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SubjectRe: [PATCH v2 2/3] seccomp: Introduce addfd ioctl to seccomp user notifier
On Thu, May 28, 2020 at 04:08:57AM -0700, Sargun Dhillon wrote:
> This adds a seccomp notifier ioctl which allows for the listener to "add"
> file descriptors to a process which originated a seccomp user
> notification. This allows calls like mount, and mknod to be "implemented",
> as the return value, and the arguments are data in memory. On the other
> hand, calls like connect can be "implemented" using pidfd_getfd.
>
> Unfortunately, there are calls which return file descriptors, like
> open, which are vulnerable to TOC-TOU attacks, and require that the
> more privileged supervisor can inspect the argument, and perform the
> syscall on behalf of the process generating the notifiation. This
> allows the file descriptor generated from that open call to be
> returned to the calling process.
>
> In addition, there is funcitonality to allow for replacement of
> specific file descriptors, following dup2-like semantics.
>
> Signed-off-by: Sargun Dhillon <sargun@sargun.me>
> Suggested-by: Matt Denton <mpdenton@google.com>

This looks mostly really clean. When I've got more brain tomorrow I want to
double-check the locking, but I think the use of notify_lock and being
in the ioctl fully protects everything from any use-after-free-like
issues.

Notes below...

> +/* valid flags for seccomp_notif_addfd */
> +#define SECCOMP_ADDFD_FLAG_SETFD (1UL << 0) /* Specify remote fd */

Nit: please use BIT()

> @@ -735,6 +770,41 @@ static u64 seccomp_next_notify_id(struct seccomp_filter *filter)
> return filter->notif->next_id++;
> }
>
> +static void seccomp_handle_addfd(struct seccomp_kaddfd *addfd)
> +{
> + struct socket *sock;
> + int ret, err;
> +
> + /*
> + * Remove the notification, and reset the list pointers, indicating
> + * that it has been handled.
> + */
> + list_del_init(&addfd->list);
> +
> + ret = security_file_receive(addfd->file);
> + if (ret)
> + goto out;
> +
> + if (addfd->fd == -1) {
> + ret = get_unused_fd_flags(addfd->flags);
> + if (ret >= 0)
> + fd_install(ret, get_file(addfd->file));
> + } else {
> + ret = replace_fd(addfd->fd, addfd->file, addfd->flags);
> + }
> +
> + /* These are the semantics from copying FDs via SCM_RIGHTS */
> + sock = sock_from_file(addfd->file, &err);
> + if (sock) {
> + sock_update_netprioidx(&sock->sk->sk_cgrp_data);
> + sock_update_classid(&sock->sk->sk_cgrp_data);
> + }

This made my eye twitch. ;) I see this is borrowed from
scm_detach_fds()... this really feels like the kind of thing that will
quickly go out of sync. I think this "receive an fd" logic needs to be
lifted out of scm_detach_fds() so it and seccomp can share it. I'm not
sure how to parameterize it quite right, though. Perhaps:

int file_receive(int fd, unsigned long flags, struct file *file)
{
struct socket *sock;
int ret;

ret = security_file_receive(file);
if (ret)
return ret;

/* Install the file. */
if (fd == -1) {
ret = get_unused_fd_flags(flags);
if (ret >= 0)
fd_install(ret, get_file(file));
} else {
ret = replace_fd(fd, file, flags);
}

/* Bump the usage count. */
sock = sock_from_file(addfd->file, &err);
if (sock) {
sock_update_netprioidx(&sock->sk->sk_cgrp_data);
sock_update_classid(&sock->sk->sk_cgrp_data);
}

return ret;
}


static void seccomp_handle_addfd(struct seccomp_kaddfd *addfd)
{
/*
* Remove the notification, and reset the list pointers, indicating
* that it has been handled.
*/
list_del_init(&addfd->list);
addfd->ret = file_receive(addfd->fd, addfd->flags, addfd->file);
complete(&addfd->completion);
}

scm_detach_fds()
...
for (i=0, cmfptr=(__force int __user *)CMSG_DATA(cm); i<fdmax;
i++, cmfptr++)
{

err = file_receive(-1, MSG_CMSG_CLOEXEC & msg->msg_flags
? O_CLOEXEC : 0, fp[i]);
if (err < 0)
break;
err = put_user(err, cmfptr);
if (err)
/* wat */
}
...

I'm not sure on the put_user() failure, though. We could check early
for faults with a put_user(0, cmfptr) before the file_receive() call, or
we could just ignore it? I'm not sure what SCM does here. I guess
worst-case:

int file_receive(int fd, unsigned long flags, struct file *file,
int __user *fdptr)
{
...
ret = get_unused_fd_flags(flags);
if (ret >= 0) {
if (cmfptr) {
int err;

err = put_user(ret, cmfptr);
if (err) {
put_unused_fd(ret);
return err;
}
}
fd_install(ret, get_file(file));
}
...
}

> @@ -763,14 +835,31 @@ static int seccomp_do_user_notification(int this_syscall,
> /*
> * This is where we wait for a reply from userspace.
> */
> +wait:
> err = wait_for_completion_interruptible(&n.ready);
> mutex_lock(&match->notify_lock);
> if (err == 0) {
> + /* Check if we were woken up by a addfd message */
> + addfd = list_first_entry_or_null(&n.addfd,
> + struct seccomp_kaddfd, list);
> + if (addfd && n.state != SECCOMP_NOTIFY_REPLIED) {
> + seccomp_handle_addfd(addfd);
> + mutex_unlock(&match->notify_lock);
> + goto wait;
> + }
> ret = n.val;
> err = n.error;
> flags = n.flags;
> }

This feels like it needs to be done differently, but when I tried to
make it "proper" loop, I think it got more ugly:

for (;;) {
err = wait_for_completion_interruptible(&n.ready);
mutex_lock(&match->notify_lock);
if (err == 0) {
/* Check if we were woken up by a addfd message */
addfd = list_first_entry_or_null(&n.addfd,
struct seccomp_kaddfd, list);
if (addfd && n.state != SECCOMP_NOTIFY_REPLIED) {
seccomp_handle_addfd(addfd);
mutex_unlock(&match->notify_lock);
continue;
}
ret = n.val;
err = n.error;
flags = n.flags;
}
break;
}

So, I guess it's fine how you have it. :)

>
> + /* If there were any pending addfd calls, clear them out */
> + list_for_each_entry_safe(addfd, tmp, &n.addfd, list) {
> + /* The process went away before we got a chance to handle it */
> + addfd->ret = -ESRCH;
> + list_del_init(&addfd->list);
> + complete(&addfd->completion);
> + }
> +
> /*
> * 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
> @@ -1174,6 +1263,95 @@ static long seccomp_notify_id_valid(struct seccomp_filter *filter,
> return ret;
> }
>
> +static long seccomp_notify_addfd(struct seccomp_filter *filter,
> + struct seccomp_notif_addfd __user *uaddfd)
> +{
> + struct seccomp_notif_addfd addfd;
> + struct seccomp_knotif *knotif;
> + struct seccomp_kaddfd kaddfd;
> + u64 size;
> + int ret;
> +
> + ret = get_user(size, &uaddfd->size);
> + if (ret)
> + return ret;
> +
> + ret = copy_struct_from_user(&addfd, sizeof(addfd), uaddfd, size);
> + if (ret)
> + return ret;
> +
> + if (addfd.newfd_flags & ~O_CLOEXEC)
> + return -EINVAL;
> +
> + if (addfd.flags & ~SECCOMP_ADDFD_FLAG_SETFD)
> + return -EINVAL;
> +
> + if (addfd.newfd && !(addfd.flags & SECCOMP_ADDFD_FLAG_SETFD))
> + return -EINVAL;
> +
> + kaddfd.file = fget(addfd.srcfd);
> + if (!kaddfd.file)
> + return -EBADF;
> +
> + kaddfd.flags = addfd.newfd_flags;
> + kaddfd.fd = (addfd.flags & SECCOMP_ADDFD_FLAG_SETFD) ?
> + addfd.newfd : -1;

Given that -1 is already illegal, do we need SECCOMP_ADDFD_FLAG_SETFD?
Could a -1 for newfd just be used instead?

> + init_completion(&kaddfd.completion);
> +
> + ret = mutex_lock_interruptible(&filter->notify_lock);
> + if (ret < 0)
> + goto out;
> +
> + knotif = find_notification(filter, addfd.id);
> + /*
> + * We do not want to allow for FD injection to occur before the
> + * notification has been picked up by a userspace handler, or after
> + * the notification has been replied to.
> + */
> + if (!knotif) {
> + ret = -ENOENT;
> + goto out_unlock;
> + }
> +
> + if (knotif->state != SECCOMP_NOTIFY_SENT) {
> + ret = -EINPROGRESS;
> + goto out_unlock;
> + }
> +
> + list_add(&kaddfd.list, &knotif->addfd);
> + complete(&knotif->ready);
> + mutex_unlock(&filter->notify_lock);
> +
> + /* Now we wait for it to be processed */
> + ret = wait_for_completion_interruptible(&kaddfd.completion);
> + if (ret == 0) {
> + /*
> + * We had a successful completion. The other side has already
> + * removed us from the addfd queue, and
> + * wait_for_completion_interruptible has a memory barrier.
> + */
> + ret = kaddfd.ret;
> + goto out;
> + }
> +
> + mutex_lock(&filter->notify_lock);
> + /*
> + * Even though we were woken up by a signal, and not a successful
> + * completion, a completion may have happened in the mean time.
> + */
> + if (list_empty(&kaddfd.list))
> + ret = kaddfd.ret;
> + else
> + list_del(&kaddfd.list);
> +
> +out_unlock:
> + mutex_unlock(&filter->notify_lock);
> +out:
> + fput(kaddfd.file);
> +
> + return ret;
> +}
> +
> static long seccomp_notify_ioctl(struct file *file, unsigned int cmd,
> unsigned long arg)
> {
> @@ -1187,6 +1365,8 @@ static long seccomp_notify_ioctl(struct file *file, unsigned int cmd,
> return seccomp_notify_send(filter, buf);
> case SECCOMP_IOCTL_NOTIF_ID_VALID:
> return seccomp_notify_id_valid(filter, buf);
> + case SECCOMP_IOCTL_NOTIF_ADDFD:
> + return seccomp_notify_addfd(filter, buf);
> default:
> return -EINVAL;
> }
> --
> 2.25.1
>

Whee! :)

--
Kees Cook

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