linux-kernel - Re: [RFC PATCH v2 14/27] mm: Handle THP/HugeTLB shadow stack page fault

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Date:   Wed, 11 Jul 2018 11:10:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
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@...capital.net>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.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>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>
Subject: Re: [RFC PATCH v2 14/27] mm: Handle THP/HugeTLB shadow stack page
 fault

On Tue, Jul 10, 2018 at 03:26:26PM -0700, Yu-cheng Yu wrote:
> diff --git a/mm/memory.c b/mm/memory.c
> index a2695dbc0418..f7c46d61eaea 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4108,7 +4108,13 @@ static int __handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
>  			if (pmd_protnone(orig_pmd) && vma_is_accessible(vma))
>  				return do_huge_pmd_numa_page(&vmf, orig_pmd);
>  
> -			if (dirty && !pmd_write(orig_pmd)) {
> +			/*
> +			 * Shadow stack trans huge PMDs are copy-on-access,
> +			 * so wp_huge_pmd() on them no mater if we have a
> +			 * write fault or not.
> +			 */
> +			if (is_shstk_mapping(vma->vm_flags) ||
> +			    (dirty && !pmd_write(orig_pmd))) {
>  				ret = wp_huge_pmd(&vmf, orig_pmd);
>  				if (!(ret & VM_FAULT_FALLBACK))
>  					return ret;

Can't we do this (and the do_wp_page thing) by setting FAULT_FLAG_WRITE
in the arch fault handler on shadow stack faults?