linux-kernel - Re: [PATCH 1/1] fanotify: check permissions when creating file descriptor

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Message-ID: <20140424090441.GA6723@quack.suse.cz>
Date:	Thu, 24 Apr 2014 11:04:41 +0200
From:	Jan Kara <jack@...e.cz>
To:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:	Jan Kara <jack@...e.cz>, Heinrich Schuchardt <xypron.glpk@....de>,
	Eric Paris <eparis@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] fanotify: check permissions when creating file
 descriptor

On Tue 22-04-14 16:07:47, Jan Kara wrote:
> On Tue 22-04-14 15:50:26, Michael Kerrisk (man-pages) wrote:
> > On Tue, Apr 22, 2014 at 3:40 PM, Jan Kara <jack@...e.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 <jack@...e.cz>
  Hum, when digging more around this code, I've found out that
fanotify_mark() checks whether it has a read permission to a watched file
when creating the mark (in fanotify_find_path()). So I don't think it's
really worth it to recheck the permissions when creating a file descriptor
for the event. Sure it may be somewhat surprising that read fd is created
after a process doesn't have access to the file anymore but OTOH it is
similar to a situation where the process has opened the file long time ago.

								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 <jack@...e.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 <jack@...e.cz>
> SUSE Labs, CR
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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