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Subject: Re: [RFC PATCH v2 14/27] mm: Handle THP/HugeTLB shadow stack page
 fault
To:     Yu-cheng Yu <yu-cheng.yu@intel.com>, 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@amacapital.net>,
        Balbir Singh <bsingharora@gmail.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Florian Weimer <fweimer@redhat.com>,
        "H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromiun.org>,
        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>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>
References: <20180710222639.8241-1-yu-cheng.yu@intel.com>
 <20180710222639.8241-15-yu-cheng.yu@intel.com>
From:   Dave Hansen <dave.hansen@linux.intel.com>
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Message-ID: <adc8f0b0-e2a5-d3e9-edaf-8d5b3be6a5b6@linux.intel.com>
Date:   Fri, 20 Jul 2018 07:20:25 -0700
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On 07/10/2018 03:26 PM, Yu-cheng Yu wrote:
> @@ -1193,6 +1195,8 @@ static int do_huge_pmd_wp_page_fallback(struct vm_fault *vmf, pmd_t orig_pmd,
>  		pte_t entry;
>  		entry = mk_pte(pages[i], vma->vm_page_prot);
>  		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> +		if (is_shstk_mapping(vma->vm_flags))
> +			entry = pte_mkdirty_shstk(entry);

Peter Z was pointing out that we should get rid of all this generic code
manipulation.  We might not easily be able to do it *all*, but we can do
better than what we've got here.

Basically, if you have code outside of arch/x86 in your patch set that
refers to shadow stacks, you should consider it a bug (for now),
especially if you have to hack .c files.

For instance, in the code above, you could move the is_shstk_mapping() into:

static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
{
        if (likely(vma->vm_flags & VM_WRITE))
                pte = pte_mkwrite(pte);
	
+	pte = arch_pte_mkwrite(pte, vma);
+
        return pte;
}

... and add an arch callback that does:

static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
{
	if (!is_shstk_mapping(vma->vm_flags))
		return pte;

	WARN_ON(... pte bits incompatible with shadow stacks?);

	/* Lots of comments of course */
	entry = pte_mkdirty_shstk(entry);
}

This is just one example.  You are probably going to need a couple of
similar things.  Just remember: the bar is very high to make changes to
.c files outside of arch/x86.  You can do a _bit_ more in non-x86
headers, but you have the most freedom to patch what you want as long as
it's in arch/x86.