linux-kernel - Re: [PATCH v7] Add a "nosymfollow" mount option.

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Message-ID: <CAGRrVHwQ4EpZy73n4NTLhDZNGN4Gi_FUL4BjWw7ruEoGHENZEg@mail.gmail.com>
Date:   Wed, 12 Aug 2020 11:59:13 -0600
From:   Ross Zwisler <zwisler@...omium.org>
To:     Aleksa Sarai <cyphar@...har.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Mattias Nissler <mnissler@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Benjamin Gordon <bmgordon@...gle.com>,
        David Howells <dhowells@...hat.com>,
        Dmitry Torokhov <dtor@...gle.com>,
        Jesse Barnes <jsbarnes@...gle.com>,
        linux-fsdevel@...r.kernel.org,
        Matthew Wilcox <willy@...radead.org>,
        Micah Morton <mortonm@...gle.com>,
        Raul Rangel <rrangel@...gle.com>
Subject: Re: [PATCH v7] Add a "nosymfollow" mount option.

On Tue, Aug 11, 2020 at 7:43 PM Aleksa Sarai <cyphar@...har.com> wrote:
> On 2020-08-11, Ross Zwisler <zwisler@...omium.org> wrote:
> > From: Mattias Nissler <mnissler@...omium.org>
> >
> > For mounts that have the new "nosymfollow" option, don't follow symlinks
> > when resolving paths. The new option is similar in spirit to the
> > existing "nodev", "noexec", and "nosuid" options, as well as to the
> > LOOKUP_NO_SYMLINKS resolve flag in the openat2(2) syscall. Various BSD
> > variants have been supporting the "nosymfollow" mount option for a long
> > time with equivalent implementations.
> >
> > Note that symlinks may still be created on file systems mounted with
> > the "nosymfollow" option present. readlink() remains functional, so
> > user space code that is aware of symlinks can still choose to follow
> > them explicitly.
> >
> > Setting the "nosymfollow" mount option helps prevent privileged
> > writers from modifying files unintentionally in case there is an
> > unexpected link along the accessed path. The "nosymfollow" option is
> > thus useful as a defensive measure for systems that need to deal with
> > untrusted file systems in privileged contexts.
> >
> > More information on the history and motivation for this patch can be
> > found here:
> >
> > https://sites.google.com/a/chromium.org/dev/chromium-os/chromiumos-design-docs/hardening-against-malicious-stateful-data#TOC-Restricting-symlink-traversal
>
> Looks good. Did you plan to add an in-tree test for this (you could
> shove it in tools/testing/selftests/mount)?

Sure, that sounds like a good idea.  I'll take a look.

> Reviewed-by: Aleksa Sarai <cyphar@...har.com>

Thank you for the review.