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From:   Florian Weimer <fweimer@redhat.com>
To:     Andrew Cooper <Andrew.Cooper3@citrix.com>
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Subject: Re: [PATCH v2 18/39] mm: Add guard pages around a shadow stack.
References: <20220929222936.14584-1-rick.p.edgecombe@intel.com>
        <20220929222936.14584-19-rick.p.edgecombe@intel.com>
        <202210031127.C6CF796@keescook>
        <37ef8d93-8bd2-ae5e-4508-9be090231d06@citrix.com>
Date:   Mon, 10 Oct 2022 14:33:30 +0200
In-Reply-To: <37ef8d93-8bd2-ae5e-4508-9be090231d06@citrix.com> (Andrew
        Cooper's message of "Wed, 5 Oct 2022 02:30:56 +0000")
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* Andrew Cooper:

> You don't actually need a hole to create a guard.=C2=A0 Any mapping of ty=
pe
> !=3D 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?

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

Thanks,
Florian