Re: [PATCH] slab: Decouple slab_debug and no_hash_pointers

[Petr Mladek]

On Thu 2025-04-10 10:44:31, Kees Cook wrote:
> Some system owners use slab_debug=FPZ (or similar) as a hardening option,
> but do not want to be forced into having kernel addresses exposed due
> to the implicit "no_hash_pointers" boot param setting.[1]
> 
> Introduce the "hash_pointers" boot param, which defaults to "auto"
> (the current behavior), but also includes "always" (forcing on hashing
> even when "slab_debug=..." is defined), and "never". The existing
> "no_hash_pointers" boot param becomes an alias for "hash_pointers=never".
> 
> This makes it possible to boot with "slab_debug=FPZ hash_pointers=always".

The idea makes sense. But it seems that the patch did not handle
the "always" mode correctly, see below.


> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -60,6 +60,20 @@
>  bool no_hash_pointers __ro_after_init;
>  EXPORT_SYMBOL_GPL(no_hash_pointers);
>  
> +/*
> + * Hashed pointers policy selected by "hash_pointers=..." boot param
> + *
> + * `auto`   - Hashed pointers enabled unless disabled by slub_debug_enabled=true
> + * `always` - Hashed pointers enabled unconditionally
> + * `never`  - Hashed pointers disabled unconditionally
> + */
> +enum hash_pointers_policy {
> +	HASH_PTR_AUTO = 0,
> +	HASH_PTR_ALWAYS,
> +	HASH_PTR_NEVER
> +};
> +static enum hash_pointers_policy hash_pointers_mode __initdata;
> +
>  noinline
>  static unsigned long long simple_strntoull(const char *startp, char **endp, unsigned int base, size_t max_chars)
>  {
> @@ -2271,12 +2285,13 @@ char *resource_or_range(const char *fmt, char *buf, char *end, void *ptr,
>  	return resource_string(buf, end, ptr, spec, fmt);
>  }
>  
> -int __init no_hash_pointers_enable(char *str)
> +void __init hash_pointers_finalize(bool slub_debug)
>  {
> -	if (no_hash_pointers)
> -		return 0;
> +	if (hash_pointers_mode == HASH_PTR_AUTO && slub_debug)
> +		no_hash_pointers = true;
>  
> -	no_hash_pointers = true;
> +	if (!no_hash_pointers)
> +		return;
>  
>  	pr_warn("**********************************************************\n");
>  	pr_warn("**   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **\n");


The mode/policy is generic but this function is ready to be called
only once. And we might actually want to call it twice, see below.

I would suggest to use a generic names and allow to call it more
times, something like:

/**
 * hash_pointers_update() - update the decision whether to hash
 *	printed pointers
 * @auto_disable: Disable hashing in auto mode
 *
 * The function allows to disable hashing printed pointers either
 * when the global mode is HASH_PTR_NEVER or when the caller
 * wants to disable it and the mode is HASH_PTR_AUTO.
 */
void __init hash_pointers_update(bool auto_disable)
{
	bool disable_hashing = false;

	switch(hash_pointers_mode) {
	case HASH_PTR_AUTO:
		disable_hashing = auto_disable;
		break;
	case HASH_PTR_ALWAYS:
		disable_hashing = true;
		break;
	case HASH_PTR_NEVER:
		if (no_hash_pointers) {
			pr_warn("Pointers were temporary printed without hashing. Force hashing again.\n");
			no_hash_pointers = false;
		}
		break;
	default:
		pr_warn("Unknown hash_pointers mode '%d' specified; assuming auto.\n",
			hash_pointers_mode);
		disable_hashing = auto_disable;
	}

	/* Nope when no change requested. */
	if (no_hash_pointers || !disable_hashing)
		return;

	no_hash_pointers = true;

	pr_warn("**********************************************************\n");
	pr_warn("**   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **\n");
	pr_warn("**                                                      **\n");
	pr_warn("** This system shows unhashed kernel memory addresses   **\n");
	pr_warn("** via the console, logs, and other interfaces. This    **\n");
	pr_warn("** might reduce the security of your system.            **\n");
	pr_warn("**                                                      **\n");
	pr_warn("** If you see this message and you are not debugging    **\n");
	pr_warn("** the kernel, report this immediately to your system   **\n");
	pr_warn("** administrator!                                       **\n");
	pr_warn("**                                                      **\n");
	pr_warn("** Use hash_pointers=always to force this mode off      **\n");
	pr_warn("**                                                      **\n");
	pr_warn("**   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **\n");
	pr_warn("**********************************************************\n");
}

> @@ -2289,11 +2304,39 @@ int __init no_hash_pointers_enable(char *str)
>  	pr_warn("** the kernel, report this immediately to your system   **\n");
>  	pr_warn("** administrator!                                       **\n");
>  	pr_warn("**                                                      **\n");
> +	pr_warn("** Use hash_pointers=always to force this mode off      **\n");
> +	pr_warn("**                                                      **\n");
>  	pr_warn("**   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **\n");
>  	pr_warn("**********************************************************\n");
> +}
> +
> +static int __init hash_pointers_mode_parse(char *str)
> +{
> +	if (!str) {
> +		pr_warn("Hash pointers mode empty; falling back to auto.\n");
> +		hash_pointers_mode = HASH_PTR_AUTO;
> +	} else if (strncmp(str, "auto", 4) == 0)   {
> +		pr_info("Hash pointers mode set to auto.\n");
> +		hash_pointers_mode = HASH_PTR_AUTO;
> +	} else if (strncmp(str, "never", 5) == 0) {
> +		pr_info("Hash pointers mode set to never.\n");
> +		hash_pointers_mode = HASH_PTR_NEVER;
> +	} else if (strncmp(str, "always", 6) == 0) {
> +		pr_info("Hash pointers mode set to always.\n");
> +		hash_pointers_mode = HASH_PTR_ALWAYS;

This mode is not handled anywhere, see below.

> +	} else {
> +		pr_warn("Unknown hash_pointers mode '%s' specified; assuming auto.\n", str);
> +		hash_pointers_mode = HASH_PTR_AUTO;
> +	}

We might handle HASH_PTR_ALWAYS by calling:

	hash_pointers_update(false);

>  	return 0;
>  }
> +early_param("hash_pointers", hash_pointers_mode_parse);
> +
> +static int __init no_hash_pointers_enable(char *str)
> +{
> +	return hash_pointers_mode_parse("never");
> +}
>  early_param("no_hash_pointers", no_hash_pointers_enable);
>  
>  /*

Best Regards,
Petr