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

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Message-ID: <143e5a85bc630d2bb0324114e78bedec8fbeb299.camel@buserror.net>
Date:   Tue, 27 Aug 2019 23:59:56 -0500
From:   Scott Wood <oss@...error.net>
To:     Jason Yan <yanaijie@...wei.com>, 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 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.

-Scott