Re: [PATCH v6 00/12] implement KASLR for powerpc/fsl_booke/32

[Jason Yan]

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
>
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>
> .