Message-ID: <63f28a0d.170a0220.21bf7.b03c@mx.google.com>
Date:   Sun, 19 Feb 2023 12:43:56 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc:     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>,
        Mike Kravetz <mike.kravetz@oracle.com>,
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        Peter Zijlstra <peterz@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        Weijiang Yang <weijiang.yang@intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        John Allen <john.allen@amd.com>, kcc@google.com,
        eranian@google.com, rppt@kernel.org, jamorris@linux.microsoft.com,
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        Andrew.Cooper3@citrix.com, christina.schimpe@intel.com,
        david@redhat.com, debug@rivosinc.com
Subject: Re: [PATCH v6 25/41] x86/mm: Introduce MAP_ABOVE4G
References: <20230218211433.26859-1-rick.p.edgecombe@intel.com>
 <20230218211433.26859-26-rick.p.edgecombe@intel.com>
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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@chromium.org>

-- 
Kees Cook