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:	Tue, 1 Dec 2009 08:15:15 -0500
From:	Jeff Layton <jlayton@...hat.com>
To:	ebiederm@...ssion.com (Eric W. Biederman)
Cc:	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 14:05:24 -0800
ebiederm@...ssion.com (Eric W. Biederman) wrote:

> Jeff Layton <jlayton@...hat.com> writes:
> 
> > There are a few situations where a lookup can end up returning a dentry
> > without revalidating it, and without checking whether the calling
> > process has permissions to access it. Two situations identified so far
> > are:
> >
> > 1) LAST_BIND symlinks (such as those under /proc/<pid>)
> >
> > 2) file bind mounts
> >
> > This patchset is intended to fix this by forcing revalidation of the
> > returned dentries at appropriate locations.
> >
> > In the case of LAST_BIND symlinks it also adds a check to verify that
> > the target of the symlink is accessible by the current process by
> > walking mounts and dentries back up to the root and checking permission
> > on each inode.
> >
> > This set fixes the reproducers I have (including the reproducer that
> > Pavel provided for the permissions bypass). It's still pretty rough
> > though and I expect that it'll need revision. At this point, I'm mainly
> > looking to get these questions answered:
> >
> > 1) what should we do if these dentries are found to be invalid? Is it ok
> > to d_invalidate them? Or is that likely to break something (particularly
> > in the case of file bind mounts)?
> 
> The normal sequence in do_revalidate should be safe.  In practice what we
> should see is d_drop().  If we access the dentries via another path today
> we already go through d_revalidate.  It is only the reference count on
> the dentry that keeps them alive and working.  The cases I have looked
> at for distributed filesystems have to call d_drop themselves so I don't
> know if it would add anything if the vfs called d_revalidate.  Especially
> since FS_REVAL_DOT doesn't have that logic.
> 

There seems to be a lot of disagreement about whether the issue that
Pavel raised is even a bug. I think what I'm going to do at this point
is respin this patchset without that patch (just add the missing
revalidations).

I'll also plan to just have force_reval_path call do_revalidate instead
so that invalid dentries get d_invalidated too. Any other thoughts on
the first two patches in this set?

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