linux-kernel - Re: [PATCH v17 11/26] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE

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Message-ID: <5f8da4cc-9c5c-73f9-7426-924c77797995@intel.com>
Date:   Tue, 26 Jan 2021 08:45:06 -0800
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     Borislav Petkov <bp@...en8.de>
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>
Subject: Re: [PATCH v17 11/26] x86/mm: Update ptep_set_wrprotect() and
 pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW

On 1/26/2021 2:24 AM, Borislav Petkov wrote:
> On Mon, Jan 25, 2021 at 02:18:37PM -0800, Yu, Yu-cheng wrote:
>> For example, when a thread reads a W=1, D=0 PTE and before changing it to
>> W=0,D=0, another thread could have written to the page and the PTE is W=1,
>> D=1 now.  When try_cmpxchg() detects the difference, old_pte is read again.
> 
> None of that is mentioned in the comment above it and if anything,
> *that* is what should be explained there - not some guarantee about some
> processors which doesn't even apply here.
> 
> Also, add the fact that try_cmpxchg() will update old_pte with any
> modified bits - D=1 for example - when it fails. As Peter just explained
> to me on IRC.
> 
> Thx.
> 

Yes, I will fix it.  Thanks!