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

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Date: Wed, 3 Apr 2024 08:36:42 +0530
From: Ratheesh Kannoth <rkannoth@...vell.com>
To: Justin Stitt <justinstitt@...gle.com>
CC: Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann
	<daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii
 Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Eduard
 Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
        Yonghong Song
	<yonghong.song@...ux.dev>,
        KP Singh <kpsingh@...nel.org>, Stanislav Fomichev
	<sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers
	<ndesaulniers@...gle.com>,
        Bill Wendling <morbo@...gle.com>, <bpf@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <llvm@...ts.linux.dev>,
        <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH] bpf: replace deprecated strncpy with strscpy

On 2024-04-03 at 05:22:50, Justin Stitt (justinstitt@...gle.com) 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.
>
> bpf sym names get looked up and compared/cleaned with various string
> apis. This suggests they need to be NUL-terminated (strncpy() suggests
> this but does not guarantee it).
>
> |	static int compare_symbol_name(const char *name, char *namebuf)
> |	{
> |		cleanup_symbol_name(namebuf);
> |		return strcmp(name, namebuf);
> |	}
>
> |	static void cleanup_symbol_name(char *s)
> |	{
> |		...
> |		res = strstr(s, ".llvm.");
> |		...
> |	}
>
> Use strscpy() as this method guarantees NUL-termination on the
> destination buffer.
>
> This patch also replaces two uses of strncpy() used in log.c. These are
> simple replacements as postfix has been zero-initialized on the stack
> and has source arguments with a size less than the destination's size.
>
> Note that this patch uses the new 2-argument version of strscpy
> introduced in Commit e6584c3964f2f ("string: Allow 2-argument
> strscpy()").
>
> 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>
> ---
> Note: build-tested only.
>
> Found with: $ rg "strncpy\("
> ---
>  kernel/bpf/core.c | 4 ++--
>  kernel/bpf/log.c  | 4 ++--
>  2 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 696bc55de8e8..8c9078f4549c 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -747,7 +747,7 @@ const char *__bpf_address_lookup(unsigned long addr, unsigned long *size,
>  		unsigned long symbol_start = ksym->start;
>  		unsigned long symbol_end = ksym->end;
>
> -		strncpy(sym, ksym->name, KSYM_NAME_LEN);
> +		strscpy(sym, ksym->name, KSYM_NAME_LEN);
You dont have to check return value of strscpy for errors ?

>
>  		ret = sym;
>  		if (size)
> @@ -813,7 +813,7 @@ int bpf_get_kallsym(unsigned int symnum, unsigned long *value, char *type,
>  		if (it++ != symnum)
>  			continue;
>
> -		strncpy(sym, ksym->name, KSYM_NAME_LEN);
> +		strscpy(sym, ksym->name, KSYM_NAME_LEN);
>
>  		*value = ksym->start;
>  		*type  = BPF_SYM_ELF_TYPE;
> diff --git a/kernel/bpf/log.c b/kernel/bpf/log.c
> index 2a243cf37c60..4bd8f17a9f24 100644
> --- a/kernel/bpf/log.c
> +++ b/kernel/bpf/log.c
> @@ -467,9 +467,9 @@ const char *reg_type_str(struct bpf_verifier_env *env, enum bpf_reg_type type)
>
>  	if (type & PTR_MAYBE_NULL) {
>  		if (base_type(type) == PTR_TO_BTF_ID)
> -			strncpy(postfix, "or_null_", 16);
> +			strscpy(postfix, "or_null_");
>  		else
> -			strncpy(postfix, "_or_null", 16);
> +			strscpy(postfix, "_or_null");
>  	}
>
>  	snprintf(prefix, sizeof(prefix), "%s%s%s%s%s%s%s",
>
> ---
> base-commit: 026e680b0a08a62b1d948e5a8ca78700bfac0e6e
> change-id: 20240402-strncpy-kernel-bpf-core-c-4d8297f95e18
>
> Best regards,
> --
> Justin Stitt <justinstitt@...gle.com>
>