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

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Message-ID: <20200827200801.GB1236603@ZenIV.linux.org.uk>
Date:   Thu, 27 Aug 2020 21:08:01 +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 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?