Re: [PATCH v7 14/41] x86/mm: Introduce _PAGE_SAVED_DIRTY

From: Borislav Petkov <bp@alien8.de>
To: Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc: 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@kernel.org>,
	Balbir Singh <bsingharora@gmail.com>,
	Cyrill Gorcunov <gorcunov@gmail.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Eugene Syromiatnikov <esyr@redhat.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@chromium.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>,
	Randy Dunlap <rdunlap@infradead.org>,
	Weijiang Yang <weijiang.yang@intel.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	John Allen <john.allen@amd.com>,
	kcc@google.com, eranian@google.com, rppt@kernel.org,
	jamorris@linux.microsoft.com, dethoma@microsoft.com,
	akpm@linux-foundation.org, Andrew.Cooper3@citrix.com,
	christina.schimpe@intel.com, david@redhat.com,
	debug@rivosinc.com, Yu-cheng Yu <yu-cheng.yu@intel.com>
Subject: Re: [PATCH v7 14/41] x86/mm: Introduce _PAGE_SAVED_DIRTY
Date: Thu, 2 Mar 2023 13:48:21 +0100	[thread overview]
Message-ID: <ZACbFc55bcVYRXpG@zn.tnic> (raw)
In-Reply-To: <20230227222957.24501-15-rick.p.edgecombe@intel.com>

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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