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(p200300cbc7058300e5194218a8b55bec.dip0.t-ipconnect.de. [2003:cb:c705:8300:e519:4218:a8b5:5bec]) by smtp.gmail.com with ESMTPSA id y16-20020a056000109000b002c596e4b3dasm1636352wrw.55.2023.02.20.03.32.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 20 Feb 2023 03:32:03 -0800 (PST) Message-ID: <70681787-0d33-a9ed-7f2a-747be1490932@redhat.com> Date: Mon, 20 Feb 2023 12:32:01 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.2 To: Rick Edgecombe , x86@kernel.org, "H . Peter Anvin" , Thomas Gleixner , Ingo Molnar , 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 , Andy Lutomirski , Balbir Singh , Borislav Petkov , Cyrill Gorcunov , Dave Hansen , Eugene Syromiatnikov , Florian Weimer , "H . J . Lu" , Jann Horn , Jonathan Corbet , Kees Cook , Mike Kravetz , Nadav Amit , Oleg Nesterov , Pavel Machek , Peter Zijlstra , Randy Dunlap , Weijiang Yang , "Kirill A . Shutemov" , John Allen , 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, debug@rivosinc.com Cc: Yu-cheng Yu References: <20230218211433.26859-1-rick.p.edgecombe@intel.com> <20230218211433.26859-15-rick.p.edgecombe@intel.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH v6 14/41] x86/mm: Introduce _PAGE_SAVED_DIRTY In-Reply-To: <20230218211433.26859-15-rick.p.edgecombe@intel.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: DB8941C000A X-Rspamd-Server: rspam01 X-Stat-Signature: 8a88oix8eu9szh6f7x6osutik1fzarfg X-HE-Tag: 1676892727-793815 X-HE-Meta: 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 vVqdg7jn 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 18.02.23 22:14, Rick Edgecombe wrote: > Some OSes have a greater dependence on software available bits in PTEs than > 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. So in order to support > shadow stack memory, Linux should avoid creating memory with this PTE bit > combination unless it intends for it to be shadow stack. > > The reason it's lightly used is that Dirty=1 is normally set by HW > _before_ a write. A write with a Write=0 PTE would typically only generate > a fault, not set Dirty=1. Hardware can (rarely) both set Dirty=1 *and* > generate the fault, resulting in a Write=0,Dirty=1 PTE. Hardware which > supports shadow stacks will no longer exhibit this oddity. > > So that leaves Write=0,Dirty=1 PTEs created in software. To achieve this, > in places where Linux normally creates Write=0,Dirty=1, it can use the > software-defined _PAGE_SAVED_DIRTY in place of the hardware _PAGE_DIRTY. > In other words, whenever Linux needs to create Write=0,Dirty=1, it instead > creates Write=0,SavedDirty=1 except for shadow stack, which is > Write=0,Dirty=1. Further differentiated by VMA flags, these PTE bit > combinations would be set as follows for various types of memory: I would simplify (see below) and not repeat what the patch contains as comments already that detailed. > > (Write=0,SavedDirty=1,Dirty=0): > - A modified, copy-on-write (COW) page. Previously when a typical > anonymous writable mapping was made COW via fork(), the kernel would > mark it Write=0,Dirty=1. Now it will instead use the SavedDirty bit. > This happens in copy_present_pte(). > - A R/O page that has been COW'ed. The user page is in a R/O VMA, > and get_user_pages(FOLL_FORCE) 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 SavedDirty=1. > - A shared shadow stack PTE. 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. > > (Write=0,SavedDirty=0,Dirty=1): > - A shadow stack PTE. > - A Cow PTE created when a processor without shadow stack support set > Dirty=1. > > There are six bits left available to software in the 64-bit PTE after > consuming a bit for _PAGE_SAVED_DIRTY. No space is consumed in 32-bit > kernels because shadow stacks are not enabled there. > > Implement only the infrastructure for _PAGE_SAVED_DIRTY. Changes to start > creating _PAGE_SAVED_DIRTY PTEs will follow once other pieces are in place. > > Tested-by: Pengfei Xu > Tested-by: John Allen > Reviewed-by: Kees Cook > Co-developed-by: Yu-cheng Yu > Signed-off-by: Yu-cheng Yu > Signed-off-by: Rick Edgecombe > > --- > v6: > - Rename _PAGE_COW to _PAGE_SAVED_DIRTY (David Hildenbrand) > - Add _PAGE_SAVED_DIRTY to _PAGE_CHG_MASK > > v5: > - Fix log, comments and whitespace (Boris) > - Remove capitalization on shadow stack (Boris) > > v4: > - Teach pte_flags_need_flush() about _PAGE_COW bit > - Break apart patch for better bisectability > > v3: > - Add comment around _PAGE_TABLE in response to comment > from (Andrew Cooper) > - Check for PSE in pmd_shstk (Andrew Cooper) > - Get to the point quicker in commit log (Andrew Cooper) > - Clarify and reorder commit log for why the PTE bit examples have > multiple entries. Apply same changes for comment. (peterz) > - Fix comment that implied dirty bit for COW was a specific x86 thing > (peterz) > - Fix swapping of Write/Dirty (PeterZ) > --- > arch/x86/include/asm/pgtable.h | 79 ++++++++++++++++++++++++++++ > arch/x86/include/asm/pgtable_types.h | 65 ++++++++++++++++++++--- > arch/x86/include/asm/tlbflush.h | 3 +- > 3 files changed, 138 insertions(+), 9 deletions(-) > > diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h > index 2b423d697490..110e552eb602 100644 > --- a/arch/x86/include/asm/pgtable.h > +++ b/arch/x86/include/asm/pgtable.h > @@ -301,6 +301,45 @@ static inline pte_t pte_clear_flags(pte_t pte, pteval_t clear) > return native_make_pte(v & ~clear); > } > > +/* > + * COW and other write protection operations can result in Dirty=1,Write=0 > + * PTEs. But in the case of X86_FEATURE_USER_SHSTK, the software SavedDirty bit > + * is used, since the Dirty=1,Write=0 will result in the memory being treated as > + * shadow stack by the HW. So when creating dirty, write-protected memory, a > + * software bit is used _PAGE_BIT_SAVED_DIRTY. The following functions > + * pte_mksaveddirty() and pte_clear_saveddirty() take a conventional dirty, > + * write-protected PTE (Write=0,Dirty=1) and transition it to the shadow stack > + * compatible version. (Write=0,SavedDirty=1). > + */ > +static inline pte_t pte_mksaveddirty(pte_t pte) > +{ > + if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK)) > + return pte; > + > + pte = pte_clear_flags(pte, _PAGE_DIRTY); > + return pte_set_flags(pte, _PAGE_SAVED_DIRTY); > +} > + > +static inline pte_t pte_clear_saveddirty(pte_t pte) > +{ > + /* > + * _PAGE_SAVED_DIRTY is unnecessary on !X86_FEATURE_USER_SHSTK kernels, > + * since the HW dirty bit can be used without creating shadow stack > + * memory. See the _PAGE_SAVED_DIRTY definition for more details. > + */ > + if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK)) > + return pte; > + > + /* > + * PTE is getting copied-on-write, so it will be dirtied > + * if writable, or made shadow stack if shadow stack and > + * being copied on access. Set the dirty bit for both > + * cases. > + */ > + pte = pte_set_flags(pte, _PAGE_DIRTY); > + return pte_clear_flags(pte, _PAGE_SAVED_DIRTY); > +} > + > #ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP > static inline int pte_uffd_wp(pte_t pte) > { > @@ -420,6 +459,26 @@ static inline pmd_t pmd_clear_flags(pmd_t pmd, pmdval_t clear) > return native_make_pmd(v & ~clear); > } > > +/* See comments above pte_mksaveddirty() */ > +static inline pmd_t pmd_mksaveddirty(pmd_t pmd) > +{ > + if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK)) > + return pmd; > + > + pmd = pmd_clear_flags(pmd, _PAGE_DIRTY); > + return pmd_set_flags(pmd, _PAGE_SAVED_DIRTY); > +} > + > +/* See comments above pte_mksaveddirty() */ > +static inline pmd_t pmd_clear_saveddirty(pmd_t pmd) > +{ > + if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK)) > + return pmd; > + > + pmd = pmd_set_flags(pmd, _PAGE_DIRTY); > + return pmd_clear_flags(pmd, _PAGE_SAVED_DIRTY); > +} > + > #ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP > static inline int pmd_uffd_wp(pmd_t pmd) > { > @@ -491,6 +550,26 @@ static inline pud_t pud_clear_flags(pud_t pud, pudval_t clear) > return native_make_pud(v & ~clear); > } > > +/* See comments above pte_mksaveddirty() */ > +static inline pud_t pud_mksaveddirty(pud_t pud) > +{ > + if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK)) > + return pud; > + > + pud = pud_clear_flags(pud, _PAGE_DIRTY); > + return pud_set_flags(pud, _PAGE_SAVED_DIRTY); > +} > + > +/* See comments above pte_mksaveddirty() */ > +static inline pud_t pud_clear_saveddirty(pud_t pud) > +{ > + if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK)) > + return pud; > + > + pud = pud_set_flags(pud, _PAGE_DIRTY); > + return pud_clear_flags(pud, _PAGE_SAVED_DIRTY); > +} > + > static inline pud_t pud_mkold(pud_t pud) > { > return pud_clear_flags(pud, _PAGE_ACCESSED); > diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h > index 0646ad00178b..3b420b6c0584 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 /* copy-on-write */ Nope, not "copy-on-write" :) It's more like "dirty bit when the hw-dirty bit cannot be used". Maybe simply drop the comment. > +#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 +127,40 @@ > #define _PAGE_SOFTW4 (_AT(pteval_t, 0)) > #endif > > +/* > + * The hardware requires shadow stack to be read-only and Dirty. > + * _PAGE_SAVED_DIRTY is a software-only bit used to separate copy-on-write > + * PTEs from shadow stack PTEs: I'd suggest phrasing this differently. COW is just one scenario where this can happen. Also, I don't think that the description of "separation" is correct. Something like the following maybe? " 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 _PAGE_SAVED_DIRTY remains unused. " The I would simply drop below (which is also too COW-specific I think). > + * > + * (Write=0,SavedDirty=1,Dirty=0): > + * - A modified, copy-on-write (COW) page. Previously when a typical > + * anonymous writable mapping was made COW via fork(), the kernel would > + * mark it Write=0,Dirty=1. Now it will instead use the Cow bit. This > + * happens in copy_present_pte(). > + * - A R/O page that has been COW'ed. The user page is in a R/O VMA, > + * and get_user_pages(FOLL_FORCE) 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 SavedDirty=1. > + * - A shared shadow stack PTE. 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. > + * > + * (Write=0,SavedDirty=0,Dirty=1): > + * - A shadow stack PTE. > + * - A Cow PTE created when a processor without shadow stack support set > + * Dirty=1. > + */ -- Thanks, David / dhildenb