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

[Mihai DonÈu]

On Thu, 02 Oct 2014 08:20:55 +0200 Yann Droneaud wrote:
> Hi,
> 
> Le mercredi 01 octobre 2014 Ã 15:36 -0700, Andrew Morton a Ãcrit :
> > 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?
> > 
> 
> In the other hand, not enabling close-on-exec might expose unwanted file
> descriptor to childs, creating security issues. YMMV.
> 

As someone who uses fanotify for content introspection, I can say that
I am _explicitly_ marking the fd obtained via read() as O_CLOEXEC,
because I have encountered a situation where a child managed to create
a deadlock because it kept the fd open after the main application
responded with FAN_ALLOW.

-- 
Mihai DonÈu
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