linux-kernel - RE: [PATCH 18/35] mm: Add guard pages around a shadow stack.

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Date:   Thu, 10 Feb 2022 22:38:51 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Dave Hansen' <dave.hansen@...el.com>,
        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>,
        Florian Weimer <fweimer@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        "joao.moreira@...el.com" <joao.moreira@...el.com>,
        John Allen <john.allen@....com>,
        "kcc@...gle.com" <kcc@...gle.com>,
        "eranian@...gle.com" <eranian@...gle.com>
CC:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: RE: [PATCH 18/35] mm: Add guard pages around a shadow stack.

From: Dave Hansen
> Sent: 09 February 2022 22:24
> 
> On 1/30/22 13:18, Rick Edgecombe wrote:
> > INCSSP(Q/D) increments shadow stack pointer and 'pops and discards' the
> > first and the last elements in the range, effectively touches those memory
> > areas.
> 
> This is a pretty close copy of the instruction reference text for
> INCSSP.  I'm feeling rather dense today, but that's just not making any
> sense.
> 
> The pseudocode is more sensible in the SDM.  I think this needs a better
> explanation:
> 
> 	The INCSSP instruction increments the shadow stack pointer.  It
> 	is the shadow stack analog of an instruction like:
> 
> 		addq	$0x80, %rsp
> 
> 	However, there is one important difference between an ADD on
> 	%rsp and INCSSP.  In addition to modifying SSP, INCSSP also
> 	reads from the memory of the first and last elements that were
> 	"popped".  You can think of it as acting like this:
> 
> 	READ_ONCE(ssp);       // read+discard top element on stack
> 	ssp += nr_to_pop * 8; // move the shadow stack
> 	READ_ONCE(ssp-8);     // read+discard last popped stack element
> 
> 
> > The maximum moving distance by INCSSPQ is 255 * 8 = 2040 bytes and
> > 255 * 4 = 1020 bytes by INCSSPD.  Both ranges are far from PAGE_SIZE.
> 
> ... That maximum distance, combined with an a guard pages at the end of
> a shadow stack ensures that INCSSP will fault before it is able to move
> across an entire guard page.
> 
> > Thus, putting a gap page on both ends of a shadow stack prevents INCSSP,
> > CALL, and RET from going beyond.

Do you need a real guard page?
Or is it just enough to ensure that the adjacent page isn't another
shadow stack page?

Any other page will cause a fault because the PTE isn't readonly+dirty.

I'm not sure how common single page allocates are in Linux.
But adjacent shadow stacks may be rare anyway.
So a check against both adjacent PTE entries would suffice.
Or maybe always allocate an even (or odd) numbered page.

	David

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