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From:   Andy Lutomirski <luto@kernel.org>
Date:   Thu, 10 Feb 2022 15:07:09 -0800
Message-ID: <CALCETrWmiNi2+sPKWDUjGtGWtP9XNryfFe-dG4fTQkXyqGqpzQ@mail.gmail.com>
Subject: Re: [PATCH 18/35] mm: Add guard pages around a shadow stack.
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Rick Edgecombe <rick.p.edgecombe@intel.com>, x86@kernel.org,
        "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-mm@kvack.org,
        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@kernel.org>,
        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
        "H . J . Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@intel.com>,
        Dave Martin <dave.martin@arm.com>,
        Weijiang Yang <weijiang.yang@intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
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        Yu-cheng Yu <yu-cheng.yu@intel.com>
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On Thu, Feb 10, 2022 at 2:44 PM Dave Hansen <dave.hansen@intel.com> wrote:
>
> 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.
> >
> > 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.
> > Thus, putting a gap page on both ends of a shadow stack prevents INCSSP,
> > CALL, and RET from going beyond.
>
> What is the downside of not applying this patch?  The shadow stack gap
> is 1MB instead of 4k?
>
> That, frankly, doesn't seem too bad.  How badly do we *need* this patch?

1MB of oer-thread guard address space in a 32-bit program may be a
show stopper.  Do we intend to support any of this for 32-bit?

--Andy