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[91.12.101.1]) by smtp.gmail.com with ESMTPSA id yw9sm3241097ejb.91.2021.05.14.01.50.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 14 May 2021 01:50:57 -0700 (PDT) To: Mike Rapoport , Andrew Morton Cc: Alexander Viro , Andy Lutomirski , Arnd Bergmann , Borislav Petkov , Catalin Marinas , Christopher Lameter , Dan Williams , Dave Hansen , Elena Reshetova , "H. Peter Anvin" , Hagen Paul Pfeifer , Ingo Molnar , James Bottomley , Kees Cook , "Kirill A. Shutemov" , Matthew Wilcox , Matthew Garrett , Mark Rutland , Michal Hocko , Mike Rapoport , Michael Kerrisk , Palmer Dabbelt , Palmer Dabbelt , Paul Walmsley , Peter Zijlstra , "Rafael J. Wysocki" , Rick Edgecombe , Roman Gushchin , Shakeel Butt , Shuah Khan , Thomas Gleixner , Tycho Andersen , Will Deacon , Yury Norov , linux-api@vger.kernel.org, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-nvdimm@lists.01.org, linux-riscv@lists.infradead.org, x86@kernel.org References: <20210513184734.29317-1-rppt@kernel.org> <20210513184734.29317-6-rppt@kernel.org> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH v19 5/8] mm: introduce memfd_secret system call to create "secret" memory areas Message-ID: Date: Fri, 14 May 2021 10:50:55 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: <20210513184734.29317-6-rppt@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On 13.05.21 20:47, Mike Rapoport wrote: > From: Mike Rapoport > > Introduce "memfd_secret" system call with the ability to create > memory areas visible only in the context of the owning process and > not mapped not only to other processes but in the kernel page tables > as well. > > The secretmem feature is off by default and the user must explicitly > enable it at the boot time. > > Once secretmem is enabled, the user will be able to create a file > descriptor using the memfd_secret() system call. The memory areas > created by mmap() calls from this file descriptor will be unmapped > from the kernel direct map and they will be only mapped in the page > table of the processes that have access to the file descriptor. > > The file descriptor based memory has several advantages over the > "traditional" mm interfaces, such as mlock(), mprotect(), madvise(). > File descriptor approach allows explict and controlled sharing of the > memory s/explict/explicit/ > areas, it allows to seal the operations. Besides, file descriptor > based memory paves the way for VMMs to remove the secret memory range > from the userpace hipervisor process, for instance QEMU. Andy > Lutomirski says: s/userpace hipervisor/userspace hypervisor/ > > "Getting fd-backed memory into a guest will take some possibly major > work in the kernel, but getting vma-backed memory into a guest > without mapping it in the host user address space seems much, much > worse." > > memfd_secret() is made a dedicated system call rather than an > extention to s/extention/extension/ > memfd_create() because it's purpose is to allow the user to create > more secure memory mappings rather than to simply allow file based > access to the memory. Nowadays a new system call cost is negligible > while it is way simpler for userspace to deal with a clear-cut system > calls than with a multiplexer or an overloaded syscall. Moreover, the > initial implementation of memfd_secret() is completely distinct from > memfd_create() so there is no much sense in overloading > memfd_create() to begin with. If there will be a need for code > sharing between these implementation it can be easily achieved > without a need to adjust user visible APIs. > > The secret memory remains accessible in the process context using > uaccess primitives, but it is not exposed to the kernel otherwise; > secret memory areas are removed from the direct map and functions in > the follow_page()/get_user_page() family will refuse to return a page > that belongs to the secret memory area. > > Once there will be a use case that will require exposing secretmem to > the kernel it will be an opt-in request in the system call flags so > that user would have to decide what data can be exposed to the > kernel. Maybe spell out an example: like page migration. > > Removing of the pages from the direct map may cause its fragmentation > on architectures that use large pages to map the physical memory > which affects the system performance. However, the original Kconfig > text for CONFIG_DIRECT_GBPAGES said that gigabyte pages in the direct > map "... can improve the kernel's performance a tiny bit ..." (commit > 00d1c5e05736 ("x86: add gbpages switches")) and the recent report [1] > showed that "... although 1G mappings are a good default choice, > there is no compelling evidence that it must be the only choice". > Hence, it is sufficient to have secretmem disabled by default with > the ability of a system administrator to enable it at boot time. Maybe add a link to the Intel performance evaluation. > > Pages in the secretmem regions are unevictable and unmovable to > avoid accidental exposure of the sensitive data via swap or during > page migration. > > Since the secretmem mappings are locked in memory they cannot exceed > RLIMIT_MEMLOCK. Since these mappings are already locked independently > from mlock(), an attempt to mlock()/munlock() secretmem range would > fail and mlockall()/munlockall() will ignore secretmem mappings. Maybe add something like "similar to pages pinned by VFIO". > > However, unlike mlock()ed memory, secretmem currently behaves more > like long-term GUP: secretmem mappings are unmovable mappings > directly consumed by user space. With default limits, there is no > excessive use of secretmem and it poses no real problem in > combination with ZONE_MOVABLE/CMA, but in the future this should be > addressed to allow balanced use of large amounts of secretmem along > with ZONE_MOVABLE/CMA. > > A page that was a part of the secret memory area is cleared when it > is freed to ensure the data is not exposed to the next user of that > page. You could skip that with init_on_free (and eventually also with init_on_alloc) set to avoid double clearing. > > The following example demonstrates creation of a secret mapping > (error handling is omitted): > > fd = memfd_secret(0); ftruncate(fd, MAP_SIZE); ptr = mmap(NULL, > MAP_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); > > [1] > https://lore.kernel.org/linux-mm/213b4567-46ce-f116-9cdf-bbd0c884eb3c@linux.intel.com/ [my mail client messed up the remainder of the mail for whatever reason, will comment in a separate mail if there is anything to comment :) ] -- Thanks, David / dhildenb