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Date:   Wed, 11 Jul 2018 11:29:51 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc:     x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
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        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>
Subject: Re: [RFC PATCH v2 16/27] mm: Modify can_follow_write_pte/pmd for
 shadow stack
Message-ID: <20180711092951.GW2476@hirez.programming.kicks-ass.net>
References: <20180710222639.8241-1-yu-cheng.yu@intel.com>
 <20180710222639.8241-17-yu-cheng.yu@intel.com>
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On Tue, Jul 10, 2018 at 03:26:28PM -0700, Yu-cheng Yu wrote:
> There are three possible shadow stack PTE settings:
> 
>   Normal SHSTK PTE: (R/O + DIRTY_HW)
>   SHSTK PTE COW'ed: (R/O + DIRTY_HW)
>   SHSTK PTE shared as R/O data: (R/O + DIRTY_SW)

I count _2_ distinct states there.

> Update can_follow_write_pte/pmd for the shadow stack.

So the below disallows can_follow_write when shstk && _PAGE_DIRTY_SW,
but this here Changelog doesn't explain why. Doesn't even get close.

Also, the code is a right mess :/ Can't we try harder to not let this
shadow stack stuff escape arch code.