linux-kernel - Re: [PATCH 2/2] x86/boot: increase maximum number of avoided KASLR regions

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Date:   Tue, 5 Feb 2019 15:44:41 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Julian Stecklina <js@...en8.de>
Cc:     x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, hpa@...or.com,
        linux-kernel@...r.kernel.org, jschoenh@...zon.de,
        Julian Stecklina <jsteckli@...zon.de>,
        Dave Jiang <dave.jiang@...el.com>,
        Kees Cook <keescook@...omium.org>, Baoquan He <bhe@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/2] x86/boot: increase maximum number of avoided KASLR
 regions

On Wed, Jan 30, 2019 at 05:40:03PM +0100, Julian Stecklina wrote:
> From: Julian Stecklina <jsteckli@...zon.de>
> 
> The boot code has a limit of 4 "non-standard" regions to avoid for
> KASLR. This limit is easy to reach when supplying memmap= parameters to
> the kernel. In this case, KASLR would be disabled.
> 
> Increase the limit to avoid turning off KASLR even when the user is
> heavily manipulating the memory map.
> 
> Signed-off-by: Julian Stecklina <jsteckli@...zon.de>
> ---
>  arch/x86/boot/compressed/kaslr.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> index 5657e34..f078d60 100644
> --- a/arch/x86/boot/compressed/kaslr.c
> +++ b/arch/x86/boot/compressed/kaslr.c
> @@ -92,8 +92,8 @@ struct mem_vector {
>  	unsigned long long size;
>  };
>  
> -/* Only supporting at most 4 unusable memmap regions with kaslr */
> -#define MAX_MEMMAP_REGIONS	4
> +/* Only supporting at most this many unusable memmap regions with kaslr */
> +#define MAX_MEMMAP_REGIONS	16
>  
>  static bool memmap_too_large;
>  
> @@ -213,7 +213,7 @@ static void mem_avoid_memmap(char *str)
>  		i++;
>  	}
>  
> -	/* More than 4 memmaps, fail kaslr */
> +	/* Can't store all regions, fail kaslr */
>  	if ((i >= MAX_MEMMAP_REGIONS) && str)
>  		memmap_too_large = true;
>  }
> -- 

Lemme add some of the folks from
f28442497b5caf7bf573ade22a7f8d3559e3ef56 to Cc.

It all looks arbitrary to me: first 4 unusable memmap regions, this
patch raises it to 16. Why are we even imposing such a limit?

Hmm.

-- 
Regards/Gruss,
    Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.