Re: [PATCH 2/5] seccomp: Introduce addfd ioctl to seccomp user notifier

[Christian Brauner]

On Sun, May 24, 2020 at 04:39:39PM -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@xxxxxxxxx>
> Suggested-by: Matt Denton <mpdenton@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxx>,
> Cc: Jann Horn <jannh@xxxxxxxxxx>,
> Cc: Robert Sesek <rsesek@xxxxxxxxxx>,
> Cc: Chris Palmer <palmer@xxxxxxxxxx>
> Cc: Christian Brauner <christian.brauner@xxxxxxxxxx>
> Cc: Tycho Andersen <tycho@xxxxxxxx>
> ---
>  include/uapi/linux/seccomp.h |  25 ++++++
>  kernel/seccomp.c             | 169 ++++++++++++++++++++++++++++++++++-
>  2 files changed, 193 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
> index c1735455bc53..7d450a9e4c29 100644
> --- a/include/uapi/linux/seccomp.h
> +++ b/include/uapi/linux/seccomp.h
> @@ -113,6 +113,27 @@ struct seccomp_notif_resp {
>  	__u32 flags;
>  };
>  
> +/* valid flags for seccomp_notif_addfd */
> +#define SECCOMP_ADDFD_FLAG_SETFD	(1UL << 0) /* Specify remote fd */
> +
> +/**
> + * struct seccomp_notif_addfd
> + * @size: The size of the seccomp_notif_addfd datastructure
> + * @fd: The local fd number
> + * @id: The ID of the seccomp notification
> + * @fd_flags: Flags the remote FD should be allocated under
> + * @remote_fd: Optional remote FD number if SETFD option is set, otherwise 0.
> + * @flags: SECCOMP_ADDFD_FLAG_*
> + */
> +struct seccomp_notif_addfd {
> +	__u32 size;
> +	__u32 fd;
> +	__u64 id;
> +	__u32 fd_flags;
> +	__u32 remote_fd;
> +	__u64 flags;
> +};

This was a little confusing to me at first. So fd is the fd from which
we take the struct file and remote_fd is either -1 at which point we
just allocate the next free fd number and if it is not we
allocate/replace a specific one. Maybe it would be clearer if we did:

struct seccomp_notif_addfd {
	__u32 size;
	__u64 id;
	__u64 flags;
	__u32 srcfd;
	__u32 newfd;
	__u32 newfd_flags;
};

No need to hide in the name that this is remote_dup2().

> +
>  #define SECCOMP_IOC_MAGIC		'!'
>  #define SECCOMP_IO(nr)			_IO(SECCOMP_IOC_MAGIC, nr)
>  #define SECCOMP_IOR(nr, type)		_IOR(SECCOMP_IOC_MAGIC, nr, type)
> @@ -124,4 +145,8 @@ struct seccomp_notif_resp {
>  #define SECCOMP_IOCTL_NOTIF_SEND	SECCOMP_IOWR(1,	\
>  						struct seccomp_notif_resp)
>  #define SECCOMP_IOCTL_NOTIF_ID_VALID	SECCOMP_IOR(2, __u64)
> +/* On success, the return value is the remote process's added fd number */
> +#define SECCOMP_IOCTL_NOTIF_ADDFD	SECCOMP_IOR(3,	\
> +						struct seccomp_notif_addfd)
> +
>  #endif /* _UAPI_LINUX_SECCOMP_H */
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index f6ce94b7a167..88940eeabaee 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -77,10 +77,42 @@ struct seccomp_knotif {
>  	long val;
>  	u32 flags;
>  
> -	/* Signals when this has entered SECCOMP_NOTIFY_REPLIED */
> +	/*
> +	 * Signals when this has changed states, such as the listener
> +	 * dying, a new seccomp addfd message, or changing to REPLIED
> +	 */
>  	struct completion ready;
>  
>  	struct list_head list;
> +
> +	/* outstanding addfd requests */
> +	struct list_head addfd;
> +};
> +
> +/**
> + * struct seccomp_kaddfd - contianer for seccomp_addfd ioctl messages

                              ^^^ typo

> + *
> + * @file: A reference to the file to install in the other task
> + * @fd: The fd number to install it at. If the fd number is -1, it means the
> + *      installing process should allocate the fd as normal.
> + * @flags: The flags for the new file descriptor. At the moment, only O_CLOEXEC
> + *         is allowed.
> + * @ret: The return value of the installing process. It is set to the fd num
> + *       upon success (>= 0).
> + * @completion: Indicates that the installing process has completed fd
> + *              installation, or gone away (either due to successful
> + *              reply, or signal)
> + *
> + */
> +struct seccomp_kaddfd {
> +	struct file *file;
> +	int fd;
> +	unsigned int flags;
> +
> +	/* To only be set on reply */
> +	int ret;
> +	struct completion completion;
> +	struct list_head list;
>  };
>  
>  /**
> @@ -735,6 +767,35 @@ static u64 seccomp_next_notify_id(struct seccomp_filter *filter)
>  	return filter->notif->next_id++;
>  }
>  
> +static void seccomp_handle_addfd(struct seccomp_kaddfd *addfd)
> +{
> +	int ret;
> +
> +	/*
> +	 * 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 >= 0) {
> +		ret = replace_fd(addfd->fd, addfd->file, addfd->flags);
> +		if (ret >= 0)
> +			fput(addfd->file);
> +	} else {
> +		ret = get_unused_fd_flags(addfd->flags);
> +		if (ret >= 0)
> +			fd_install(ret, addfd->file);
> +	}
> +
> +out:
> +	addfd->ret = ret;
> +	complete(&addfd->completion);
> +}
> +
>  static int seccomp_do_user_notification(int this_syscall,
>  					struct seccomp_filter *match,
>  					const struct seccomp_data *sd)
> @@ -743,6 +804,7 @@ static int seccomp_do_user_notification(int this_syscall,
>  	u32 flags = 0;
>  	long ret = 0;
>  	struct seccomp_knotif n = {};
> +	struct seccomp_kaddfd *addfd, *tmp;
>  
>  	mutex_lock(&match->notify_lock);
>  	err = -ENOSYS;
> @@ -755,6 +817,7 @@ static int seccomp_do_user_notification(int this_syscall,
>  	n.id = seccomp_next_notify_id(match);
>  	init_completion(&n.ready);
>  	list_add(&n.list, &match->notif->notifications);
> +	INIT_LIST_HEAD(&n.addfd);
>  
>  	up(&match->notif->request);
>  	wake_up_poll(&match->notif->wqh, EPOLLIN | EPOLLRDNORM);
> @@ -763,14 +826,31 @@ 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;
>  	}
>  
> +	/* 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 = -ENOENT;

Looks like it should be -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
> @@ -1179,6 +1259,91 @@ 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;
> +	u32 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.fd_flags & (~O_CLOEXEC))
> +		return -EINVAL;
> +
> +	if (addfd.flags & ~(SECCOMP_ADDFD_FLAG_SETFD))
> +		return -EINVAL;

I'd suggest to remove the brackets and just do

if (addfd.flags & ~SECCOMP_ADDFD_FLAG_SETFD)

that's more common in the kernel.

> +
> +	if (addfd.remote_fd && !(addfd.flags & SECCOMP_ADDFD_FLAG_SETFD))
> +		return -EINVAL;
> +
> +	kaddfd.file = fget(addfd.fd);
> +	if (!kaddfd.file)
> +		return -EBADF;
> +
> +	kaddfd.flags = addfd.fd_flags;
> +	kaddfd.fd = (addfd.flags & SECCOMP_ADDFD_FLAG_SETFD) ?
> +		    addfd.remote_fd : -1;
> +	init_completion(&kaddfd.completion);
> +
> +	ret = mutex_lock_interruptible(&filter->notify_lock);
> +	if (ret)

The patter in all other places in seccomp is to compare against < 0, so

if (ret < 0)
	goto out;

I think.

> +		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 || knotif->state != SECCOMP_NOTIFY_SENT) {
> +		ret = -ENOENT;
> +		goto out_unlock;
> +	}

if (!knotif) {
	ret = -ESRCH;
	goto out_unlock;
}

if (knotif->state != SECCOMP_NOTIFY_SENT) {
	ret = -EBUSY/-EINVAL;
	goto out_unlock;
}

so we don't overload errors?

> +
> +	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:
> +	if (ret < 0)
> +		fput(kaddfd.file);
> +
> +	return ret;
> +}
> +
>  static long seccomp_notify_ioctl(struct file *file, unsigned int cmd,
>  				 unsigned long arg)
>  {
> @@ -1192,6 +1357,8 @@ 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
>