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

[Florian Weimer]

* 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.

Indeed, I hope shadow stacks can be used to put the discussion around
frame pointers to a rest, at least when it comes to profiling. 8-)

>> 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.

I guess we can update the manual page proactively.  It does look like a
tempting optimization.

Thanks,
Florian