linux-kernel - Re: [PATCH v4 2/4] namei: O_BENEATH-style path resolution flags

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Date:   Fri, 23 Nov 2018 13:10:17 +0100
From:   Jürg Billeter <j@...ron.ch>
To:     Aleksa Sarai <cyphar@...har.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>
Cc:     Eric Biederman <ebiederm@...ssion.com>,
        Christian Brauner <christian@...uner.io>,
        linux-api@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Jann Horn <jannh@...gle.com>,
        David Drysdale <drysdale@...gle.com>,
        Aleksa Sarai <asarai@...e.de>,
        containers@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org
Subject: Re: [PATCH v4 2/4] namei: O_BENEATH-style path resolution flags

Hi Aleksa,

On Tue, 2018-11-13 at 01:26 +1100, Aleksa Sarai wrote:
> * O_BENEATH: Disallow "escapes" from the starting point of the
>   filesystem tree during resolution (you must stay "beneath" the
>   starting point at all times). Currently this is done by disallowing
>   ".." and absolute paths (either in the given path or found during
>   symlink resolution) entirely, as well as all "magic link" jumping.

With open_tree(2) and OPEN_TREE_CLONE, will O_BENEATH still be
necessary? As I understand it, O_BENEATH could be replaced by a much
simpler flag that only disallows absolute paths (incl. absolute
symlinks). And it would have the benefit that you can actually pass the
tree/directory fd to another process and escaping would not be possible
even if that other process doesn't use O_BENEATH (after calling
mount_setattr(2) to make sure it's locked down).

This approach would also make it easy to restrict writes via a cloned
tree/directory fd by marking it read-only via mount_setattr(2) (and
locking down the read-only flag). This would again be especially useful
when passing tree/directory fds across processes, or for voluntary
self-lockdown within a process for robustness against security bugs.

This wouldn't affect any of the other flags in this patch. And for full
equivalence to O_BENEATH you'd have to use O_NOMAGICLINKS in addition
to O_NOABSOLUTE, or whatever that new flag would be called.

Or is OPEN_TREE_CLONE too expensive for this use case? Or is there
anything else I'm missing?

Jürg