On Wed, Feb 14, 2018 at 11:06 AM, Laura Abbott [off-list ref] wrote:

On 02/13/2018 01:43 PM, Kees Cook wrote:

quoted

On Tue, Feb 13, 2018 at 8:09 AM, Laura Abbott [off-list ref] wrote:

quoted

No, arm64 doesn't fixup the aliases, mostly because arm64 uses larger
page sizes which can't be broken down at runtime. CONFIG_PAGE_POISONING
does use 4K pages which could be adjusted at runtime. So yes, you are
right we would have physmap exposure on arm64 as well.

Errr, so that means even modules and kernel code are writable via the
arm64 physmap? That seems extraordinarily bad. :(

-Kees

(adding linux-arm-kernel and changing the subject)

Kernel code should be fine, if it isn't that is a bug that should be
fixed. Modules yes are not fully protected. The conclusion from past

I think that's a pretty serious problem: we can't have aliases with
mismatched permissions; this degrades a deterministic protection
(read-only) to a probabilistic protection (knowing where the alias of
a target is mapped). Having an attack be "needs some info leaks"
instead of "need execution control to change perms" is a much lower
bar, IMO.

experience has been that we cannot safely break down larger page sizes
at runtime like x86 does. We could theoretically
add support for fixing up the alias if PAGE_POISONING is enabled but
I don't know who would actually use that in production. Performance
is very poor at that point.

Why does using finer granularity on the physmap degrade performance? I
assume TLB pressure, but what is heavily using that area? (I must not
be understanding what physmap actually gets used for -- I thought it
was just a convenience to have a 1:1 virt/phys map for some lookups?)

-Kees


-- 
Kees Cook
Pixel Security