linux-kernel - Re: [PATCH 0/3] vfs: plug some holes involving LAST_BIND symlinks and file bind mounts (try #5)

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Date:	Mon, 23 Nov 2009 18:15:45 -0500
From:	Jeff Layton <jlayton@...hat.com>
To:	Jamie Lokier <jamie@...reable.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	pavel@....cz, miklos@...redi.hu, viro@...IV.linux.org.uk
Subject: Re: [PATCH 0/3] vfs: plug some holes involving LAST_BIND symlinks
 and file bind mounts (try #5)

On Mon, 23 Nov 2009 22:49:48 +0000
Jamie Lokier <jamie@...reable.org> wrote:

> Jeff Layton wrote:
> > > check_path_accessible seems pretty horrible.   If a process is running
> > > inside of a subdirectory it doesn't have permissions to access, say
> > > a chroot, /proc/self/fd/XXX becomes completely unusable.
> > > 
> > 
> > Hmm...I have this in there:
> > 
> > +		/* are we at global root or root of namespace? */
> > +		if ((tdentry == root.dentry && vfsmnt == root.mnt) ||
> > +		    vfsmnt->mnt_parent == vfsmnt)
> > +			break;
> > 
> > ...In the case of a chroot, wouldn't "current->fs->root" point to the
> > root of the process' namespace? Or am I misunderstanding what
> > current->fs actually represents?
> 
> A process can run inside a subdirectory it doesn't have permissions to
> access without that being a chroot.
> 

Certainly.

> It can also run inside a subdirectory that isn't accessible from it's
> root, if that's how it was started - as well as having other
> descriptors pointing to things outside its root.
> 

Yes.

> It can also be passed file descriptors from outside it's root while
> it's running.
> 

Yep.

> Really, I think the /proc/PID/fd/N check should restrict the open to
> the O_* limitations that were used to open fd N before, and not have
> any connection to actual paths at the time of this check.
> 

The big question with all of this is: Should a task have the ability
to follow a /proc/pid symlink to a path that it wouldn't ordinarily be
able to resolve with a path lookup. The concensus that I got from the
bugtraq discussion was that it should not, and this patch is an attempt
to prevent that.

I take it from you and Eric's comments that you disagree? If so, what's
your rationale for allowing a task to resolve this symlink when it
wouldn't ordinarily be able to do so if it were a "normal" symlink?

-- 
Jeff Layton <jlayton@...hat.com>
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