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Message-ID: <66b7f5ef-2554-0b2d-13d3-a3f3467d968c@intel.com> Date: Mon, 15 Feb 2021 08:49:05 -0800 From: "Yu, Yu-cheng" <yu-cheng.yu@...el.com> To: Kees Cook <keescook@...omium.org> 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 2/10/2021 12:28 PM, Yu, Yu-cheng wrote: > On 2/10/2021 11:42 AM, Kees Cook wrote: >> 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? >> > > For example, in... > > static inline int pte_write(pte_t pte) > { > if (cpu_feature_enabled(X86_FEATURE_SHSTK)) > return pte_flags(pte) & (_PAGE_RW | _PAGE_DIRTY); > else > return pte_flags(pte) & _PAGE_RW; > } > > There are four cases: > > (a) RW=1, Dirty=1 -> writable > (b) RW=1, Dirty=0 -> writable > (c) RW=0, Dirty=0 -> not writable > (d) RW=0, Dirty=1 -> shadow stack, or not-writable if !X86_FEATURE_SHSTK > > Case (d) is ture only when shadow stack is enabled, otherwise it is not > writable. With shadow stack feature, the usual dirty, copy-on-write PTE > becomes RW=0, Cow=1. > > We can get this changed universally, but all usual dirty, copy-on-write > PTEs need the Dirty/Cow swapping, always. Is that desirable? > Instead of working on _PAGE_COW directly, create three helpers: pte_make_cow(), pte_clear_cow(), pte_shstk(). The decision of swapping _PAGE_DIRTY/_PAGE_COW is now in the helpers. For example, pte_wrprotect() is now: static inline pte_t pte_wrprotect(pte_t pte) { pte = pte_clear_flags(pte, _PAGE_RW); if (pte_dirty(pte)) pte = pte_make_cow(pte); return pte; } How is that? The revise patch is the following. Yu-cheng diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h index a02c67291cfc..1a6c0784af0a 100644 --- a/arch/x86/include/asm/pgtable.h +++ b/arch/x86/include/asm/pgtable.h @@ -121,11 +121,21 @@ 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; } +static inline bool pte_shstk(pte_t pte) +{ + if (!cpu_feature_enabled(X86_FEATURE_SHSTK)) + return false; + + return (pte_flags(pte) & (_PAGE_RW | _PAGE_DIRTY)) == _PAGE_DIRTY; +} static inline u32 read_pkru(void) { @@ -160,9 +170,20 @@ 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 bool pmd_shstk(pmd_t pmd) +{ + if (!cpu_feature_enabled(X86_FEATURE_SHSTK)) + return false; + + return (pmd_flags(pmd) & (_PAGE_RW | _PAGE_DIRTY)) == _PAGE_DIRTY; } static inline int pmd_young(pmd_t pmd) @@ -170,9 +191,12 @@ 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 +206,7 @@ static inline int pud_young(pud_t pud) static inline int pte_write(pte_t pte) { - return pte_flags(pte) & _PAGE_RW; + return (pte_flags(pte) & _PAGE_RW) || pte_shstk(pte); } static inline int pte_huge(pte_t pte) @@ -314,6 +338,24 @@ static inline pte_t pte_clear_flags(pte_t pte, pteval_t clear) return native_make_pte(v & ~clear); } +static inline pte_t pte_make_cow(pte_t pte) +{ + if (!cpu_feature_enabled(X86_FEATURE_SHSTK)) + return pte; + + pte = pte_clear_flags(pte, _PAGE_DIRTY); + return pte_set_flags(pte, _PAGE_COW); +} + +static inline pte_t pte_clear_cow(pte_t pte) +{ + if (!cpu_feature_enabled(X86_FEATURE_SHSTK)) + return pte; + + pte = pte_set_flags(pte, _PAGE_DIRTY); + return pte_clear_flags(pte, _PAGE_COW); +} + #ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP static inline int pte_uffd_wp(pte_t pte) { @@ -333,7 +375,7 @@ 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) @@ -343,7 +385,16 @@ static inline pte_t pte_mkold(pte_t pte) static inline pte_t pte_wrprotect(pte_t pte) { - return pte_clear_flags(pte, _PAGE_RW); + pte = pte_clear_flags(pte, _PAGE_RW); + + /* + * 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 (pte_dirty(pte)) + pte = pte_make_cow(pte); + return pte; } static inline pte_t pte_mkexec(pte_t pte) @@ -353,7 +404,18 @@ static inline pte_t pte_mkexec(pte_t pte) static inline pte_t pte_mkdirty(pte_t pte) { - return pte_set_flags(pte, _PAGE_DIRTY | _PAGE_SOFT_DIRTY); + 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) +{ + return pte_clear_cow(pte); } static inline pte_t pte_mkyoung(pte_t pte) @@ -363,7 +425,12 @@ static inline pte_t pte_mkyoung(pte_t pte) static inline pte_t pte_mkwrite(pte_t pte) { - return pte_set_flags(pte, _PAGE_RW); + pte = pte_set_flags(pte, _PAGE_RW); + + if (pte_dirty(pte)) + pte = pte_clear_cow(pte); + + return pte; } static inline pte_t pte_mkhuge(pte_t pte) @@ -410,6 +477,24 @@ static inline pmd_t pmd_clear_flags(pmd_t pmd, pmdval_t clear) return native_make_pmd(v & ~clear); } +static inline pmd_t pmd_make_cow(pmd_t pmd) +{ + if (!cpu_feature_enabled(X86_FEATURE_SHSTK)) + return pmd; + + pmd = pmd_clear_flags(pmd, _PAGE_DIRTY); + return pmd_set_flags(pmd, _PAGE_COW); +} + +static inline pmd_t pmd_clear_cow(pmd_t pmd) +{ + if (!cpu_feature_enabled(X86_FEATURE_SHSTK)) + return pmd; + + pmd = pmd_set_flags(pmd, _PAGE_DIRTY); + return pmd_clear_flags(pmd, _PAGE_COW); +} + #ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP static inline int pmd_uffd_wp(pmd_t pmd) { @@ -434,17 +519,36 @@ 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) { - return pmd_clear_flags(pmd, _PAGE_RW); + pmd = pmd_clear_flags(pmd, _PAGE_RW); + /* + * 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 (pmd_dirty(pmd)) + pmd = pmd_make_cow(pmd); + return pmd; } static inline pmd_t pmd_mkdirty(pmd_t pmd) { - return pmd_set_flags(pmd, _PAGE_DIRTY | _PAGE_SOFT_DIRTY); + 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) +{ + return pmd_clear_cow(pmd); } static inline pmd_t pmd_mkdevmap(pmd_t pmd) @@ -464,7 +568,11 @@ static inline pmd_t pmd_mkyoung(pmd_t pmd) static inline pmd_t pmd_mkwrite(pmd_t pmd) { - return pmd_set_flags(pmd, _PAGE_RW); + pmd = pmd_set_flags(pmd, _PAGE_RW); + + if (pmd_dirty(pmd)) + pmd = pmd_clear_cow(pmd); + return pmd; } static inline pud_t pud_set_flags(pud_t pud, pudval_t set) @@ -481,6 +589,24 @@ static inline pud_t pud_clear_flags(pud_t pud, pudval_t clear) return native_make_pud(v & ~clear); } +static inline pud_t pud_make_cow(pud_t pud) +{ + if (!cpu_feature_enabled(X86_FEATURE_SHSTK)) + return pud; + + pud = pud_clear_flags(pud, _PAGE_DIRTY); + return pud_set_flags(pud, _PAGE_COW); +} + +static inline pud_t pud_clear_cow(pud_t pud) +{ + if (!cpu_feature_enabled(X86_FEATURE_SHSTK)) + return pud; + + pud = pud_set_flags(pud, _PAGE_DIRTY); + return pud_clear_flags(pud, _PAGE_COW); +} + static inline pud_t pud_mkold(pud_t pud) { return pud_clear_flags(pud, _PAGE_ACCESSED); @@ -488,17 +614,32 @@ 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) { - return pud_clear_flags(pud, _PAGE_RW); + pud = pud_clear_flags(pud, _PAGE_RW); + + /* + * 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 (pud_dirty(pud)) + pud = pud_make_cow(pud); + return pud; } 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) @@ -518,7 +659,11 @@ static inline pud_t pud_mkyoung(pud_t pud) static inline pud_t pud_mkwrite(pud_t pud) { - return pud_set_flags(pud, _PAGE_RW); + pud = pud_set_flags(pud, _PAGE_RW); + + if (pud_dirty(pud)) + pud = pud_clear_cow(pud); + return pud; } #ifdef CONFIG_HAVE_ARCH_SOFT_DIRTY @@ -1131,7 +1276,7 @@ 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; + return (pmd_flags(pmd) & _PAGE_RW) || pmd_shstk(pmd); } #define __HAVE_ARCH_PMDP_HUGE_GET_AND_CLEAR
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