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Date:   Mon, 3 Oct 2022 11:22:52 -0700
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>,
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        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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        "Ravi V . Shankar" <ravi.v.shankar@intel.com>,
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        jamorris@linux.microsoft.com, dethoma@microsoft.com,
        Yu-cheng Yu <yu-cheng.yu@intel.com>
Subject: Re: [PATCH v2 16/39] x86/mm: Update maybe_mkwrite() for shadow stack
Message-ID: <202210031122.A27CE24D39@keescook>
References: <20220929222936.14584-1-rick.p.edgecombe@intel.com>
 <20220929222936.14584-17-rick.p.edgecombe@intel.com>
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On Thu, Sep 29, 2022 at 03:29:13PM -0700, 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().
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>

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

-- 
Kees Cook