Re: [PATCH v2 18/39] mm: Add guard pages around a shadow stack.

[Andrew Cooper]

On 10/10/2022 13:33, Florian Weimer wrote:
> * Andrew Cooper:
>
>> You don't actually need a hole to create a guard.  Any mapping of type
>> != shstk will do.
>>
>> If you've got a load of threads, you can tightly pack stack / shstk /
>> stack / shstk with no holes, and they each act as each other guard pages.
> Can userspace read the shadow stack directly?  Writing is obviously
> blocked, but reading?

Yes - regular reads are permitted to shstk memory.

It's actually a great way to get backtraces with no extra metadata needed.

> GCC's stack-clash probing uses OR instructions, so it would be fine with
> a readable mapping.

It's `or $0, (%rsp)` which is a read/modify/write and will fault when
hitting a shstk mapping.

> POSIX does not appear to require PROT_NONE mappings
> for the stack guard region, either.  However, the
> pthread_attr_setguardsize manual page pretty clearly says that it's got
> to be unreadable and unwriteable.  Hence my question.

Hmm.  If that's what the manuals say, then fine.

But honestly, you don't get very far at all without faulting on a
read-only stack.

~Andrew