linux-kernel - Re: [PATCH v6 25/41] x86/mm: Introduce MAP

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Message-ID: <63f28a0d.170a0220.21bf7.b03c@mx.google.com>
Date:   Sun, 19 Feb 2023 12:43:56 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     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-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...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>,
        Mike Kravetz <mike.kravetz@...cle.com>,
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        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Allen <john.allen@....com>, kcc@...gle.com,
        eranian@...gle.com, rppt@...nel.org, jamorris@...ux.microsoft.com,
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        david@...hat.com, debug@...osinc.com
Subject: Re: [PATCH v6 25/41] x86/mm: Introduce MAP_ABOVE4G

On Sat, Feb 18, 2023 at 01:14:17PM -0800, Rick Edgecombe wrote:
> The x86 Control-flow Enforcement Technology (CET) feature includes a new
> type of memory called shadow stack. This shadow stack memory has some
> unusual properties, which require some core mm changes to function
> properly.
> 
> One of the properties is that the shadow stack pointer (SSP), which is a
> CPU register that points to the shadow stack like the stack pointer points
> to the stack, can't be pointing outside of the 32 bit address space when
> the CPU is executing in 32 bit mode. It is desirable to prevent executing
> in 32 bit mode when shadow stack is enabled because the kernel can't easily
> support 32 bit signals.
> 
> On x86 it is possible to transition to 32 bit mode without any special
> interaction with the kernel, by doing a "far call" to a 32 bit segment.
> So the shadow stack implementation can use this address space behavior
> as a feature, by enforcing that shadow stack memory is always crated
> outside of the 32 bit address space. This way userspace will trigger a
> general protection fault which will in turn trigger a segfault if it
> tries to transition to 32 bit mode with shadow stack enabled.
> 
> This provides a clean error generating border for the user if they try
> attempt to do 32 bit mode shadow stack, rather than leave the kernel in a
> half working state for userspace to be surprised by.
> 
> So to allow future shadow stack enabling patches to map shadow stacks
> out of the 32 bit address space, introduce MAP_ABOVE4G. The behavior
> is pretty much like MAP_32BIT, except that it has the opposite address
> range. The are a few differences though.
> 
> If both MAP_32BIT and MAP_ABOVE4G are provided, the kernel will use the
> MAP_ABOVE4G behavior. Like MAP_32BIT, MAP_ABOVE4G is ignored in a 32 bit
> syscall.

Should the interface refuse to accept both set instead?

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook