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Date:   Thu, 4 Feb 2021 12:22:13 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc:     x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        Dave Martin <Dave.Martin@arm.com>,
        Weijiang Yang <weijiang.yang@intel.com>,
        Pengfei Xu <pengfei.xu@intel.com>
Subject: Re: [PATCH v19 14/25] x86/mm: Update maybe_mkwrite() for shadow stack
Message-ID: <202102041222.B111A20A9@keescook>
References: <20210203225547.32221-1-yu-cheng.yu@intel.com>
 <20210203225547.32221-15-yu-cheng.yu@intel.com>
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In-Reply-To: <20210203225547.32221-15-yu-cheng.yu@intel.com>
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On Wed, Feb 03, 2021 at 02:55:36PM -0800, Yu-cheng Yu wrote:
> When serving a page fault, maybe_mkwrite() makes a PTE writable if its vma
> has VM_WRITE.
> 
> A shadow stack vma has VM_SHSTK.  Its PTEs have _PAGE_DIRTY, but not
> _PAGE_WRITE.  In fork(), _PAGE_DIRTY is cleared to effect copy-on-write,
> and in page fault, _PAGE_DIRTY is restored and the shadow stack page is
> writable again.
> 
> Update maybe_mkwrite() by introducing arch_maybe_mkwrite(), which sets
> _PAGE_DIRTY for a shadow stack PTE.
> 
> 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