linux-kernel - Re: [RFC PATCH v2 16/27] mm: Modify can_follow_write

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Date:   Fri, 13 Jul 2018 11:26:46 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>, 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@...capital.net>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.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@...omiun.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>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>
Subject: Re: [RFC PATCH v2 16/27] mm: Modify can_follow_write_pte/pmd for
 shadow stack

On 07/11/2018 10:05 AM, Yu-cheng Yu wrote:
> My understanding is that we don't want to follow write pte if the page
> is shared as read-only.  For a SHSTK page, that is (R/O + DIRTY_SW),
> which means the SHSTK page has not been COW'ed.  Is that right?

Let's look at the code again:

> -static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
> +static inline bool can_follow_write_pte(pte_t pte, unsigned int flags,
> +					bool shstk)
>  {
> +	bool pte_cowed = shstk ? is_shstk_pte(pte):pte_dirty(pte);
> +
>  	return pte_write(pte) ||
> -		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
> +		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_cowed);
>  }

This is another case where the naming of pte_*() is biting us vs. the
perversion of the PTE bits.  The lack of comments and explanation inthe
patch is compounding the confusion.

We need to find a way to differentiate "someone can write to this PTE"
from "the write bit is set in this PTE".

In this particular hunk, we need to make it clear that pte_write() is
*never* true for shadowstack PTEs.  In other words, shadow stack VMAs
will (should?) never even *see* a pte_write() PTE.

I think this is a case where you just need to bite the bullet and
bifurcate can_follow_write_pte().  Just separate the shadowstack and
non-shadowstack parts.