linux-kernel - Re: [PATCH 1/3] namei: implement O_BENEATH-style AT

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Date:   Sun, 30 Sep 2018 14:38:23 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>, Shuah Khan <shuah@...nel.org>
Cc:     David Howells <dhowells@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Christian Brauner <christian@...uner.io>,
        Eric Biederman <ebiederm@...ssion.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Tycho Andersen <tycho@...ho.ws>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-kselftest@...r.kernel.org, dev@...ncontainers.org,
        containers@...ts.linux-foundation.org
Subject: Re: [PATCH 1/3] namei: implement O_BENEATH-style AT_* flags

On 2018-09-29, Aleksa Sarai <cyphar@...har.com> wrote:
> * AT_XDEV: Disallow mount-point crossing (both *down* into one, or *up*
>   from one). The primary "scoping" use is to blocking resolution that
>   crosses a bind-mount, which has a similar property to a symlink (in
>   the way that it allows for escape from the starting-point). Since it
>   is not possible to differentiate bind-mounts However since
>   bind-mounting requires privileges (in ways symlinks don't) this has
>   been split from LOOKUP_BENEATH. The naming is based on "find -xdev"
>   (though find(1) doesn't walk upwards, the semantics seem obvious).

I've just noticed that the mountpoint-crossing code for AT_XDEV doesn't
detect things like:

   % ln -s / /tmp/jumpup
   % vfs_helper -o open -F xdev -d /tmp jumpup
   /

I will fix that in v2.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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