Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.34 as permitted sender) client-ip=23.128.96.34;
Date:   Mon, 25 Sep 2023 11:03:34 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Justin Stitt <justinstitt@google.com>
Cc:     Alasdair Kergon <agk@redhat.com>,
        Mike Snitzer <snitzer@kernel.org>, dm-devel@redhat.com,
        linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
Subject: Re: [PATCH] dm cache metadata: replace deprecated strncpy with
 strscpy
Message-ID: <202309251103.4FBC42C4@keescook>
References: <20230925-strncpy-drivers-md-dm-cache-metadata-c-v1-1-4b75c7db0cfe@google.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20230925-strncpy-drivers-md-dm-cache-metadata-c-v1-1-4b75c7db0cfe@google.com>
Precedence: bulk

On Mon, Sep 25, 2023 at 06:13:12AM +0000, Justin Stitt wrote:
> `strncpy` is deprecated for use on NUL-terminated destination strings
> [1] and as such we should prefer more robust and less ambiguous string
> interfaces.
> 
> It seems `cmd->policy_name` is intended to be NUL-terminated based on a
> now changed line of code from Commit (c6b4fcbad044e6ff "dm: add cache
> target"):
> |       if (strcmp(cmd->policy_name, policy_name)) { // ...
> 
> However, now a length-bounded strncmp is used:
> |       if (strncmp(cmd->policy_name, policy_name, sizeof(cmd->policy_name)))
> ... which means NUL-terminated may not strictly be required. However, I
> believe the intent of the code is clear and we should maintain
> NUL-termination of policy_names.
> 
> Moreover, __begin_transaction_flags() zero-allocates `cmd` before
> calling read_superblock_fields():
> |       cmd = kzalloc(sizeof(*cmd), GFP_KERNEL);
> 
> Also, `disk_super->policy_name` is zero-initialized
> |       memset(disk_super->policy_name, 0, sizeof(disk_super->policy_name));
> ... therefore any NUL-padding is redundant.
> 
> Considering the above, a suitable replacement is `strscpy` [2] due to
> the fact that it guarantees NUL-termination on the destination buffer
> without unnecessarily NUL-padding.
> 
> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
> Link: https://github.com/KSPP/linux/issues/90
> Cc: linux-hardening@vger.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@google.com>

Agreed about the %NUL termination and padding assessment.

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

-- 
Kees Cook