linux-kernel - Re: [PATCH v3 20/37] mm/mprotect: Exclude shadow stack from preserve

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Message-ID: <Y3OAnl9nZ4CHvhVr@hirez.programming.kicks-ass.net>
Date:   Tue, 15 Nov 2022 13:05:50 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rick Edgecombe <rick.p.edgecombe@...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@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.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@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Allen <john.allen@....com>, kcc@...gle.com,
        eranian@...gle.com, rppt@...nel.org, jamorris@...ux.microsoft.com,
        dethoma@...rosoft.com, akpm@...ux-foundation.org,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v3 20/37] mm/mprotect: Exclude shadow stack from
 preserve_write

On Fri, Nov 04, 2022 at 03:35:47PM -0700, Rick Edgecombe wrote:
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 73b9b78f8cf4..7643a4db1b50 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1803,6 +1803,13 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>  		return 0;
>  
>  	preserve_write = prot_numa && pmd_write(*pmd);
> +
> +	/*
> +	 * Preserve only normal writable huge PMD, but not shadow
> +	 * stack (RW=0, Dirty=1).
> +	 */
> +	if (vma->vm_flags & VM_SHADOW_STACK)
> +		preserve_write = false;
>  	ret = 1;
>  
>  #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 668bfaa6ed2a..ea82ce5f38fe 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -115,6 +115,13 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
>  			pte_t ptent;
>  			bool preserve_write = prot_numa && pte_write(oldpte);
>  
> +			/*
> +			 * Preserve only normal writable PTE, but not shadow
> +			 * stack (RW=0, Dirty=1).
> +			 */
> +			if (vma->vm_flags & VM_SHADOW_STACK)
> +				preserve_write = false;
> +
>  			/*
>  			 * Avoid trapping faults against the zero or KSM
>  			 * pages. See similar comment in change_huge_pmd.

These comments lack a why component; someone is going to wonder wtf this
code is doing in the near future -- that someone might be you.