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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>
        <87bkqj26zp.fsf@oldenburg.str.redhat.com>
        <6e75eb27-c16b-ccfe-08b9-856edeff51eb@citrix.com>
Date:   Mon, 10 Oct 2022 15:40:18 +0200
In-Reply-To: <6e75eb27-c16b-ccfe-08b9-856edeff51eb@citrix.com> (Andrew
        Cooper's message of "Mon, 10 Oct 2022 13:32:51 +0000")
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* Andrew Cooper:

> On 10/10/2022 13:33, Florian Weimer wrote:
>> * Andrew Cooper:
>>
>>> You don't actually need a hole to create a guard.=C2=A0 Any mapping of =
type
>>> !=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 page=
s.
>> 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.=C2=A0 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