On 2019/8/28 12:59, Scott Wood wrote:

On Tue, 2019-08-27 at 23:05 -0500, Scott Wood wrote:

quoted

On Fri, 2019-08-09 at 18:07 +0800, Jason Yan wrote:

quoted

  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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