linux-kernel - Re: [PATCH v6 00/12] implement KASLR for powerpc/fsl

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Date:   Thu, 29 Aug 2019 10:41:20 +0800
From:   Jason Yan <yanaijie@...wei.com>
To:     Scott Wood <oss@...error.net>, <mpe@...erman.id.au>,
        <linuxppc-dev@...ts.ozlabs.org>, <diana.craciun@....com>,
        <christophe.leroy@....fr>, <benh@...nel.crashing.org>,
        <paulus@...ba.org>, <npiggin@...il.com>, <keescook@...omium.org>,
        <kernel-hardening@...ts.openwall.com>
CC:     <wangkefeng.wang@...wei.com>, <linux-kernel@...r.kernel.org>,
        <jingxiangfeng@...wei.com>, <zhaohongjiang@...wei.com>,
        <thunder.leizhen@...wei.com>, <fanchengyang@...wei.com>,
        <yebin10@...wei.com>
Subject: Re: [PATCH v6 00/12] implement KASLR for powerpc/fsl_booke/32



On 2019/8/28 12:59, Scott Wood wrote:
> On Tue, 2019-08-27 at 23:05 -0500, Scott Wood wrote:
>> On Fri, 2019-08-09 at 18:07 +0800, Jason Yan wrote:
>>>   Freescale Book-E
>>> parts expect lowmem to be mapped by fixed TLB entries(TLB1). The TLB1
>>> entries are not suitable to map the kernel directly in a randomized
>>> region, so we chose to copy the kernel to a proper place and restart to
>>> relocate.
>>>
>>> Entropy is derived from the banner and timer base, which will change every
>>> build and boot. This not so much safe so additionally the bootloader may
>>> pass entropy via the /chosen/kaslr-seed node in device tree.
>>
>> How complicated would it be to directly access the HW RNG (if present) that
>> early in the boot?  It'd be nice if a U-Boot update weren't required (and
>> particularly concerning that KASLR would appear to work without a U-Boot
>> update, but without decent entropy).
> 
> OK, I see that kaslr-seed is used on some other platforms, though arm64 aborts
> KASLR if it doesn't get a seed.  I'm not sure if that's better than a loud
> warning message (or if it was a conscious choice rather than just not having
> an alternative implemented), but silently using poor entropy for something
> like this seems bad.
> 

It can still make the attacker's cost higher with not so good entropy.
The same strategy exists in X86 when X86 KASLR uses RDTSC if without
X86_FEATURE_RDRAND supported. I agree that having a warning message
looks better for reminding people in this situation.

> -Scott
> 
> 
> 
> .
>