linux-kernel - Re: [PATCH] dm cache metadata: replace deprecated strncpy with strscpy

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Message-ID: <202309251103.4FBC42C4@keescook>
Date:   Mon, 25 Sep 2023 11:03:34 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Justin Stitt <justinstitt@...gle.com>
Cc:     Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...nel.org>, dm-devel@...hat.com,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] dm cache metadata: replace deprecated strncpy with
 strscpy

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@...r.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>

Agreed about the %NUL termination and padding assessment.

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

-Kees

-- 
Kees Cook