linux-kernel - Re: [PATCH v23 15/28] x86/mm: Update maybe

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Date:   Wed, 17 Mar 2021 16:56:08 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Yu-cheng Yu <yu-cheng.yu@...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>,
        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>,
        Kees Cook <keescook@...omium.org>,
        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>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>,
        Haitao Huang <haitao.huang@...el.com>
Subject: Re: [PATCH v23 15/28] x86/mm: Update maybe_mkwrite() for shadow stack

On Tue, Mar 16, 2021 at 08:10:41AM -0700, 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,

						  to cause

> and in page fault, _PAGE_DIRTY is restored and the shadow stack page is

      in the page fault handler...

> 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@...el.com>
> Reviewed-by: Kees Cook <keescook@...omium.org>
> ---
>  arch/x86/Kconfig        |  4 ++++
>  arch/x86/mm/pgtable.c   | 18 ++++++++++++++++++
>  include/linux/mm.h      |  2 ++
>  include/linux/pgtable.h | 24 ++++++++++++++++++++++++
>  mm/huge_memory.c        |  2 ++
>  5 files changed, 50 insertions(+)

Looks straightforward to me but I guess it needs a mm person's ack.

-- 
Regards/Gruss,
    Boris.

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