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Tue, 26 Jan 2021 09:53:19 +0000 (UTC) Received: from [10.36.114.192] (ovpn-114-192.ams2.redhat.com [10.36.114.192]) by smtp.corp.redhat.com (Postfix) with ESMTP id 024AB19D80; Tue, 26 Jan 2021 09:53:09 +0000 (UTC) Subject: Re: [PATCH v16 06/11] mm: introduce memfd_secret system call to create "secret" memory areas To: Michal Hocko , Mike Rapoport References: <20210121122723.3446-1-rppt@kernel.org> <20210121122723.3446-7-rppt@kernel.org> <20210125170122.GU827@dhcp22.suse.cz> <20210125213618.GL6332@kernel.org> <20210126071614.GX827@dhcp22.suse.cz> <20210126083311.GN6332@kernel.org> <20210126090013.GF827@dhcp22.suse.cz> <20210126092011.GP6332@kernel.org> <20210126094903.GI827@dhcp22.suse.cz> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: <23850371-a19f-51fa-d813-6e78624ee8f8@redhat.com> Date: Tue, 26 Jan 2021 10:53:08 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.5.0 MIME-Version: 1.0 In-Reply-To: <20210126094903.GI827@dhcp22.suse.cz> Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210126_045330_704910_AC1B5BCE X-CRM114-Status: GOOD ( 30.29 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Mark Rutland , Peter Zijlstra , Catalin Marinas , Dave Hansen , linux-mm@kvack.org, linux-kselftest@vger.kernel.org, "H. Peter Anvin" , Christopher Lameter , Shuah Khan , Thomas Gleixner , Elena Reshetova , linux-arch@vger.kernel.org, Tycho Andersen , linux-nvdimm@lists.01.org, Will Deacon , x86@kernel.org, Matthew Wilcox , Mike Rapoport , Ingo Molnar , Michael Kerrisk , Palmer Dabbelt , Arnd Bergmann , James Bottomley , Hagen Paul Pfeifer , Borislav Petkov , Alexander Viro , Andy Lutomirski , Paul Walmsley , "Kirill A. Shutemov" , Dan Williams , linux-arm-kernel@lists.infradead.org, linux-api@vger.kernel.org, linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org, Palmer Dabbelt , linux-fsdevel@vger.kernel.org, Shakeel Butt , Andrew Morton , Rick Edgecombe , Roman Gushchin Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 26.01.21 10:49, Michal Hocko wrote: > On Tue 26-01-21 11:20:11, Mike Rapoport wrote: >> On Tue, Jan 26, 2021 at 10:00:13AM +0100, Michal Hocko wrote: >>> On Tue 26-01-21 10:33:11, Mike Rapoport wrote: >>>> On Tue, Jan 26, 2021 at 08:16:14AM +0100, Michal Hocko wrote: >>>>> On Mon 25-01-21 23:36:18, Mike Rapoport wrote: >>>>>> On Mon, Jan 25, 2021 at 06:01:22PM +0100, Michal Hocko wrote: >>>>>>> On Thu 21-01-21 14:27:18, 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 user will 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 owning mm. >>>>>>>> >>>>>>>> The secret memory remains accessible in the process context using uaccess >>>>>>>> primitives, but it is not accessible using direct/linear map addresses. >>>>>>>> >>>>>>>> Functions in the follow_page()/get_user_page() family will refuse to return >>>>>>>> a page that belongs to the secret memory area. >>>>>>>> >>>>>>>> A page that was a part of the secret memory area is cleared when it is >>>>>>>> freed. >>>>>>>> >>>>>>>> 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); >>>>>>> >>>>>>> I do not see any access control or permission model for this feature. >>>>>>> Is this feature generally safe to anybody? >>>>>> >>>>>> The mappings obey memlock limit. Besides, this feature should be enabled >>>>>> explicitly at boot with the kernel parameter that says what is the maximal >>>>>> memory size secretmem can consume. >>>>> >>>>> Why is such a model sufficient and future proof? I mean even when it has >>>>> to be enabled by an admin it is still all or nothing approach. Mlock >>>>> limit is not really useful because it is per mm rather than per user. >>>>> >>>>> Is there any reason why this is allowed for non-privileged processes? >>>>> Maybe this has been discussed in the past but is there any reason why >>>>> this cannot be done by a special device which will allow to provide at >>>>> least some permission policy? >>>> >>>> Why this should not be allowed for non-privileged processes? This behaves >>>> similarly to mlocked memory, so I don't see a reason why secretmem should >>>> have different permissions model. >>> >>> Because appart from the reclaim aspect it fragments the direct mapping >>> IIUC. That might have an impact on all others, right? >> >> It does fragment the direct map, but first it only splits 1G pages to 2M >> pages and as was discussed several times already it's not that clear which >> page size in the direct map is the best and this is very much workload >> dependent. > > I do appreciate this has been discussed but this changelog is not > specific on any of that reasoning and I am pretty sure nobody will > remember details in few years in the future. Also some numbers would be > appropriate. > >> These are the results of the benchmarks I've run with the default direct >> mapping covered with 1G pages, with disabled 1G pages using "nogbpages" in >> the kernel command line and with the entire direct map forced to use 4K >> pages using a simple patch to arch/x86/mm/init.c. >> >> https://docs.google.com/spreadsheets/d/1tdD-cu8e93vnfGsTFxZ5YdaEfs2E1GELlvWNOGkJV2U/edit?usp=sharing > > A good start for the data I am asking above. I assume you've seen the benchmark results provided by Xing Zhengjun https://lore.kernel.org/linux-mm/213b4567-46ce-f116-9cdf-bbd0c884eb3c@linux.intel.com/ -- Thanks, David / dhildenb _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel