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Date:   Thu, 19 Jan 2023 17:01:18 -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>,
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        Arnd Bergmann <arnd@arndb.de>,
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        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
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        Weijiang Yang <weijiang.yang@intel.com>,
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        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 v5 19/39] mm: Fixup places that call pte_mkwrite()
 directly
Message-ID: <202301191701.ADBD82708A@keescook>
References: <20230119212317.8324-1-rick.p.edgecombe@intel.com>
 <20230119212317.8324-20-rick.p.edgecombe@intel.com>
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On Thu, Jan 19, 2023 at 01:22:57PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@intel.com>
> 
> 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 requires some core mm changes to function
> properly.
> 
> With the introduction of shadow stack memory there are two ways a pte can
> be writable: regular writable memory and shadow stack memory.
> 
> In past patches, maybe_mkwrite() has been updated to apply pte_mkwrite()
> or pte_mkwrite_shstk() depending on the VMA flag. This covers most cases
> where a PTE is made writable. However, there are places where pte_mkwrite()
> is called directly and the logic should now also create a shadow stack PTE
> in the case of a shadow stack VMA.
> 
> - do_anonymous_page() and migrate_vma_insert_page() check VM_WRITE
>   directly and call pte_mkwrite(). Teach it about pte_mkwrite_shstk()
> 
> - When userfaultfd is creating a PTE after userspace handles the fault
>   it calls pte_mkwrite() directly. Teach it about pte_mkwrite_shstk()
> 
> To make the code cleaner, introduce is_shstk_write() which simplifies
> checking for VM_WRITE | VM_SHADOW_STACK together.
> 
> In other cases where pte_mkwrite() is called directly, the VMA will not
> be VM_SHADOW_STACK, and so shadow stack memory should not be created.
>  - In the case of pte_savedwrite(), shadow stack VMA's are excluded.
>  - In the case of the "dirty_accountable" optimization in mprotect(),
>    shadow stack VMA's won't be VM_SHARED, so it is not necessary.
> 
> 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