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

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Date:   Wed, 06 Feb 2019 13:50:57 +0100
From:   Julian Stecklina <jsteckli@...zon.de>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Julian Stecklina <js@...en8.de>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, hpa@...or.com,
        linux-kernel@...r.kernel.org, jschoenh@...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

Borislav Petkov <bp@...en8.de> writes:

>> @@ -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?

Because at this point, we are not in a good position to handle an
unlimited amount of regions.

As for the choice of "16", I took our usecase and multiplied it by two.
FWIW, this could be even larger.

Julian