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From: Dave Hansen <dave.hansen@intel.com>
To: Yu-cheng Yu <yu-cheng.yu@intel.com>,
	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@amacapital.net>,
	Balbir Singh <bsingharora@gmail.com>,
	Borislav Petkov <bp@alien8.de>,
	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>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
	Dave Martin <Dave.Martin@arm.com>
Subject: Re: [PATCH v8 11/27] x86/mm: Introduce _PAGE_DIRTY_SW
Date: Tue, 13 Aug 2019 16:02:22 -0700
Message-ID: <dac2d62b-9045-4767-87dd-eac12e7abafd@intel.com> (raw)
In-Reply-To: <20190813205225.12032-12-yu-cheng.yu@intel.com>

> +#if defined(CONFIG_X86_INTEL_SHADOW_STACK_USER)
> +static inline pte_t pte_move_flags(pte_t pte, pteval_t from, pteval_t to)
> +{
> +	if (pte_flags(pte) & from)
> +		pte = pte_set_flags(pte_clear_flags(pte, from), to);
> +	return pte;

Why is this conditional on the compile option and not a runtime check?

> +}
> +#else
> +static inline pte_t pte_move_flags(pte_t pte, pteval_t from, pteval_t to)
> +{
> +	return pte;
> +}
> +#endif

Why do we need this function?  It's not mentioned in the changelog or
commented.

>  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)
> @@ -322,6 +336,7 @@ static inline pte_t pte_mkold(pte_t pte)
>  
>  static inline pte_t pte_wrprotect(pte_t pte)
>  {
> +	pte = pte_move_flags(pte, _PAGE_DIRTY_HW, _PAGE_DIRTY_SW);
>  	return pte_clear_flags(pte, _PAGE_RW);
>  }

Please comment what this is doing and why.

> @@ -332,9 +347,24 @@ static inline pte_t pte_mkexec(pte_t pte)
>  
>  static inline pte_t pte_mkdirty(pte_t pte)
>  {
> +	pteval_t dirty = (!IS_ENABLED(CONFIG_X86_INTEL_SHADOW_STACK_USER) ||
> +			   pte_write(pte)) ? _PAGE_DIRTY_HW:_PAGE_DIRTY_SW;

This is *really* hard for me to read and parse.  How about:

	pte_t dirty = _PAGE_DIRTY_HW;

	/*
	 * When Shadow Stacks are enabled, read-only PTEs can
	 * not have the hardware dirty bit set and must use
	 * the software bit.
	 */
	if (IS_ENABLED(CONFIG_X86_INTEL_SHADOW_STACK_USER) &&
	    !pte_write(pte))
		dirty = _PAGE_DIRTY_SW;


> +	return pte_set_flags(pte, dirty | _PAGE_SOFT_DIRTY);
> +}
> +
> +#ifdef CONFIG_ARCH_HAS_SHSTK
> +static inline pte_t pte_mkdirty_shstk(pte_t pte)
> +{
> +	pte = pte_clear_flags(pte, _PAGE_DIRTY_SW);
>  	return pte_set_flags(pte, _PAGE_DIRTY_HW | _PAGE_SOFT_DIRTY);
>  }

Why does the _PAGE_DIRTY_SW *HAVE* to be cleared on shstk pages?

> +static inline bool pte_dirty_hw(pte_t pte)
> +{
> +	return pte_flags(pte) & _PAGE_DIRTY_HW;
> +}
> +#endif

Why are these #ifdef'd?

>  static inline pte_t pte_mkyoung(pte_t pte)
>  {
>  	return pte_set_flags(pte, _PAGE_ACCESSED);
> @@ -342,6 +372,7 @@ static inline pte_t pte_mkyoung(pte_t pte)
>  
>  static inline pte_t pte_mkwrite(pte_t pte)
>  {
> +	pte = pte_move_flags(pte, _PAGE_DIRTY_SW, _PAGE_DIRTY_HW);
>  	return pte_set_flags(pte, _PAGE_RW);
>  }

It also isn't clear to me why this *must* move bits here.  Its doubly
unclear why you would need to do this on systems when shadow stacks are
compiled in but disabled.

<snip>

Same comments for pmds and puds.

> -
>  /* mprotect needs to preserve PAT bits when updating vm_page_prot */
>  #define pgprot_modify pgprot_modify
>  static inline pgprot_t pgprot_modify(pgprot_t oldprot, pgprot_t newprot)
> @@ -1178,6 +1254,19 @@ static inline int pmd_write(pmd_t pmd)
>  	return pmd_flags(pmd) & _PAGE_RW;
>  }
>  
> +static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot)
> +{
> +	pmdval_t val = pmd_val(pmd), oldval = val;
> +
> +	val &= _HPAGE_CHG_MASK;
> +	val |= check_pgprot(newprot) & ~_HPAGE_CHG_MASK;
> +	val = flip_protnone_guard(oldval, val, PHYSICAL_PMD_PAGE_MASK);
> +	if ((pmd_write(pmd) && !(pgprot_val(newprot) & _PAGE_RW)))
> +		return pmd_move_flags(__pmd(val), _PAGE_DIRTY_HW,
> +				      _PAGE_DIRTY_SW);
> +	return __pmd(val);
> +}

Why was this function moved?  This makes it really hard to review what
you changed

I'm going to stop reading this code now.  It needs a lot more care and
feeding to make it reviewable.  Please go back, double-check your
changelogs and flesh them out, then please try to make the code more
readable and understandable by commenting it.

Please take all of the compile-time checks and ask yourself whether they
need to be or *can* be runtime checks.  Consider what the overhead is of
non-shadowstack systems running shadowstack-required code.

Please also reconcile the supervisor XSAVE portion of your patches with
the ones that Fenghua has been sending around.  I've given quite a bit
of feedback to improve those.  Please consolidate and agree on a common
set of patches with him.


  reply index

Thread overview: 53+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-13 20:51 [PATCH v8 00/27] Control-flow Enforcement: Shadow Stack Yu-cheng Yu
2019-08-13 20:51 ` [PATCH v8 01/27] Documentation/x86: Add CET description Yu-cheng Yu
2019-08-14  8:07   ` Florian Weimer
2019-08-14 15:57     ` Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 02/27] x86/cpufeatures: Add CET CPU feature flags for Control-flow Enforcement Technology (CET) Yu-cheng Yu
2019-08-21 10:20   ` Borislav Petkov
2019-08-21 14:46     ` Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 03/27] x86/fpu/xstate: Change names to separate XSAVES system and user states Yu-cheng Yu
2019-08-21 14:45   ` Borislav Petkov
2019-08-13 20:52 ` [PATCH v8 04/27] x86/fpu/xstate: Introduce XSAVES system states Yu-cheng Yu
2019-08-16 19:56   ` Thomas Gleixner
2019-08-16 19:59     ` Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 05/27] x86/fpu/xstate: Introduce CET MSR " Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 06/27] x86/cet: Add control protection exception handler Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 07/27] x86/cet/shstk: Add Kconfig option for user-mode shadow stack Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 08/27] mm: Introduce VM_SHSTK for shadow stack memory Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 09/27] mm/mmap: Prevent Shadow Stack VMA merges Yu-cheng Yu
2019-08-13 22:34   ` Dave Hansen
2019-08-14 16:20     ` Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 10/27] x86/mm: Change _PAGE_DIRTY to _PAGE_DIRTY_HW Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 11/27] x86/mm: Introduce _PAGE_DIRTY_SW Yu-cheng Yu
2019-08-13 23:02   ` Dave Hansen [this message]
2019-08-13 23:49     ` Andy Lutomirski
2019-08-14 16:42     ` Yu-cheng Yu
2019-08-14 16:58       ` Dave Hansen
2019-08-23 14:02   ` Peter Zijlstra
2019-08-27 22:37     ` Yu-cheng Yu
2019-08-28  7:03       ` Peter Zijlstra
2019-08-28 14:57         ` Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 12/27] drm/i915/gvt: Update _PAGE_DIRTY to _PAGE_DIRTY_BITS Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 13/27] x86/mm: Modify ptep_set_wrprotect and pmdp_set_wrprotect for _PAGE_DIRTY_SW Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 14/27] x86/mm: Shadow stack page fault error checking Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 15/27] mm: Handle shadow stack page fault Yu-cheng Yu
2019-08-13 22:55   ` Andy Lutomirski
2019-08-14 16:27     ` Yu-cheng Yu
2019-08-14 16:48       ` Dave Hansen
2019-08-14 17:00         ` Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 16/27] mm: Handle THP/HugeTLB " Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 17/27] mm: Update can_follow_write_pte/pmd for shadow stack Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 18/27] mm: Introduce do_mmap_locked() Yu-cheng Yu
2019-08-20  1:02   ` Sean Christopherson
2019-08-20 16:08     ` Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 19/27] x86/cet/shstk: User-mode shadow stack support Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 20/27] x86/cet/shstk: Introduce WRUSS instruction Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 21/27] x86/cet/shstk: Handle signals for shadow stack Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 22/27] binfmt_elf: Extract .note.gnu.property from an ELF file Yu-cheng Yu
2019-08-16 13:55   ` Dave Martin
2019-08-20 10:02   ` Dave Martin
2019-08-13 20:52 ` [PATCH v8 23/27] x86/cet/shstk: ELF header parsing of Shadow Stack Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 24/27] x86/cet/shstk: Handle thread shadow stack Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 25/27] mm/mmap: Add Shadow stack pages to memory accounting Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 26/27] x86/cet/shstk: Add arch_prctl functions for Shadow Stack Yu-cheng Yu
2019-08-13 20:52 ` [PATCH v8 27/27] x86/cet/shstk: Add Shadow Stack instructions to opcode map Yu-cheng Yu

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