Date:   Wed, 10 Feb 2021 11:42:35 -0800
From:   Kees Cook <keescook@...omium.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@...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>,
        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>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>, haitao.huang@...el.com
Subject: Re: [PATCH v20 08/25] x86/mm: Introduce _PAGE_COW

On Wed, Feb 10, 2021 at 09:56:46AM -0800, Yu-cheng Yu wrote:
> There is essentially no room left in the x86 hardware PTEs on some OSes
> (not Linux).  That left the hardware architects looking for a way to
> represent a new memory type (shadow stack) within the existing bits.
> They chose to repurpose a lightly-used state: Write=0, Dirty=1.
> 
> The reason it's lightly used is that Dirty=1 is normally set by hardware
> and cannot normally be set by hardware on a Write=0 PTE.  Software must
> normally be involved to create one of these PTEs, so software can simply
> opt to not create them.
> 
> In places where Linux normally creates Write=0, Dirty=1, it can use the
> software-defined _PAGE_COW in place of the hardware _PAGE_DIRTY.  In other
> words, whenever Linux needs to create Write=0, Dirty=1, it instead creates
> Write=0, Cow=1, except for shadow stack, which is Write=0, Dirty=1.  This
> clearly separates shadow stack from other data, and results in the
> following:
> 
> (a) A modified, copy-on-write (COW) page: (Write=0, Cow=1)
> (b) A R/O page that has been COW'ed: (Write=0, Cow=1)
>     The user page is in a R/O VMA, and get_user_pages() needs a writable
>     copy.  The page fault handler creates a copy of the page and sets
>     the new copy's PTE as Write=0 and Cow=1.
> (c) A shadow stack PTE: (Write=0, Dirty=1)
> (d) A shared shadow stack PTE: (Write=0, Cow=1)
>     When a shadow stack page is being shared among processes (this happens
>     at fork()), its PTE is made Dirty=0, so the next shadow stack access
>     causes a fault, and the page is duplicated and Dirty=1 is set again.
>     This is the COW equivalent for shadow stack pages, even though it's
>     copy-on-access rather than copy-on-write.
> (e) A page where the processor observed a Write=1 PTE, started a write, set
>     Dirty=1, but then observed a Write=0 PTE.  That's possible today, but
>     will not happen on processors that support shadow stack.
> 
> Define _PAGE_COW and update pte_*() helpers and apply the same changes to
> pmd and pud.

I still find this commit confusing mostly due to _PAGE_COW being 0
without CET enabled. Shouldn't this just get changed universally? Why
should this change depend on CET?

-Kees

> 
> After this, there are six free bits left in the 64-bit PTE, and no more
> free bits in the 32-bit PTE (except for PAE) and Shadow Stack is not
> implemented for the 32-bit kernel.
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
> ---
>  arch/x86/include/asm/pgtable.h       | 136 ++++++++++++++++++++++++---
>  arch/x86/include/asm/pgtable_types.h |  42 ++++++++-
>  2 files changed, 165 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index a02c67291cfc..2309fa9d412e 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -121,9 +121,12 @@ extern pmdval_t early_pmd_flags;
>   * The following only work if pte_present() is true.
>   * Undefined behaviour if not..
>   */
> -static inline int pte_dirty(pte_t pte)
> +static inline bool pte_dirty(pte_t pte)
>  {
> -	return pte_flags(pte) & _PAGE_DIRTY;
> +	/*
> +	 * A dirty PTE has Dirty=1 or Cow=1.
> +	 */
> +	return pte_flags(pte) & _PAGE_DIRTY_BITS;
>  }
>  
>  
> @@ -160,9 +163,12 @@ static inline int pte_young(pte_t pte)
>  	return pte_flags(pte) & _PAGE_ACCESSED;
>  }
>  
> -static inline int pmd_dirty(pmd_t pmd)
> +static inline bool pmd_dirty(pmd_t pmd)
>  {
> -	return pmd_flags(pmd) & _PAGE_DIRTY;
> +	/*
> +	 * A dirty PMD has Dirty=1 or Cow=1.
> +	 */
> +	return pmd_flags(pmd) & _PAGE_DIRTY_BITS;
>  }
>  
>  static inline int pmd_young(pmd_t pmd)
> @@ -170,9 +176,12 @@ static inline int pmd_young(pmd_t pmd)
>  	return pmd_flags(pmd) & _PAGE_ACCESSED;
>  }
>  
> -static inline int pud_dirty(pud_t pud)
> +static inline bool pud_dirty(pud_t pud)
>  {
> -	return pud_flags(pud) & _PAGE_DIRTY;
> +	/*
> +	 * A dirty PUD has Dirty=1 or Cow=1.
> +	 */
> +	return pud_flags(pud) & _PAGE_DIRTY_BITS;
>  }
>  
>  static inline int pud_young(pud_t pud)
> @@ -182,7 +191,15 @@ static inline int pud_young(pud_t pud)
>  
>  static inline int pte_write(pte_t pte)
>  {
> -	return pte_flags(pte) & _PAGE_RW;
> +	/*
> +	 * When shadow stack is enabled, a directly writable PTE has RW=1.
> +	 * A shadow stack PTE is logically writable and has RW=0, Dirty=1.
> +	 * See comments for _PAGE_COW.
> +	 */
> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK))
> +		return pte_flags(pte) & (_PAGE_RW | _PAGE_DIRTY);
> +	else
> +		return pte_flags(pte) & _PAGE_RW;
>  }
>  
>  static inline int pte_huge(pte_t pte)
> @@ -333,7 +350,7 @@ static inline pte_t pte_clear_uffd_wp(pte_t pte)
>  
>  static inline pte_t pte_mkclean(pte_t pte)
>  {
> -	return pte_clear_flags(pte, _PAGE_DIRTY);
> +	return pte_clear_flags(pte, _PAGE_DIRTY_BITS);
>  }
>  
>  static inline pte_t pte_mkold(pte_t pte)
> @@ -343,6 +360,18 @@ static inline pte_t pte_mkold(pte_t pte)
>  
>  static inline pte_t pte_wrprotect(pte_t pte)
>  {
> +	/*
> +	 * Blindly clearing _PAGE_RW might accidentally create
> +	 * a shadow stack PTE (RW=0, Dirty=1).  Move the hardware
> +	 * dirty value to the software bit.
> +	 */
> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
> +		if (pte_flags(pte) & _PAGE_DIRTY) {
> +			pte = pte_clear_flags(pte, _PAGE_DIRTY);
> +			pte = pte_set_flags(pte, _PAGE_COW);
> +		}
> +	}
> +
>  	return pte_clear_flags(pte, _PAGE_RW);
>  }
>  
> @@ -353,6 +382,18 @@ static inline pte_t pte_mkexec(pte_t pte)
>  
>  static inline pte_t pte_mkdirty(pte_t pte)
>  {
> +	pteval_t dirty = _PAGE_DIRTY;
> +
> +	/* Avoid creating (HW)Dirty=1, Write=0 PTEs */
> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK) && !pte_write(pte))
> +		dirty = _PAGE_COW;
> +
> +	return pte_set_flags(pte, dirty | _PAGE_SOFT_DIRTY);
> +}
> +
> +static inline pte_t pte_mkwrite_shstk(pte_t pte)
> +{
> +	pte = pte_clear_flags(pte, _PAGE_COW);
>  	return pte_set_flags(pte, _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
>  }
>  
> @@ -363,6 +404,13 @@ static inline pte_t pte_mkyoung(pte_t pte)
>  
>  static inline pte_t pte_mkwrite(pte_t pte)
>  {
> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
> +		if (pte_flags(pte) & _PAGE_COW) {
> +			pte = pte_clear_flags(pte, _PAGE_COW);
> +			pte = pte_set_flags(pte, _PAGE_DIRTY);
> +		}
> +	}
> +
>  	return pte_set_flags(pte, _PAGE_RW);
>  }
>  
> @@ -434,16 +482,40 @@ static inline pmd_t pmd_mkold(pmd_t pmd)
>  
>  static inline pmd_t pmd_mkclean(pmd_t pmd)
>  {
> -	return pmd_clear_flags(pmd, _PAGE_DIRTY);
> +	return pmd_clear_flags(pmd, _PAGE_DIRTY_BITS);
>  }
>  
>  static inline pmd_t pmd_wrprotect(pmd_t pmd)
>  {
> +	/*
> +	 * Blindly clearing _PAGE_RW might accidentally create
> +	 * a shadow stack PMD (RW=0, Dirty=1).  Move the hardware
> +	 * dirty value to the software bit.
> +	 */
> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
> +		if (pmd_flags(pmd) & _PAGE_DIRTY) {
> +			pmd = pmd_clear_flags(pmd, _PAGE_DIRTY);
> +			pmd = pmd_set_flags(pmd, _PAGE_COW);
> +		}
> +	}
> +
>  	return pmd_clear_flags(pmd, _PAGE_RW);
>  }
>  
>  static inline pmd_t pmd_mkdirty(pmd_t pmd)
>  {
> +	pmdval_t dirty = _PAGE_DIRTY;
> +
> +	/* Avoid creating (HW)Dirty=1, Write=0 PMDs */
> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK) && !(pmd_flags(pmd) & _PAGE_RW))
> +		dirty = _PAGE_COW;
> +
> +	return pmd_set_flags(pmd, dirty | _PAGE_SOFT_DIRTY);
> +}
> +
> +static inline pmd_t pmd_mkwrite_shstk(pmd_t pmd)
> +{
> +	pmd = pmd_clear_flags(pmd, _PAGE_COW);
>  	return pmd_set_flags(pmd, _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
>  }
>  
> @@ -464,6 +536,13 @@ static inline pmd_t pmd_mkyoung(pmd_t pmd)
>  
>  static inline pmd_t pmd_mkwrite(pmd_t pmd)
>  {
> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
> +		if (pmd_flags(pmd) & _PAGE_COW) {
> +			pmd = pmd_clear_flags(pmd, _PAGE_COW);
> +			pmd = pmd_set_flags(pmd, _PAGE_DIRTY);
> +		}
> +	}
> +
>  	return pmd_set_flags(pmd, _PAGE_RW);
>  }
>  
> @@ -488,17 +567,35 @@ static inline pud_t pud_mkold(pud_t pud)
>  
>  static inline pud_t pud_mkclean(pud_t pud)
>  {
> -	return pud_clear_flags(pud, _PAGE_DIRTY);
> +	return pud_clear_flags(pud, _PAGE_DIRTY_BITS);
>  }
>  
>  static inline pud_t pud_wrprotect(pud_t pud)
>  {
> +	/*
> +	 * Blindly clearing _PAGE_RW might accidentally create
> +	 * a shadow stack PUD (RW=0, Dirty=1).  Move the hardware
> +	 * dirty value to the software bit.
> +	 */
> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
> +		if (pud_flags(pud) & _PAGE_DIRTY) {
> +			pud = pud_clear_flags(pud, _PAGE_DIRTY);
> +			pud = pud_set_flags(pud, _PAGE_COW);
> +		}
> +	}
> +
>  	return pud_clear_flags(pud, _PAGE_RW);
>  }
>  
>  static inline pud_t pud_mkdirty(pud_t pud)
>  {
> -	return pud_set_flags(pud, _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
> +	pudval_t dirty = _PAGE_DIRTY;
> +
> +	/* Avoid creating (HW)Dirty=1, Write=0 PUDs */
> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK) && !(pud_flags(pud) & _PAGE_RW))
> +		dirty = _PAGE_COW;
> +
> +	return pud_set_flags(pud, dirty | _PAGE_SOFT_DIRTY);
>  }
>  
>  static inline pud_t pud_mkdevmap(pud_t pud)
> @@ -518,6 +615,13 @@ static inline pud_t pud_mkyoung(pud_t pud)
>  
>  static inline pud_t pud_mkwrite(pud_t pud)
>  {
> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
> +		if (pud_flags(pud) & _PAGE_COW) {
> +			pud = pud_clear_flags(pud, _PAGE_COW);
> +			pud = pud_set_flags(pud, _PAGE_DIRTY);
> +		}
> +	}
> +
>  	return pud_set_flags(pud, _PAGE_RW);
>  }
>  
> @@ -1131,7 +1235,15 @@ extern int pmdp_clear_flush_young(struct vm_area_struct *vma,
>  #define pmd_write pmd_write
>  static inline int pmd_write(pmd_t pmd)
>  {
> -	return pmd_flags(pmd) & _PAGE_RW;
> +	/*
> +	 * When shadow stack is enabled, a directly writable PMD has RW=1.
> +	 * A shadow stack PMD is logically writable and has RW=0, Dirty=1.
> +	 * See comments for _PAGE_COW.
> +	 */
> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK))
> +		return pmd_flags(pmd) & (_PAGE_RW | _PAGE_DIRTY);
> +	else
> +		return pmd_flags(pmd) & _PAGE_RW;
>  }
>  
>  #define __HAVE_ARCH_PMDP_HUGE_GET_AND_CLEAR
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index b8b79d618bbc..437d7ff0ae80 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -23,7 +23,8 @@
>  #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 */
> @@ -36,6 +37,15 @@
>  #define _PAGE_BIT_SOFT_DIRTY	_PAGE_BIT_SOFTW3 /* software dirty tracking */
>  #define _PAGE_BIT_DEVMAP	_PAGE_BIT_SOFTW4
>  
> +/*
> + * Indicates a copy-on-write page.
> + */
> +#ifdef CONFIG_X86_CET
> +#define _PAGE_BIT_COW		_PAGE_BIT_SOFTW5 /* copy-on-write */
> +#else
> +#define _PAGE_BIT_COW		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 +127,36 @@
>  #define _PAGE_DEVMAP	(_AT(pteval_t, 0))
>  #endif
>  
> +/*
> + * The hardware requires shadow stack to be read-only and Dirty.
> + * _PAGE_COW is a software-only bit used to separate copy-on-write PTEs
> + * from shadow stack PTEs:
> + * (a) A modified, copy-on-write (COW) page: (Write=0, Cow=1)
> + * (b) A R/O page that has been COW'ed: (Write=0, Cow=1)
> + *     The user page is in a R/O VMA, and get_user_pages() needs a
> + *     writable copy.  The page fault handler creates a copy of the page
> + *     and sets the new copy's PTE as Write=0, Cow=1.
> + * (c) A shadow stack PTE: (Write=0, Dirty=1)
> + * (d) A shared (copy-on-access) shadow stack PTE: (Write=0, Cow=1)
> + *     When a shadow stack page is being shared among processes (this
> + *     happens at fork()), its PTE is cleared of _PAGE_DIRTY, so the next
> + *     shadow stack access causes a fault, and the page is duplicated and
> + *     _PAGE_DIRTY is set again.  This is the COW equivalent for shadow
> + *     stack pages, even though it's copy-on-access rather than
> + *     copy-on-write.
> + * (e) A page where the processor observed a Write=1 PTE, started a write,
> + *     set Dirty=1, but then observed a Write=0 PTE (changed by another
> + *     thread).  That's possible today, but will not happen on processors
> + *     that support shadow stack.
> + */
> +#ifdef CONFIG_X86_CET
> +#define _PAGE_COW	(_AT(pteval_t, 1) << _PAGE_BIT_COW)
> +#else
> +#define _PAGE_COW	(_AT(pteval_t, 0))
> +#endif
> +
> +#define _PAGE_DIRTY_BITS (_PAGE_DIRTY | _PAGE_COW)
> +
>  #define _PAGE_PROTNONE	(_AT(pteval_t, 1) << _PAGE_BIT_PROTNONE)
>  
>  /*
> -- 
> 2.21.0
> 

-- 
Kees Cook

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