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(p200300cbc704110065a0c03a142af914.dip0.t-ipconnect.de. [2003:cb:c704:1100:65a0:c03a:142a:f914]) by smtp.gmail.com with ESMTPSA id b10-20020a05600c4e0a00b003db0cab0844sm10235533wmq.40.2023.01.23.01.28.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 23 Jan 2023 01:28:24 -0800 (PST) Message-ID: <634aa365-1f51-8684-24ae-3b68aba1e12a@redhat.com> Date: Mon, 23 Jan 2023 10:28:23 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.6.0 Subject: Re: [PATCH v5 10/39] x86/mm: Introduce _PAGE_COW Content-Language: en-US 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 Cc: Yu-cheng Yu References: <20230119212317.8324-1-rick.p.edgecombe@intel.com> <20230119212317.8324-11-rick.p.edgecombe@intel.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20230119212317.8324-11-rick.p.edgecombe@intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org On 19.01.23 22:22, 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_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. > Further differentiated by VMA flags, these PTE bit combinations would be > set as follows for various types of memory: > > (Write=0,Cow=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 Cow=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,Cow=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_COW. No space is consumed in 32-bit kernels > because shadow stacks are not enabled there. > > Implement only the infrastructure for _PAGE_COW. Changes to start > creating _PAGE_COW PTEs will follow once other pieces are in place. > > Tested-by: Pengfei Xu > Tested-by: John Allen > Co-developed-by: Yu-cheng Yu > Signed-off-by: Yu-cheng Yu > Signed-off-by: Rick Edgecombe > --- > > 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) > > v2: > - Update commit log with comments (Dave Hansen) > - Add comments in code to explain pte modification code better (Dave) > - Clarify info on the meaning of various Write,Cow,Dirty combinations > > arch/x86/include/asm/pgtable.h | 78 ++++++++++++++++++++++++++++ > arch/x86/include/asm/pgtable_types.h | 59 +++++++++++++++++++-- > arch/x86/include/asm/tlbflush.h | 3 +- > 3 files changed, 134 insertions(+), 6 deletions(-) > > diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h > index b39f16c0d507..6d2f612c04b5 100644 > --- a/arch/x86/include/asm/pgtable.h > +++ b/arch/x86/include/asm/pgtable.h > @@ -301,6 +301,44 @@ static inline pte_t pte_clear_flags(pte_t pte, pteval_t clear) > return native_make_pte(v & ~clear); > } > > +/* > + * Normally COW memory can result in Dirty=1,Write=0 PTEs. But in the case > + * of X86_FEATURE_USER_SHSTK, the software COW 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 COW memory, a software bit is used > + * _PAGE_BIT_COW. The following functions pte_mkcow() and pte_clear_cow() > + * take a PTE marked conventionally COW (Dirty=1) and transition it to the > + * shadow stack compatible version of COW (Cow=1). > + */ TBH, I find that all highly confusing. Dirty=1,Write=0 does not indicate a COW page reliably. You could have both, false negatives and false positives. False negative: fork() on a clean anon page. False positives: wrpotect() of a dirty anon page. I wonder if it really has to be that complicated: what you really want to achieve is to disallow "Dirty=1,Write=0" if it's not a shadow stack page, correct? -- Thanks, David / dhildenb