linux-kernel - Re: [PATCH v9 1/2] Add a "nosymfollow" mount option.

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Date:   Thu, 27 Aug 2020 21:10:15 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Ross Zwisler <zwisler@...omium.org>
Cc:     linux-kernel@...r.kernel.org,
        Mattias Nissler <mnissler@...omium.org>,
        Aleksa Sarai <cyphar@...har.com>,
        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, linux-kselftest@...r.kernel.org,
        Matthew Wilcox <willy@...radead.org>,
        Micah Morton <mortonm@...gle.com>,
        Raul Rangel <rrangel@...gle.com>,
        Shuah Khan <shuah@...nel.org>,
        Ross Zwisler <zwisler@...gle.com>
Subject: Re: [PATCH v9 1/2] Add a "nosymfollow" mount option.

On Thu, Aug 27, 2020 at 09:08:01PM +0100, Al Viro wrote:
> On Thu, Aug 27, 2020 at 11:09:46AM -0600, Ross Zwisler 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
> > 
> > Signed-off-by: Mattias Nissler <mnissler@...omium.org>
> > Signed-off-by: Ross Zwisler <zwisler@...gle.com>
> > Reviewed-by: Aleksa Sarai <cyphar@...har.com>
> > ---
> > Changes since v8 [1]:
> >  * Look for MNT_NOSYMFOLLOW in link->mnt->mnt_flags so we are testing
> >    the link itself rather than the directory holding the link. (Al Viro)
> >  * Rebased onto v5.9-rc2.
> 
> AFAICS, it applies clean to -rc1; what was the rebase about?

Applied (to -rc1) and pushed