On Tue 22-04-14 15:50:26, Michael Kerrisk (man-pages) wrote:
> On Tue, Apr 22, 2014 at 3:40 PM, Jan Kara <j...@suse.cz> wrote:
> > On Sat 19-04-14 22:53:53, Heinrich Schuchardt wrote:
> >> When monitoring a directory or a mount with the fanotify API
> >> the call to fanotify_init checks,
> >> * the process has cap_sys_admin capability
> >>
> >> The call to fanotify_mark checks,
> >> * the process has read authorization for directory or mount
> >>
> >> A directory or mount may contain files for which the process
> >> has no read or write authorization.
> >> Yet when reading from the fanotify file descriptor, structures
> >> fanotify_event_metadata are returned, which contain a file
> >> descriptor for these files, and will allow to read or write.
> >>
> >> The patch adds an authorization check for read and write
> >> permission. In case of missing permission, reading from the
> >> fanotify file descriptor returns EACCES.
> > OK, am I right you are concerned about a situation where fanotify group
> > descriptor is passed to an unpriviledged process which handles all the
> > incoming events? I'm asking because the permission checking can be
> > relatively expensive (think of acls) so we better do it for a reason.
> > I'd prefer to hear from Eric what the original intention regarding
> > permissions was...
>
> If I understand correctly, passing to an unprivileged process is the
> point. The point is I think that supposedly one only needs to
> CAP_SYS_ADMIN to use fanotify. However, once you have that capability,
> then you implicitly get the effect of CAP_DAC_READ_SEARCH and
> CAP_DAC_OVERRIDE as well.
Ah, OK. Thanks for explanation. Then I'm OK with the patch. So feel free
to add:
Reviewed-by: Jan Kara <j...@suse.cz>
Honza
> >> fs/notify/fanotify/fanotify_user.c | 20 +++++++++++++++-----
> >> 1 file changed, 15 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/fs/notify/fanotify/fanotify_user.c
> >> b/fs/notify/fanotify/fanotify_user.c
> >> index 4e565c8..5d22a20 100644
> >> --- a/fs/notify/fanotify/fanotify_user.c
> >> +++ b/fs/notify/fanotify/fanotify_user.c
> >> @@ -62,6 +62,8 @@ static int create_fd(struct fsnotify_group *group,
> >> {
> >> int client_fd;
> >> struct file *new_file;
> >> + int mask;
> >> + int ret;
> >>
> >> pr_debug("%s: group=%p event=%p\n", __func__, group, event);
> >>
> >> @@ -75,11 +77,19 @@ static int create_fd(struct fsnotify_group *group,
> >> */
> >> /* it's possible this event was an overflow event. in that case
> >> dentry and mnt
> >> * are NULL; That's fine, just don't call dentry open */
> >> - if (event->path.dentry && event->path.mnt)
> >> - new_file = dentry_open(&event->path,
> >> - group->fanotify_data.f_flags |
> >> FMODE_NONOTIFY,
> >> - current_cred());
> >> - else
> >> + if (event->path.dentry && event->path.mnt) {
> >> + /* check permissions before granting access to file */
> >> + mask = MAY_READ;
> >> + if (group->fanotify_data.f_flags & (O_RDWR | O_WRONLY))
> >> + mask |= MAY_WRITE;
> >> + ret = inode_permission(event->path.dentry->d_inode, mask);
> >> + if (ret)
> >> + new_file = ERR_PTR(ret);
> >> + else
> >> + new_file = dentry_open(&event->path,
> >> + group->fanotify_data.f_flags
> >> | FMODE_NONOTIFY,
> >> + current_cred());
> >> + } else
> >> new_file = ERR_PTR(-EOVERFLOW);
> >> if (IS_ERR(new_file)) {
> >> /*
> >> --
> >> 1.9.1
> >>
> > --
> > Jan Kara <j...@suse.cz>
> > SUSE Labs, CR
>
>
>
> --
> Michael Kerrisk
> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
> Linux/UNIX System Programming Training: http://man7.org/training/
--
Jan Kara <j...@suse.cz>
SUSE Labs, CR
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