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Date:   Fri, 2 Dec 2022 18:34:59 -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,
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        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
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        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
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        Nadav Amit <nadav.amit@gmail.com>,
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        Randy Dunlap <rdunlap@infradead.org>,
        Weijiang Yang <weijiang.yang@intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
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        eranian@google.com, rppt@kernel.org, jamorris@linux.microsoft.com,
        dethoma@microsoft.com, akpm@linux-foundation.org,
        Andrew.Cooper3@citrix.com, christina.schimpe@intel.com,
        Yu-cheng Yu <yu-cheng.yu@intel.com>
Subject: Re: [PATCH v4 17/39] x86/mm: Update maybe_mkwrite() for shadow stack
Message-ID: <202212021834.200AFA92BC@keescook>
References: <20221203003606.6838-1-rick.p.edgecombe@intel.com>
 <20221203003606.6838-18-rick.p.edgecombe@intel.com>
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On Fri, Dec 02, 2022 at 04:35:44PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@intel.com>
> 
> When serving a page fault, maybe_mkwrite() makes a PTE writable if there is
> a write access to it, and its vma has VM_WRITE. Shadow stack accesses to
> shadow stack vma's are also treated as write accesses by the fault handler.
> This is because setting shadow stack memory makes it writable via some
> instructions, so COW has to happen even for shadow stack reads.
> 
> So maybe_mkwrite() should continue to set VM_WRITE vma's as normally
> writable, but also set VM_WRITE|VM_SHADOW_STACK vma's as shadow stack.
> 
> Do this by adding a pte_mkwrite_shstk() and a cross-arch stub. Check for
> VM_SHADOW_STACK in maybe_mkwrite() and call pte_mkwrite_shstk()
> accordingly.
> 
> Apply the same changes to maybe_pmd_mkwrite().
> 
> Tested-by: Pengfei Xu <pengfei.xu@intel.com>
> Tested-by: John Allen <john.allen@amd.com>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>

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

-- 
Kees Cook