On Sun 24-01-21 20:42:04, Amir Goldstein wrote:
> Add limited support for unprivileged fanotify event listener.
> An unprivileged event listener does not get an open file descriptor in
> the event nor the process pid of another process. An unprivileged event
> listener cannot request permission events, cannot set mount/filesystem
> marks and cannot request unlimited queue/marks.
>
> This enables the limited functionality similar to inotify when watching a
> set of files and directories for OPEN/ACCESS/MODIFY/CLOSE events, without
> requiring SYS_CAP_ADMIN privileges.
>
> The FAN_REPORT_DFID_NAME init flag, provide a method for an unprivileged
> event listener watching a set of directories (with FAN_EVENT_ON_CHILD)
> to monitor all changes inside those directories.
>
> This typically requires that the listener keeps a map of watched directory
> fid to dirfd (O_PATH), where fid is obtained with name_to_handle_at()
> before starting to watch for changes.
>
> When getting an event, the reported fid of the parent should be resolved
> to dirfd and fstatsat(2) with dirfd and name should be used to query the
> state of the filesystem entry.
>
> Note that even though events do not report the event creator pid,
> fanotify does not merge similar events on the same object that were
> generated by different processes. This is aligned with exiting behavior
> when generating processes are outside of the listener pidns (which
> results in reporting 0 pid to listener).
>
> Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
The patch looks mostly good to me. Just two questions:
a) Remind me please, why did we decide pid isn't safe to report to
unpriviledged listeners?
b) Why did we decide returning open file descriptors isn't safe for
unpriviledged listeners? Is it about FMODE_NONOTIFY?
I'm not opposed to either but I'm wondering. Also with b) old style
fanotify events are not very useful so maybe we could just disallow all
notification groups without FID/DFID reporting? In the future if we ever
decide returning open fds is safe or how to do it, we can enable that group
type for unpriviledged users. However just starting to return open fds
later won't fly because listener has to close these fds when receiving
events.
Honza
> ---
> fs/notify/fanotify/fanotify_user.c | 49 +++++++++++++++++++++++++++---
> fs/notify/fdinfo.c | 3 +-
> include/linux/fanotify.h | 16 ++++++++++
> 3 files changed, 62 insertions(+), 6 deletions(-)
>
> diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> index 4ade3f9df337..b70de273eedb 100644
> --- a/fs/notify/fanotify/fanotify_user.c
> +++ b/fs/notify/fanotify/fanotify_user.c
> @@ -397,9 +397,21 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
> metadata.vers = FANOTIFY_METADATA_VERSION;
> metadata.reserved = 0;
> metadata.mask = event->mask & FANOTIFY_OUTGOING_EVENTS;
> - metadata.pid = pid_vnr(event->pid);
> + /*
> + * An unprivileged event listener does not get an open file descriptor
> + * in the event nor another generating process pid. If the event was
> + * generated by the unprivileged process itself, self pid is reported.
> + * We may relax this in the future by checking calling process access
> + * permissions to the object.
> + */
> + if (!FAN_GROUP_FLAG(group, FANOTIFY_UNPRIV) ||
> + task_tgid(current) == event->pid)
> + metadata.pid = pid_vnr(event->pid);
> + else
> + metadata.pid = 0;
>
> - if (path && path->mnt && path->dentry) {
> + if (!FAN_GROUP_FLAG(group, FANOTIFY_UNPRIV) &&
> + path && path->mnt && path->dentry) {
> fd = create_fd(group, path, &f);
> if (fd < 0)
> return fd;
> @@ -995,12 +1007,29 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
> int f_flags, fd;
> unsigned int fid_mode = flags & FANOTIFY_FID_BITS;
> unsigned int class = flags & FANOTIFY_CLASS_BITS;
> + unsigned int internal_flags = 0;
>
> pr_debug("%s: flags=%x event_f_flags=%x\n",
> __func__, flags, event_f_flags);
>
> - if (!capable(CAP_SYS_ADMIN))
> - return -EPERM;
> + if (!capable(CAP_SYS_ADMIN)) {
> + /*
> + * An unprivileged user can setup an unprivileged listener with
> + * limited functionality - an unprivileged event listener cannot
> + * request permission events, cannot set mount/filesystem marks
> + * and cannot request unlimited queue/marks.
> + */
> + if ((flags & ~FANOTIFY_UNPRIV_INIT_FLAGS) ||
> + class != FAN_CLASS_NOTIF)
> + return -EPERM;
> +
> + /*
> + * We set the internal flag FANOTIFY_UNPRIV on the group, so we
> + * know that we need to limit setting mount/filesystem marks on
> + * this group and avoid providing pid and open fd in the event.
> + */
> + internal_flags |= FANOTIFY_UNPRIV;
> + }
>
> #ifdef CONFIG_AUDITSYSCALL
> if (flags & ~(FANOTIFY_INIT_FLAGS | FAN_ENABLE_AUDIT))
> @@ -1051,7 +1080,7 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
> goto out_destroy_group;
> }
>
> - group->fanotify_data.flags = flags;
> + group->fanotify_data.flags = flags | internal_flags;
> group->memcg = get_mem_cgroup_from_mm(current->mm);
>
> group->overflow_event = fanotify_alloc_overflow_event();
> @@ -1247,6 +1276,15 @@ static int do_fanotify_mark(int fanotify_fd, unsigned int flags, __u64 mask,
> goto fput_and_out;
> group = f.file->private_data;
>
> + /*
> + * An unprivileged event listener is not allowed to watch a mount
> + * point nor a filesystem.
> + */
> + ret = -EPERM;
> + if (FAN_GROUP_FLAG(group, FANOTIFY_UNPRIV) &&
> + mark_type != FAN_MARK_INODE)
> + goto fput_and_out;
> +
> /*
> * group->priority == FS_PRIO_0 == FAN_CLASS_NOTIF. These are not
> * allowed to set permissions events.
> @@ -1379,6 +1417,7 @@ SYSCALL32_DEFINE6(fanotify_mark,
> */
> static int __init fanotify_user_setup(void)
> {
> + BUILD_BUG_ON(FANOTIFY_INIT_FLAGS & FANOTIFY_INTERNAL_FLAGS);
> BUILD_BUG_ON(HWEIGHT32(FANOTIFY_INIT_FLAGS) != 10);
> BUILD_BUG_ON(HWEIGHT32(FANOTIFY_MARK_FLAGS) != 9);
>
> diff --git a/fs/notify/fdinfo.c b/fs/notify/fdinfo.c
> index f0d6b54be412..57f0d5d9f934 100644
> --- a/fs/notify/fdinfo.c
> +++ b/fs/notify/fdinfo.c
> @@ -144,7 +144,8 @@ void fanotify_show_fdinfo(struct seq_file *m, struct file *f)
> struct fsnotify_group *group = f->private_data;
>
> seq_printf(m, "fanotify flags:%x event-flags:%x\n",
> - group->fanotify_data.flags, group->fanotify_data.f_flags);
> + group->fanotify_data.flags & FANOTIFY_INIT_FLAGS,
> + group->fanotify_data.f_flags);
>
> show_fdinfo(m, f, fanotify_fdinfo);
> }
> diff --git a/include/linux/fanotify.h b/include/linux/fanotify.h
> index 031a97d8369a..a573c1028c14 100644
> --- a/include/linux/fanotify.h
> +++ b/include/linux/fanotify.h
> @@ -28,6 +28,22 @@ extern struct ctl_table fanotify_table[]; /* for sysctl */
> FAN_CLOEXEC | FAN_NONBLOCK | \
> FAN_UNLIMITED_QUEUE | FAN_UNLIMITED_MARKS)
>
> +/* Internal flags */
> +#define FANOTIFY_UNPRIV 0x80000000
> +#define FANOTIFY_INTERNAL_FLAGS (FANOTIFY_UNPRIV)
> +
> +/*
> + * fanotify_init() flags allowed for unprivileged listener.
> + * FAN_CLASS_NOTIF in this mask is purely semantic because it is zero,
> + * but it is the only class we allow for unprivileged listener.
> + * Since unprivileged listener does not provide file descriptors in events,
> + * reporting file handles makes sense, but it is not a must.
> + * FAN_REPORT_TID does not make sense for unprivileged listener, which uses
> + * event->pid only to filter out events generated by listener process itself.
> + */
> +#define FANOTIFY_UNPRIV_INIT_FLAGS (FAN_CLOEXEC | FAN_NONBLOCK | \
> + FAN_CLASS_NOTIF | FANOTIFY_FID_BITS)
> +
> #define FANOTIFY_MARK_TYPE_BITS (FAN_MARK_INODE | FAN_MARK_MOUNT | \
> FAN_MARK_FILESYSTEM)
>
> --
> 2.25.1
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
