kernel will restart the target's system
call. Consequently, the supervisor will receive another user-
space notification. Thus, depending on how many times the
blocked system call is interrupted by a signal handler, the
supervisor may receive multiple notifications for the same
instance of a system call in the target.

One oddity is that system call restarting as described in this
scenario will occur even for the blocking system calls listed in
signal(7) that would never normally be restarted by the
SA_RESTART flag.

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│About the above, Kees Cook commented: │
│ │
│Does this need fixing? I imagine the correct │
│behavior for this case would be a response to _SEND │
│of EINPROGRESS and the target would see EINTR │
│normally? │
│ │
│I mean, it's not like seccomp doesn't already expose │
│weirdness with syscall restarts. Not even arm64 │
│compat agrees[3] with arm32 in this regard. :( │
└─────────────────────────────────────────────────────┘

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Michael Kerrisk: I wonder about the effect of this │
│oddity for system calls that are normally │
│nonrestartable because they have timeouts. My │
│understanding is that the kernel doesn't restart │
│those system calls because it's impossible for the │
│kernel to restart the call with the right timeout │
│value. I wonder what happens when those system calls │
│are restarted in the scenario we're discussing.) │
└─────────────────────────────────────────────────────┘

BUGS
If a SECCOMP_IOCTL_NOTIF_RECV ioctl(2) operation is performed
after the target terminates, then the ioctl(2) call simply blocks
(rather than returning an error to indicate that the target no
longer exists).

┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Comment from Kees Cook: │
│ │
│I want this fixed. It caused me no end of pain when │
│building the selftests, and ended up spawning my │
│implementing a global test timeout in kselftest. :P │
│Before the usage counter refactor, there was no sane │
│way to deal with this, but now I think we're close. │
│ │
└─────────────────────────────────────────────────────┘

EXAMPLES
The (somewhat contrived) program shown below demonstrates the use
of the interfaces described in this page. The program creates a
child process that serves as the "target" process. The child
process installs a seccomp filter that returns the
SECCOMP_RET_USER_NOTIF action value if a call is made to
mkdir(2). The child process then calls mkdir(2) once for each of
the supplied command-line arguments, and reports the result
returned by the call. After processing all arguments, the child
process terminates.

The parent process acts as the supervisor, listening for the
notifications that are generated when the target process calls
mkdir(2). When such a notification occurs, the supervisor
examines the memory of the target process (using /proc/[pid]/mem)
to discover the pathname argument that was supplied to the
mkdir(2) call, and performs one of the following actions:

· If the pathname begins with the prefix "/tmp/", then the
supervisor attempts to create the specified directory, and then
spoofs a return for the target process based on the return
value of the supervisor's mkdir(2) call. In the event that
that call succeeds, the spoofed success return value is the
length of the pathname.

· If the pathname begins with "./" (i.e., it is a relative
pathname), the supervisor sends a
SECCOMP_USER_NOTIF_FLAG_CONTINUE response to the kernel to say
that the kernel should execute the target process's mkdir(2)
call.

· If the pathname begins with some other prefix, the supervisor
spoofs an error return for the target process, so that the
target process's mkdir(2) call appears to fail with the error
EOPNOTSUPP ("Operation not supported"). Additionally, if the
specified pathname is exactly "/bye", then the supervisor
terminates.

This program can be used to demonstrate various aspects of the
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:").

In the following example, the target attempts to create the
directory /tmp/x. Upon receiving the notification, the
supervisor creates the directory on the target's behalf, and
spoofs a success return to be received by the target process's
mkdir(2) call.

$ ./seccomp_unotify /tmp/x
T: PID = 23168

T: about to mkdir("/tmp/x")
S: got notification (ID 0x17445c4a0f4e0e3c) for PID 23168
S: executing: mkdir("/tmp/x", 0700)
S: success! spoofed return = 6
S: sending response (flags = 0; val = 6; error = 0)
T: SUCCESS: mkdir(2) returned 6

T: terminating
S: target has terminated; bye

In the above output, note that the spoofed return value seen by
the target process is 6 (the length of the pathname /tmp/x),
whereas a normal mkdir(2) call returns 0 on success.

In the next example, the target attempts to create a directory
using the relative pathname ./sub. Since this pathname starts
with "./", the supervisor sends a
SECCOMP_USER_NOTIF_FLAG_CONTINUE response to the kernel, and the
kernel then (successfully) executes the target process's mkdir(2)
call.

$ ./seccomp_unotify ./sub
T: PID = 23204

T: about to mkdir("./sub")
S: got notification (ID 0xddb16abe25b4c12) for PID 23204
S: target can execute system call
S: sending response (flags = 0x1; val = 0; error = 0)
T: SUCCESS: mkdir(2) returned 0

T: terminating
S: target has terminated; bye

If the target process attempts to create a directory with a
pathname that doesn't start with "." and doesn't begin with the
prefix "/tmp/", then the supervisor spoofs an error return
(EOPNOTSUPP, "Operation not supported") for the target's
mkdir(2) call (which is not executed):

$ ./seccomp_unotify /xxx
T: PID = 23178

T: about to mkdir("/xxx")
S: got notification (ID 0xe7dc095d1c524e80) for PID 23178
S: spoofing error response (Operation not supported)
S: sending response (flags = 0; val = 0; error = -95)
T: ERROR: mkdir(2): Operation not supported

T: terminating
S: target has terminated; bye

In the next example, the target process attempts to create a
directory with the pathname /tmp/nosuchdir/b. Upon receiving the
notification, the supervisor attempts to create that directory,
but the mkdir(2) call fails because the directory /tmp/nosuchdir
does not exist. Consequently, the supervisor spoofs an error
return that passes the error that it received back to the target
process's mkdir(2) call.

$ ./seccomp_unotify /tmp/nosuchdir/b
T: PID = 23199

T: about to mkdir("/tmp/nosuchdir/b")
S: got notification (ID 0x8744454293506046) for PID 23199
S: executing: mkdir("/tmp/nosuchdir/b", 0700)
S: failure! (errno = 2; No such file or directory)
S: sending response (flags = 0; val = 0; error = -2)
T: ERROR: mkdir(2): No such file or directory

T: terminating
S: target has terminated; bye

If the supervisor receives a notification and sees that the
argument of the target's mkdir(2) is the string "/bye", then (as
well as spoofing an EOPNOTSUPP error), the supervisor terminates.
If the target process subsequently executes another mkdir(2) that
triggers its seccomp filter to return the SECCOMP_RET_USER_NOTIF
action value, then the kernel causes the target process's system
call to fail with the error ENOSYS ("Function not implemented").
This is demonstrated by the following example:

$ ./seccomp_unotify /bye /tmp/y
T: PID = 23185

T: about to mkdir("/bye")
S: got notification (ID 0xa81236b1d2f7b0f4) for PID 23185
S: spoofing error response (Operation not supported)
S: sending response (flags = 0; val = 0; error = -95)
S: terminating **********
T: ERROR: mkdir(2): Operation not supported

T: about to mkdir("/tmp/y")
T: ERROR: mkdir(2): Function not implemented

T: terminating

Program source
#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/prctl.h>
#include <fcntl.h>
#include <limits.h>
#include <signal.h>
#include <stddef.h>
#include <stdint.h>
#include <stdbool.h>
#include <linux/audit.h>
#include <sys/syscall.h>
#include <sys/stat.h>
#include <linux/filter.h>
#include <linux/seccomp.h>
#include <sys/ioctl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <sys/socket.h>
#include <sys/un.h>

#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)

/* 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 suitably aligned. */
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;
}

/* Receive a file descriptor on a connected UNIX domain socket. Returns
the received file descriptor on success, or -1 on error. */

static int
recvfd(int sockfd)
{
struct msghdr msgh;
struct iovec iov;
int data, fd;
ssize_t nr;

/* Allocate a char buffer for the ancillary data. See the comments
in sendfd() */
union {
char buf[CMSG_SPACE(sizeof(int))];
struct cmsghdr align;
} controlMsg;
struct cmsghdr *cmsgp;

/* The 'msg_name' field can be used to obtain the address of the
sending socket. However, we do not need this information. */

msgh.msg_name = NULL;
msgh.msg_namelen = 0;

/* Specify buffer for receiving real data */

msgh.msg_iov = &iov;
msgh.msg_iovlen = 1;
iov.iov_base = &data; /* Real data is an 'int' */
iov.iov_len = sizeof(int);

/* Set 'msghdr' fields that describe ancillary data */

msgh.msg_control = controlMsg.buf;
msgh.msg_controllen = sizeof(controlMsg.buf);

/* Receive real plus ancillary data; real data is ignored */

nr = recvmsg(sockfd, &msgh, 0);
if (nr == -1)
return -1;

cmsgp = CMSG_FIRSTHDR(&msgh);

/* Check the validity of the 'cmsghdr' */

if (cmsgp == NULL ||
cmsgp->cmsg_len != CMSG_LEN(sizeof(int)) ||
cmsgp->cmsg_level != SOL_SOCKET ||
cmsgp->cmsg_type != SCM_RIGHTS) {
errno = EINVAL;
return -1;
}

/* Return the received file descriptor to our caller */

memcpy(&fd, CMSG_DATA(cmsgp), sizeof(int));
return fd;
}

static void
sigchldHandler(int sig)
{
char msg[] = "\tS: target has terminated; bye\n";

write(STDOUT_FILENO, msg, sizeof(msg) - 1);
_exit(EXIT_SUCCESS);
}

static int
seccomp(unsigned int operation, unsigned int flags, void *args)
{
return syscall(__NR_seccomp, operation, flags, args);
}

/* The following is the x86-64-specific BPF boilerplate code for checking
that the BPF program is running on the right architecture + ABI. At
completion of these instructions, the accumulator contains the system
call number. */

/* For the x32 ABI, all system call numbers have bit 30 set */

#define X32_SYSCALL_BIT 0x40000000

#define X86_64_CHECK_ARCH_AND_LOAD_SYSCALL_NR \
BPF_STMT(BPF_LD | BPF_W | BPF_ABS, \
(offsetof(struct seccomp_data, arch))), \
BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, AUDIT_ARCH_X86_64, 0, 2), \
BPF_STMT(BPF_LD | BPF_W | BPF_ABS, \
(offsetof(struct seccomp_data, nr))), \
BPF_JUMP(BPF_JMP | BPF_JGE | BPF_K, X32_SYSCALL_BIT, 0, 1), \
BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_KILL_PROCESS)

/* installNotifyFilter() installs a seccomp filter that generates
user-space notifications (SECCOMP_RET_USER_NOTIF) when the process
calls mkdir(2); the filter allows all other system calls.

The function return value is a file descriptor from which the
user-space notifications can be fetched. */

static int
installNotifyFilter(void)
{
struct sock_filter filter[] = {
X86_64_CHECK_ARCH_AND_LOAD_SYSCALL_NR,

/* mkdir() triggers notification to user-space supervisor */

BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_mkdir, 0, 1),
BPF_STMT(BPF_RET + BPF_K, SECCOMP_RET_USER_NOTIF),

/* Every other system call is allowed */

BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW),
};

struct sock_fprog prog = {
.len = sizeof(filter) / sizeof(filter[0]),
.filter = filter,
};

/* Install the filter with the SECCOMP_FILTER_FLAG_NEW_LISTENER flag;
as a result, seccomp() returns a notification file descriptor. */

int notifyFd = seccomp(SECCOMP_SET_MODE_FILTER,
SECCOMP_FILTER_FLAG_NEW_LISTENER, &prog);
if (notifyFd == -1)
errExit("seccomp-install-notify-filter");

return notifyFd;
}

/* Close a pair of sockets created by socketpair() */

static void
closeSocketPair(int sockPair[2])
{
if (close(sockPair[0]) == -1)
errExit("closeSocketPair-close-0");
if (close(sockPair[1]) == -1)
errExit("closeSocketPair-close-1");
}

/* Implementation of the target process; create a child process that:

(1) installs a seccomp filter with the
SECCOMP_FILTER_FLAG_NEW_LISTENER flag;
(2) writes the seccomp notification file descriptor returned from
the previous step onto the UNIX domain socket, 'sockPair[0]';
(3) calls mkdir(2) for each element of 'argv'.

The function return value in the parent is the PID of the child
process; the child does not return from this function. */

static pid_t
targetProcess(int sockPair[2], char *argv[])
{
pid_t targetPid = fork();
if (targetPid == -1)
errExit("fork");

if (targetPid > 0) /* In parent, return PID of child */
return targetPid;

/* Child falls through to here */

printf("T: PID = %ld\n", (long) getpid());

/* Install seccomp filter(s) */

if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0))
errExit("prctl");

int notifyFd = installNotifyFilter();

/* Pass the notification file descriptor to the tracing process over
a UNIX domain socket */

if (sendfd(sockPair[0], notifyFd) == -1)
errExit("sendfd");

/* Notification and socket FDs are no longer needed in target */

if (close(notifyFd) == -1)
errExit("close-target-notify-fd");

closeSocketPair(sockPair);

/* Perform a mkdir() call for each of the command-line arguments */

for (char **ap = argv; *ap != NULL; ap++) {
printf("\nT: about to mkdir(\"%s\")\n", *ap);

int s = mkdir(*ap, 0700);
if (s == -1)
perror("T: ERROR: mkdir(2)");
else
printf("T: SUCCESS: mkdir(2) returned %d\n", s);
}

printf("\nT: terminating\n");
exit(EXIT_SUCCESS);
}

/* Check that the notification ID provided by a SECCOMP_IOCTL_NOTIF_RECV
operation is still valid. It will no longer be valid if the process
has terminated. This operation can be used when accessing /proc/PID
files in the target process in order to avoid TOCTOU race conditions
where the PID that is returned by SECCOMP_IOCTL_NOTIF_RECV terminates
and is reused by another process. */

static void
checkNotificationIdIsValid(int notifyFd, uint64_t id)
{
if (ioctl(notifyFd, SECCOMP_IOCTL_NOTIF_ID_VALID, &id) == -1)
errExit("\tS: notification ID check: "
"target has terminated!!!\n");
}

/* Access the memory of the target process in order to discover the
pathname that was given to mkdir() */

static bool
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);
if (procMemFd == -1)
errExit("\tS: 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);

/* Read bytes at the location containing the pathname argument
(i.e., the first argument) of the mkdir(2) call */

ssize_t nread = pread(procMemFd, path, len, req->data.args[0]);
if (nread == -1)
errExit("pread");

if (nread == 0) {
fprintf(stderr, "\tS: pread() of /proc/PID/mem "
"returned 0 (EOF)\n");
exit(EXIT_FAILURE);
}

if (close(procMemFd) == -1)
errExit("close-/proc/PID/mem");

/* We have no guarantees about what was in the memory of the target
process. We therefore treat the buffer returned by pread() as
untrusted input. The buffer should be terminated by a null byte;
if not, then we will trigger an error for the target process. */

if (strnlen(path, nread) < nread)
return true;

return false;
}

/* 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];

/* 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");

/* When allocating the response buffer, we must allow for the fact
that the user-space binary may have been built with user-space
headers where 'struct seccomp_notif_resp' is bigger than the
response buffer expected by the (older) kernel. Therefore, we
allocate a buffer that is the maximum of the two sizes. This
ensures that if the supervisor places bytes into the response
structure that are past the response size that the kernel expects,
then the supervisor is not touching an invalid memory location. */

size_t resp_size = sizes.seccomp_notif_resp;
if (sizeof(struct seccomp_notif_resp) > resp_size)
resp_size = sizeof(struct seccomp_notif_resp);

struct seccomp_notif_resp *resp = malloc(resp_size);
if (resp == NULL)
errExit("\tS: malloc");

/* Loop handling notifications */

for (;;) {
/* Wait for next notification, returning info in '*req' */

memset(req, 0, sizes.seccomp_notif);
if (ioctl(notifyFd, SECCOMP_IOCTL_NOTIF_RECV, req) == -1) {
if (errno == EINTR)
continue;
errExit("\tS: ioctl-SECCOMP_IOCTL_NOTIF_RECV");
}

printf("\tS: got notification (ID %#llx) for PID %d\n",
req->id, req->pid);

/* The only system call that can generate a notification event
is mkdir(2). Nevertheless, we check that the notified system
call is indeed mkdir() as kind of future-proofing of this
code in case the seccomp filter is later modified to
generate notifications for other system calls. */

if (req->data.nr != __NR_mkdir) {
printf("\tS: notification contained unexpected "
"system call number; bye!!!\n");
exit(EXIT_FAILURE);
}

bool pathOK = getTargetPathname(req, notifyFd, path,
sizeof(path));

/* Prepopulate some fields of the response */

resp->id = req->id; /* Response includes notification ID */
resp->flags = 0;
resp->val = 0;

/* If the target pathname was not valid, trigger an EINVAL error;
if the directory is in /tmp, then create it on behalf of the
supervisor; if the pathname starts with '.', tell the kernel
to let the target process execute the mkdir(); otherwise, give
an error for a directory pathname in any other location. */

if (!pathOK) {
resp->error = -EINVAL;
printf("\tS: spoofing error for invalid pathname (%s)\n",
strerror(-resp->error));
} else if (strncmp(path, "/tmp/", strlen("/tmp/")) == 0) {
printf("\tS: executing: mkdir(\"%s\", %#llo)\n",
path, req->data.args[1]);

if (mkdir(path, req->data.args[1]) == 0) {
resp->error = 0; /* "Success" */
resp->val = strlen(path); /* Used as return value of
mkdir() in target */
printf("\tS: success! spoofed return = %lld\n",
resp->val);
} 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) {
resp->error = resp->val = 0;
resp->flags = SECCOMP_USER_NOTIF_FLAG_CONTINUE;
printf("\tS: target can execute system call\n");
} else {
resp->error = -EOPNOTSUPP;
printf("\tS: spoofing error response (%s)\n",
strerror(-resp->error));
}

/* Send a response to the notification */

printf("\tS: sending response "
"(flags = %#x; val = %lld; error = %d)\n",
resp->flags, resp->val, resp->error);

if (ioctl(notifyFd, SECCOMP_IOCTL_NOTIF_SEND, resp) == -1) {
if (errno == ENOENT)
printf("\tS: response failed with ENOENT; "
"perhaps target process's syscall was "
"interrupted by a signal?\n");
else
perror("ioctl-SECCOMP_IOCTL_NOTIF_SEND");
}

/* If the pathname is just "/bye", then the supervisor
terminates. This allows us to see what happens if the
target process makes further calls to mkdir(2). */

if (strcmp(path, "/bye") == 0) {
printf("\tS: terminating **********\n");
exit(EXIT_FAILURE);
}
}
}

/* Implementation of the supervisor process:

(1) obtains the notification file descriptor from 'sockPair[1]'
(2) handles notifications that arrive on that file descriptor. */

static void
supervisor(int sockPair[2])
{
int notifyFd = recvfd(sockPair[1]);
if (notifyFd == -1)
errExit("recvfd");

closeSocketPair(sockPair); /* We no longer need the socket pair */

handleNotifications(notifyFd);
}

int
main(int argc, char *argv[])
{
int sockPair[2];

setbuf(stdout, NULL);

if (argc < 2) {
fprintf(stderr, "At least one pathname argument is required\n");
exit(EXIT_FAILURE);
}

/* Create a UNIX domain socket that is used to pass the seccomp
notification file descriptor from the target process to the
supervisor process. */

if (socketpair(AF_UNIX, SOCK_STREAM, 0, sockPair) == -1)
errExit("socketpair");

/* Create a child process--the "target"--that installs seccomp
filtering. The target process writes the seccomp notification
file descriptor onto 'sockPair[0]' and then calls mkdir(2) for
each directory in the command-line arguments. */

(void) targetProcess(sockPair, &argv[optind]);

/* Catch SIGCHLD when the target terminates, so that the
supervisor can also terminate. */

struct sigaction sa;
sa.sa_handler = sigchldHandler;
sa.sa_flags = 0;
sigemptyset(&sa.sa_mask);
if (sigaction(SIGCHLD, &sa, NULL) == -1)
errExit("sigaction");

supervisor(sockPair);

exit(EXIT_SUCCESS);
}

SEE ALSO
ioctl(2), seccomp(2)

A further example program can be found in the kernel source file
samples/seccomp/user-trap.c.

Linux 2020-10-01 SECCOMP_USER_NOTIF(2)

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/