linux-kernel - Re: [PATCH RFC 0/1] mount: universally disallow mounting over symlinks

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Message-ID: <19fa114ef619057c0d14dc1a587d0ae9ad67dc6d.camel@themaw.net>
Date:   Tue, 14 Jan 2020 08:25:19 +0800
From:   Ian Kent <raven@...maw.net>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Aleksa Sarai <cyphar@...har.com>,
        David Howells <dhowells@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        stable <stable@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Serge Hallyn <serge@...lyn.com>, dev@...ncontainers.org,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC 0/1] mount: universally disallow mounting over
 symlinks

On Mon, 2020-01-13 at 10:59 +0800, Ian Kent wrote:
> 
> > 3) is _anything_ besides root directory ever created in direct
> > autofs
> > superblocks by anyone?  If not, why does autofs_lookup() even
> > bother
> > to
> > do anything there?  IOW, why not have it return ERR_PTR(-ENOENT)
> > immediately
> > for direct ones?  Or am I missing something and it is, in fact,
> > possible
> > to have the daemon create something in those?
> 
> Short answer is no, longer answer is directories "shouldn't" ever
> be created inside direct mount points.
> 
> The thing is that the multi-mount map construct can be used with
> direct mounts too, but they must always have a real mount at the
> base because they are direct mounts. So processes should not be
> able to walk into them while they are being mounted (constructed).
> 
> But I'm pretty sure it's rare (maybe not done at all) that this
> map construct is used with direct mounts.

This isn't right.

There's actually nothing stopping a user from using a direct map
entry that's a multi-mount without an actual mount at its root.
So there could be directories created under these, it's just not
usually done.

I'm pretty sure I don't check and disallow this.

Ian