linux-kernel - Re: [PATCH v7 14/41] x86/mm: Introduce _PAGE_SAVED

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Message-ID: <ZACbFc55bcVYRXpG@zn.tnic>
Date:   Thu, 2 Mar 2023 13:48:21 +0100
From:   Borislav Petkov <bp@...en8.de>
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>,
        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>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        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,
        Andrew.Cooper3@...rix.com, christina.schimpe@...el.com,
        david@...hat.com, debug@...osinc.com,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v7 14/41] x86/mm: Introduce _PAGE_SAVED_DIRTY

On Mon, Feb 27, 2023 at 02:29:30PM -0800, Rick Edgecombe wrote:
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index 0646ad00178b..56b374d1bffb 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -21,7 +21,8 @@
>  #define _PAGE_BIT_SOFTW2	10	/* " */
>  #define _PAGE_BIT_SOFTW3	11	/* " */
>  #define _PAGE_BIT_PAT_LARGE	12	/* On 2MB or 1GB pages */
> -#define _PAGE_BIT_SOFTW4	58	/* available for programmer */
> +#define _PAGE_BIT_SOFTW4	57	/* available for programmer */
> +#define _PAGE_BIT_SOFTW5	58	/* available for programmer */
>  #define _PAGE_BIT_PKEY_BIT0	59	/* Protection Keys, bit 1/4 */
>  #define _PAGE_BIT_PKEY_BIT1	60	/* Protection Keys, bit 2/4 */
>  #define _PAGE_BIT_PKEY_BIT2	61	/* Protection Keys, bit 3/4 */
> @@ -34,6 +35,15 @@
>  #define _PAGE_BIT_SOFT_DIRTY	_PAGE_BIT_SOFTW3 /* software dirty tracking */
>  #define _PAGE_BIT_DEVMAP	_PAGE_BIT_SOFTW4
>  
> +/*
> + * Indicates a Saved Dirty bit page.
> + */
> +#ifdef CONFIG_X86_USER_SHADOW_STACK
> +#define _PAGE_BIT_SAVED_DIRTY		_PAGE_BIT_SOFTW5 /* Saved Dirty bit */
> +#else
> +#define _PAGE_BIT_SAVED_DIRTY		0
> +#endif
> +
>  /* If _PAGE_BIT_PRESENT is clear, we use these: */
>  /* - if the user mapped it with PROT_NONE; pte_present gives true */
>  #define _PAGE_BIT_PROTNONE	_PAGE_BIT_GLOBAL
> @@ -117,6 +127,25 @@
>  #define _PAGE_SOFTW4	(_AT(pteval_t, 0))
>  #endif
>  
> +/*
> + * The hardware requires shadow stack to be Write=0,Dirty=1. However,
> + * there are valid cases where the kernel might create read-only PTEs that
> + * are dirty (e.g., fork(), mprotect(), uffd-wp(), soft-dirty  tracking). In
> + * this case, the _PAGE_SAVED_DIRTY bit is used instead of the HW-dirty bit,
> + * to avoid creating a wrong "shadow stack" PTEs. Such PTEs have
> + * (Write=0,SavedDirty=1,Dirty=0) set.
> + *
> + * Note that on processors without shadow stack support, the 

.git/rebase-apply/patch:154: trailing whitespace.
 * Note that on processors without shadow stack support, the 
warning: 1 line adds whitespace errors.

Hm, apparently git checks for that too - not only trailing empty lines.

-- 
Regards/Gruss,
    Boris.

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