linux-hardening - Re: [PATCH v2] soc: qcom: cmd-db: replace deprecated strncpy with strtomem

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Message-ID: <ag7oslzfcuiyuehxq5n5jth4adrhel4xafby5whxsvyybbm6dc@6rtwaolbmm45>
Date: Mon, 18 Mar 2024 22:37:49 -0500
From: Bjorn Andersson <andersson@...nel.org>
To: Justin Stitt <justinstitt@...gle.com>
Cc: Konrad Dybcio <konrad.dybcio@...aro.org>, 
	linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] soc: qcom: cmd-db: replace deprecated strncpy with
 strtomem

On Mon, Mar 18, 2024 at 10:49:23PM +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.
> 

I don't mind changing the strncpy() in this function, but I don't think
this problem description adequately describes the problem you're
solving.

If the motivation is that we want 0 users of strncpy() in the kernel,
then say so.

> @query is already marked as __nonstring and doesn't need to be
> NUL-terminated.

You're not wrong, but in the event that strlen(id) < sizeof(ent->id) the
destination should be NUL-padded - exactly one of the well known,
normally unwanted, effects of strncpy(). strtomem() does explicitly not
do this.

> Since @id is a string, we can use the self-describing
> string API strtomem().

"self-describing"?

> 
> 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>
> ---
> Changes in v2:
> - use strtomem instead of memcpy (thanks Kees)
> - Link to v1: https://lore.kernel.org/r/20240314-strncpy-drivers-soc-qcom-cmd-db-c-v1-1-70f5d5e70732@google.com
> ---
> Note: build-tested only.
> 
> Found with: $ rg "strncpy\("
> ---
>  drivers/soc/qcom/cmd-db.c | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/soc/qcom/cmd-db.c b/drivers/soc/qcom/cmd-db.c
> index a5fd68411bed..d05f35d175bd 100644
> --- a/drivers/soc/qcom/cmd-db.c
> +++ b/drivers/soc/qcom/cmd-db.c
> @@ -141,18 +141,13 @@ static int cmd_db_get_header(const char *id, const struct entry_header **eh,
>  	const struct rsc_hdr *rsc_hdr;
>  	const struct entry_header *ent;
>  	int ret, i, j;
> -	u8 query[sizeof(ent->id)] __nonstring;
> +	u8 query[sizeof(ent->id)] __nonstring = { 0 };
>  
>  	ret = cmd_db_ready();
>  	if (ret)
>  		return ret;
>  
> -	/*
> -	 * Pad out query string to same length as in DB. NOTE: the output
> -	 * query string is not necessarily '\0' terminated if it bumps up
> -	 * against the max size. That's OK and expected.
> -	 */
> -	strncpy(query, id, sizeof(query));
> +	strtomem(query, id);

query needs to be NUL-padded to sizeof(ent->id) bytes (like strncpy
does), something you recognized by adding the zero-initialization above.
But why split this requirement across two non-adjacent lines? Isn't this
what strtomem_pad() is supposed to do?

Regards,
Bjorn

>  
>  	for (i = 0; i < MAX_SLV_ID; i++) {
>  		rsc_hdr = &cmd_db_header->header[i];
> 
> ---
> base-commit: fe46a7dd189e25604716c03576d05ac8a5209743
> change-id: 20240314-strncpy-drivers-soc-qcom-cmd-db-c-284f3abaabb8
> 
> Best regards,
> --
> Justin Stitt <justinstitt@...gle.com>
>