From: "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
To: Borislav Petkov <bp@alien8.de>
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>,
"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
Dave Martin <Dave.Martin@arm.com>,
Weijiang Yang <weijiang.yang@intel.com>,
Pengfei Xu <pengfei.xu@intel.com>
Subject: Re: [PATCH v15 08/26] x86/mm: Introduce _PAGE_COW
Date: Tue, 8 Dec 2020 10:25:15 -0800 [thread overview]
Message-ID: <218503f6-eec1-94b0-8404-6f92c55799e3@intel.com> (raw)
In-Reply-To: <20201208175014.GD27920@zn.tnic>
On 12/8/2020 9:50 AM, Borislav Petkov wrote:
> On Tue, Nov 10, 2020 at 08:21:53AM -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.
>
> It is not clear to me what the definition and semantics of that bit is.
>
> +#define _PAGE_BIT_COW _PAGE_BIT_SOFTW5 /* copy-on-write */
>
> Is it set by hw or by sw and hw uses it to know it is a shadow stack
> page, and so on.
>
> I think you should lead with its definition.
Ok.
...
>> Write=0,Dirty=1 PTEs. In places where we do create them, we need to find
>> an alternative way to represent them _without_ using the same hardware bit
>> combination. Thus, enter _PAGE_COW. This results in the following:
>>
>> (a) A modified, copy-on-write (COW) page: (R/O + _PAGE_COW)
>> (b) A R/O page that has been COW'ed: (R/O + _PAGE_COW)
>
> Both are "R/O + _PAGE_COW". Where's the difference? The dirty bit?
The PTEs are the same for both (a) and (b), but come from different routes.
>> 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 R/O and _PAGE_COW.
>> (c) A shadow stack PTE: (R/O + _PAGE_DIRTY_HW)
>
> So W=0, D=1 ?
Yes.
>> (d) A shared shadow stack PTE: (R/O + _PAGE_COW)
>> When a shadow stack page is being shared among processes (this happens
>> at fork()), its PTE is cleared of _PAGE_DIRTY_HW, so the next shadow
>> stack access causes a fault, and the page is duplicated and
>> _PAGE_DIRTY_HW 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.
>
> How does that happen? Something changed the PTE's W bit to 0 in-between?
Yes.
...
>> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
>> index b23697658b28..c88c7ccf0318 100644
>> --- a/arch/x86/include/asm/pgtable.h
>> +++ b/arch/x86/include/asm/pgtable.h
>> @@ -121,9 +121,9 @@ 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_HW;
>> + return pte_flags(pte) & _PAGE_DIRTY_BITS;
>
> Why?
>
> Does _PAGE_COW mean dirty too?
Yes. Basically [read-only & dirty] is created by software. Now the
software uses a different bit.
>> @@ -343,6 +349,17 @@ 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)) {
>> + pte.pte |= (pte.pte & _PAGE_DIRTY_HW) >>
>> + _PAGE_BIT_DIRTY_HW << _PAGE_BIT_COW;
>
> Let that line stick out. And that shifting is not grokkable at a quick
> glance, at least not to me. Simplify?
Ok.
>> 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.
>
> This whole carefully sidestepping the possiblity of creating a shadow
> stack pXd is kinda sucky...
>
>> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
>> index 7462a574fc93..5f764d8d9bae 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 +37,16 @@
>> #define _PAGE_BIT_SOFT_DIRTY _PAGE_BIT_SOFTW3 /* software dirty tracking */
>> #define _PAGE_BIT_DEVMAP _PAGE_BIT_SOFTW4
>>
>> +/*
>> + * This bit indicates a copy-on-write page, and is different from
>> + * _PAGE_BIT_SOFT_DIRTY, which tracks which pages a task writes to.
>> + */
>> +#ifdef CONFIG_X86_64
>
> CONFIG_X86_64 ? Do all x86 machines out there support CET?
>
> If anything, CONFIG_X86_CET...
Ok.
--
Yu-cheng
next prev parent reply other threads:[~2020-12-08 18:25 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-10 16:21 [PATCH v15 00/26] Control-flow Enforcement: Shadow Stack Yu-cheng Yu
2020-11-10 16:21 ` [PATCH v15 01/26] Documentation/x86: Add CET description Yu-cheng Yu
2020-11-30 18:26 ` Nick Desaulniers
2020-11-30 18:34 ` Yu, Yu-cheng
2020-11-30 19:38 ` Fāng-ruì Sòng
2020-11-30 19:47 ` Yu, Yu-cheng
2020-11-10 16:21 ` [PATCH v15 02/26] x86/cpufeatures: Add CET CPU feature flags for Control-flow Enforcement Technology (CET) Yu-cheng Yu
2020-11-10 16:21 ` [PATCH v15 03/26] x86/fpu/xstate: Introduce CET MSR XSAVES supervisor states Yu-cheng Yu
2020-11-26 11:02 ` Borislav Petkov
2020-11-30 17:45 ` [NEEDS-REVIEW] " Dave Hansen
2020-11-30 18:06 ` Yu, Yu-cheng
2020-11-30 18:12 ` Dave Hansen
2020-11-30 18:17 ` Yu, Yu-cheng
2020-11-30 23:16 ` Yu, Yu-cheng
2020-12-01 22:26 ` Dave Hansen
2020-12-01 22:35 ` Yu, Yu-cheng
2020-11-10 16:21 ` [PATCH v15 04/26] x86/cet: Add control-protection fault handler Yu-cheng Yu
2020-11-26 18:49 ` Borislav Petkov
2020-11-10 16:21 ` [PATCH v15 05/26] x86/cet/shstk: Add Kconfig option for user-mode Shadow Stack Yu-cheng Yu
2020-11-27 17:10 ` Borislav Petkov
2020-11-28 16:23 ` Yu, Yu-cheng
2020-11-30 18:15 ` Borislav Petkov
2020-11-30 22:48 ` Yu, Yu-cheng
2020-12-01 16:02 ` Borislav Petkov
2020-11-30 19:56 ` Nick Desaulniers
2020-11-30 20:30 ` Yu, Yu-cheng
2020-11-10 16:21 ` [PATCH v15 06/26] x86/mm: Change _PAGE_DIRTY to _PAGE_DIRTY_HW Yu-cheng Yu
2020-12-03 9:19 ` Borislav Petkov
2020-12-03 15:12 ` Dave Hansen
2020-12-03 15:56 ` Yu, Yu-cheng
2020-11-10 16:21 ` [PATCH v15 07/26] x86/mm: Remove _PAGE_DIRTY_HW from kernel RO pages Yu-cheng Yu
2020-12-07 16:36 ` Borislav Petkov
2020-12-07 17:11 ` Yu, Yu-cheng
2020-11-10 16:21 ` [PATCH v15 08/26] x86/mm: Introduce _PAGE_COW Yu-cheng Yu
2020-12-08 17:50 ` Borislav Petkov
2020-12-08 18:25 ` Yu, Yu-cheng [this message]
[not found] ` <20201208184727.GF27920@zn.tnic>
2020-12-08 19:24 ` Yu, Yu-cheng
2020-12-10 17:41 ` Borislav Petkov
2020-12-10 18:10 ` Yu, Yu-cheng
2020-11-10 16:21 ` [PATCH v15 09/26] drm/i915/gvt: Change _PAGE_DIRTY to _PAGE_DIRTY_BITS Yu-cheng Yu
2020-11-10 16:21 ` [PATCH v15 10/26] x86/mm: Update pte_modify for _PAGE_COW Yu-cheng Yu
2020-11-10 16:21 ` [PATCH v15 11/26] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY_HW to _PAGE_COW Yu-cheng Yu
2020-11-10 16:21 ` [PATCH v15 12/26] mm: Introduce VM_SHSTK for shadow stack memory Yu-cheng Yu
2020-11-10 16:21 ` [PATCH v15 13/26] x86/mm: Shadow Stack page fault error checking Yu-cheng Yu
2020-11-10 16:21 ` [PATCH v15 14/26] x86/mm: Update maybe_mkwrite() for shadow stack Yu-cheng Yu
2020-11-10 16:22 ` [PATCH v15 15/26] mm: Fixup places that call pte_mkwrite() directly Yu-cheng Yu
2020-11-10 16:22 ` [PATCH v15 16/26] mm: Add guard pages around a shadow stack Yu-cheng Yu
2020-11-10 16:22 ` [PATCH v15 17/26] mm/mmap: Add shadow stack pages to memory accounting Yu-cheng Yu
2020-11-10 16:22 ` [PATCH v15 18/26] mm: Update can_follow_write_pte() for shadow stack Yu-cheng Yu
2020-11-10 16:22 ` [PATCH v15 19/26] mm: Re-introduce vm_flags to do_mmap() Yu-cheng Yu
2020-11-10 16:22 ` [PATCH v15 20/26] x86/cet/shstk: User-mode shadow stack support Yu-cheng Yu
2020-11-10 16:22 ` [PATCH v15 21/26] x86/cet/shstk: Handle signals for shadow stack Yu-cheng Yu
2020-11-10 16:22 ` [PATCH v15 22/26] binfmt_elf: Define GNU_PROPERTY_X86_FEATURE_1_AND properties Yu-cheng Yu
2020-11-10 16:22 ` [PATCH v15 23/26] ELF: Introduce arch_setup_elf_property() Yu-cheng Yu
2020-11-10 16:22 ` [PATCH v15 24/26] x86/cet/shstk: Handle thread shadow stack Yu-cheng Yu
2020-11-10 16:22 ` [PATCH v15 25/26] x86/cet/shstk: Add arch_prctl functions for " Yu-cheng Yu
2020-11-10 16:22 ` [PATCH v15 26/26] mm: Introduce PROT_SHSTK " Yu-cheng Yu
2020-11-27 9:29 ` [PATCH v15 00/26] Control-flow Enforcement: Shadow Stack Balbir Singh
2020-11-28 16:31 ` Yu, Yu-cheng
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