Re: [PATCHv8.1] fanotify: enable close-on-exec on events' fd when requested in fanotify_init()

[Andrew Morton]

On Thu, 2 Oct 2014 12:44:10 +0200 Jan Kara <jack@xxxxxxx> wrote:

> On Wed 01-10-14 15:36:21, Andrew Morton wrote:
> > On Mon, 29 Sep 2014 10:49:15 +0200 Yann Droneaud <ydroneaud@xxxxxxxxxx> wrote:
> > 
> > > According to commit 80af258867648 ('fanotify: groups can specify
> > > their f_flags for new fd'), file descriptors created as part of
> > > file access notification events inherit flags from the
> > > event_f_flags argument passed to syscall fanotify_init(2).
> > > 
> > > So while it is legal for userspace to call fanotify_init() with
> > > O_CLOEXEC as part of its second argument, O_CLOEXEC is currently
> > > silently ignored.
> > > 
> > > Indeed event_f_flags are only given to dentry_open(), which only
> > > seems to care about O_ACCMODE and O_PATH in do_dentry_open(),
> > > O_DIRECT in open_check_o_direct() and O_LARGEFILE in
> > > generic_file_open().
> > > 
> > > But it seems logical to set close-on-exec flag on the file
> > > descriptor if userspace is allowed to request it with O_CLOEXEC.
> > > 
> > > In fact, according to some lookup on http://codesearch.debian.net/
> > > and various search engine, there's already some userspace code
> > > requesting it:
> > > 
> > > - in systemd's readahead[2]:
> > > 
> > >     fanotify_fd = fanotify_init(FAN_CLOEXEC|FAN_NONBLOCK, O_RDONLY|O_LARGEFILE|O_CLOEXEC|O_NOATIME);
> > > 
> > > - in clsync[3]:
> > > 
> > >     #define FANOTIFY_EVFLAGS (O_LARGEFILE|O_RDONLY|O_CLOEXEC)
> > > 
> > >     int fanotify_d = fanotify_init(FANOTIFY_FLAGS, FANOTIFY_EVFLAGS);
> > > 
> > > - in examples [4] from "Filesystem monitoring in the Linux
> > >   kernel" article[5] by Aleksander Morgado:
> > > 
> > >     if ((fanotify_fd = fanotify_init (FAN_CLOEXEC,
> > >                                       O_RDONLY | O_CLOEXEC | O_LARGEFILE)) < 0)
> > 
> > So we have a number of apps which are setting O_CLOEXEC, but it doesn't
> > actually work.  With this change it *will* work, so the behaviour of
> > those apps might change, possibly breaking them?
>   Possibly. OTOH I'd dare to say that most of the apps specifying O_CLOEXEC
> want that behavior and their security may be weakened by the fact that
> O_CLOEXEC is ignored. So we are weighting possible security issues for apps
> doing things right (and Mihai mentioned in this thread that at least he has
> an application which needs O_CLOEXEC working) against possible breakage for
> apps which just randomly set O_CLOEXEC without wanting. So I'm really for
> fixing O_CLOEXEC behavior.

Fair enough, it sounds like the risk is acceptable.

Can we get a new version sent out with all this new info appropriately
changelogged?

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