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SubjectRe: [PATCH 2/5] seccomp: Introduce addfd ioctl to seccomp user notifier
On Mon, May 25, 2020 at 6:50 AM Christian Brauner
<christian.brauner@ubuntu.com> wrote:
>
> 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@sargun.me>
> > Suggested-by: Matt Denton <mpdenton@google.com>
> > Cc: Kees Cook <keescook@google.com>,
> > Cc: Jann Horn <jannh@google.com>,
> > Cc: Robert Sesek <rsesek@google.com>,
> > Cc: Chris Palmer <palmer@google.com>
> > Cc: Christian Brauner <christian.brauner@ubuntu.com>
> > Cc: Tycho Andersen <tycho@tycho.ws>
> > ---
> > 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?
>
I'm a little confused on where we use ESRCH vs. ENOENT. It looks like
in the cookie
check (SECCOMP_IOCTL_NOTIF_ID_VALID), we return ENOENT on both error paths
-- whether the notification is missing, or whether the notification
was already replied to.

I originally had this as ESRCH, but switched to ENOENT to be
consistent with that API.
Do we want the API to disclose information about half-done /
incomplete notifications?

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