linux-kernel - Re: [PATCH v3] vfs: new O

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Date:	Mon, 7 Dec 2009 17:16:38 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Andrew Lutomirski <luto@....edu>
Cc:	Miklos Szeredi <miklos@...redi.hu>, akpm@...ux-foundation.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] vfs: new O_NODE open flag

> >        while(1)
> >                fchmod(fd, 0666);
> >
> > wait for device to unload, reload and be intended for another user
> > Race udev to a real open. You have a similar problem with vhangup() and
> > ttys.
> 
> Huh?  I would've thought that udev would (and already does?), on
> device unload, chown to 0:0, then chmod to 0000, then unlink, in which
> case that attack doesn't work.

udev doesn't control the device unload/reload. It responds to messages
from the kernel which are to some extent asynchronous to actual events.
It may be ok if udev is very careful but the fact it requires a close
inspection of the kernel and user space sides doesn't bode well (with or
without O_NODE). The fact we currently have an implied revoke by the
device refcounts is a big helper at the moment.

The tty cases using vhangup() assume that the handle is killed and would
also need addressing.

> Would you be okay with a patch that prevented opening
> /proc/self/fd/xxx on O_NODE handles?  I personally don't care about

I'd like to see what Al Viro has to say on the subject first.
The /proc/self stuff bothers me less - I've not seen a convincing
description of it being misuable where ptrace wouldn't allow the same
actions. Even the constructed scenarios share that property.

> O_NODE all that much, but I'd like a decent in-kernel AFS
> implementation (and a decent revoke() implementation, and especially
> the ability to revoke whole filesystems would be really nice too).

The AFS case is probably the easier one - its things like device files
where one handle can change completely what it references (due to device
loads/unloads and dynamic major/minor assignment) that make it evil.

CIFS/SMB is horrible for different reasons (a handle open on some piece
of namespace isn't going to always been the same actual file) but you
could simply decide CIFS/SMB and any other problematic cases don't
support it.

I don't really have a problem with it providing its restricted to
ordinary files on a file system where having a local inode reference
means you have a stable reference to an object on the remote system or
the local media.

The way to start this is firstly to convince Al Viro (always a good
sanity check), and then to start with the obviously safe cases only -
regular files, only file systems with stable inode references.

Devices are hard - why do we need O_NODE on devices anyway ?

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