[PATCH 3/4] arm64/kasan: don't allocate extra shadow memory

On Thu, Jun 1, 2017 at 7:05 PM, Dmitry Vyukov [off-list ref] wrote:

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We used to read several bytes of the shadow memory in advance.
Therefore additional shadow memory mapped to prevent crash if
speculative load would happen near the end of the mapped shadow memory.

Now we don't have such speculative loads, so we no longer need to map
additional shadow memory.

I see that patch 1 fixed up the Linux helpers for outline
instrumentation.

Just to check, is it also true that the inline instrumentation never
performs unaligned accesses to the shadow memory?

Correct, inline instrumentation assumes that all accesses are properly aligned as it
required by C standard. I knew that the kernel violates this rule in many places,
therefore I decided to add checks for unaligned accesses in outline case.

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Inline instrumentation generally accesses only a single byte.

Sorry to be a little pedantic, but does that mean we'll never access the
additional shadow, or does that mean it's very unlikely that we will?

I'm guessing/hoping it's the former!

Outline will never access additional shadow byte: https://github.com/google/sanitizers/wiki/AddressSanitizerAlgorithm#unaligned-accesses

s/Outline/inline  of course.

I suspect that actual implementations have diverged from that
description. Trying to follow asan_expand_check_ifn in:
https://gcc.gnu.org/viewcvs/gcc/trunk/gcc/asan.c?revision=246703&view=markup
but it's not trivial.

+Yuri, maybe you know off the top of your head if asan instrumentation
in gcc ever accesses off-by-one shadow byte (i.e. 1 byte after actual
object end)?

Thinking of this more. There is at least 1 case in user-space asan
where off-by-one shadow access would lead to similar crashes: for
mmap-ed regions we don't have redzones and map shadow only for the
region itself, so any off-by-one access would lead to crashes. So I
guess we are safe here. Or at least any crash would be gcc bug.