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

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Date:   Mon, 10 Oct 2022 15:40:18 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Andrew Cooper <Andrew.Cooper3@...rix.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
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        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
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        John Allen <john.allen@....com>,
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        "dethoma@...rosoft.com" <dethoma@...rosoft.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: 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.

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