linux-hardening - Re: [PATCH] nvme-fc: replace deprecated strncpy with strscpy

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Date: Thu, 19 Oct 2023 16:20:28 -0700
From: Kees Cook <keescook@...omium.org>
To: Justin Stitt <justinstitt@...gle.com>
Cc: James Smart <james.smart@...adcom.com>, Keith Busch <kbusch@...nel.org>,
	Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
	Sagi Grimberg <sagi@...mberg.me>, linux-nvme@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] nvme-fc: replace deprecated strncpy with strscpy

On Thu, Oct 19, 2023 at 09:34:35PM +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.
> 
> Let's instead use strscpy() [2] as it guarantees NUL-termination on the
> destination buffer.
> 
> Moreover, there is no need to use:
> 
> |       min(FCNVME_ASSOC_HOSTNQN_LEN, NVMF_NQN_SIZE));
> 
> I imagine this was originally done to make sure the destination buffer
> is NUL-terminated by ensuring we copy a number of bytes less than the
> size of our destination, thus leaving some NUL-bytes at the end.

Yeah, this is odd, but I agree that the resulting strscpy does the
intended copy, since we've seen that other nqn strings are expected to
be %NUL terminated.

> 
> However, with strscpy(), we no longer need to do this and we can instead
> opt for the more idiomatic strscpy() usage of:
> 
> | strscpy(dest, src, sizeof(dest))
> 
> Also, no NUL-padding is required as lsop is zero-allocated:
> 
> |       lsop = kzalloc((sizeof(*lsop) +
> |                        sizeof(*assoc_rqst) + sizeof(*assoc_acc) +
> |                        ctrl->lport->ops->lsrqst_priv_sz), GFP_KERNEL);
> 
> ... and assoc_rqst points to a field in lsop:
> 
> |       assoc_rqst = (struct fcnvme_ls_cr_assoc_rqst *)&lsop[1];
> 
> Therefore, any additional NUL-byte assignments (like the ones that
> strncpy() makes) are redundant.
> 
> 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>
> Similar-to: https://lore.kernel.org/all/20231018-strncpy-drivers-nvme-host-fabrics-c-v1-1-b6677df40a35@google.com/

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

-- 
Kees Cook