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Subject: Re: [PATCH v19 08/25] x86/mm: Introduce _PAGE_COW
To:     Kees Cook <keescook@chromium.org>,
        Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc:     x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-mm@kvack.org,
        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@kernel.org>,
        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
        "H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Weijiang Yang <weijiang.yang@intel.com>,
        Pengfei Xu <pengfei.xu@intel.com>
References: <20210203225547.32221-1-yu-cheng.yu@intel.com>
 <20210203225547.32221-9-yu-cheng.yu@intel.com>
 <202102041215.B54FCA552F@keescook>
From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <2e43bf0b-e1a9-99f6-8d5d-d6e6886b4217@intel.com>
Date:   Thu, 4 Feb 2021 12:27:52 -0800
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On 2/4/21 12:19 PM, Kees Cook wrote:
>> (e) A page where the processor observed a Write=1 PTE, started a write, set
>>     Dirty=1, but then observed a Write=0 PTE.  That's possible today, but
>>     will not happen on processors that support shadow stack.
> What happens for "e" with/without CET? It sounds like direct writes to
> such pages will be (correctly) rejected by the MMU?

A page fault would be generated regardless of CET support.

If CET were not around, the fault would be reported as a present, write
fault.

If this happened and CET were around (which shouldn't happen in
practice, it means we have a hardware issue) a page fault exception is
generated.  Yu-cheng, I'm not sure there's enough debugging around to
tell us if this happens.  Would we even notice?