linux-kernel - Re: [PATCH v4 18/39] mm: Fixup places that call pte

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Date:   Fri, 2 Dec 2022 18:37:26 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Allen <john.allen@....com>, kcc@...gle.com,
        eranian@...gle.com, rppt@...nel.org, jamorris@...ux.microsoft.com,
        dethoma@...rosoft.com, akpm@...ux-foundation.org,
        Andrew.Cooper3@...rix.com, christina.schimpe@...el.com,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v4 18/39] mm: Fixup places that call pte_mkwrite()
 directly

On Fri, Dec 02, 2022 at 04:35:45PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@...el.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 nessary.
> 
> Tested-by: Pengfei Xu <pengfei.xu@...el.com>
> Tested-by: John Allen <john.allen@....com>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>

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

-- 
Kees Cook