linux-kernel - Re: [PATCH] x86/resctrl: Prevent possible overrun during bitmap operations

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Message-ID: <20190620134429.GD28032@zn.tnic>
Date:   Thu, 20 Jun 2019 15:44:29 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     tglx@...utronix.de, fenghua.yu@...el.com, tony.luck@...el.com,
        mingo@...hat.com, hpa@...or.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] x86/resctrl: Prevent possible overrun during bitmap
 operations

On Wed, Jun 19, 2019 at 01:27:16PM -0700, Reinette Chatre wrote:
> @@ -2494,26 +2498,19 @@ static int mkdir_mondata_all(struct kernfs_node *parent_kn,
>   */
>  static void cbm_ensure_valid(u32 *_val, struct rdt_resource *r)
>  {
> -	/*
> -	 * Convert the u32 _val to an unsigned long required by all the bit
> -	 * operations within this function. No more than 32 bits of this
> -	 * converted value can be accessed because all bit operations are
> -	 * additionally provided with cbm_len that is initialized during
> -	 * hardware enumeration using five bits from the EAX register and
> -	 * thus never can exceed 32 bits.
> -	 */
> -	unsigned long *val = (unsigned long *)_val;
> +	unsigned long val = *_val;
>  	unsigned int cbm_len = r->cache.cbm_len;
>  	unsigned long first_bit, zero_bit;

Please sort function local variables declaration in a reverse christmas
tree order:

	<type A> longest_variable_name;
	<type B> shorter_var_name;
	<type C> even_shorter;
	<type D> i;

> -	if (*val == 0)
> +	if (val == 0)

	if (!val)

>  		return;
>  
> -	first_bit = find_first_bit(val, cbm_len);
> -	zero_bit = find_next_zero_bit(val, cbm_len, first_bit);
> +	first_bit = find_first_bit(&val, cbm_len);
> +	zero_bit = find_next_zero_bit(&val, cbm_len, first_bit);
>  
>  	/* Clear any remaining bits to ensure contiguous region */
> -	bitmap_clear(val, zero_bit, cbm_len - zero_bit);
> +	bitmap_clear(&val, zero_bit, cbm_len - zero_bit);
> +	*_val = (u32)val;

... and also, that function should simply return the u32 value instead
of using @_val as an input and output var.

But that should be a separate cleanup patch anyway.

Thx.

-- 
Regards/Gruss,
    Boris.

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