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

* Andrew Cooper:

On 10/10/2022 13:33, Florian Weimer wrote:

quoted

* Andrew Cooper:

quoted

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-)

quoted

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