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Tue, 09 Jul 2024 20:27:36 -0700 (PDT) Received: from [172.20.2.228] ([4.28.11.157]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-1fbb6ac321asm23853985ad.220.2024.07.09.20.27.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 09 Jul 2024 20:27:36 -0700 (PDT) Message-ID: <378f23cb-362e-413a-b221-09a5352e79f2@redhat.com> Date: Wed, 10 Jul 2024 05:27:34 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v22 1/4] mm: add MAP_DROPPABLE for designating always lazily freeable mappings To: "Jason A. Donenfeld" , linux-kernel@vger.kernel.org, patches@lists.linux.dev, tglx@linutronix.de Cc: linux-crypto@vger.kernel.org, linux-api@vger.kernel.org, x86@kernel.org, Linus Torvalds , Greg Kroah-Hartman , Adhemerval Zanella Netto , Carlos O'Donell , Florian Weimer , Arnd Bergmann , Jann Horn , Christian Brauner , David Hildenbrand , linux-mm@kvack.org References: <20240709130513.98102-1-Jason@zx2c4.com> <20240709130513.98102-2-Jason@zx2c4.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: <20240709130513.98102-2-Jason@zx2c4.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-Server: rspam04 X-Rspamd-Queue-Id: 3592440008 X-Stat-Signature: d19jm61k3juiafx8jdd9nywzoe5uhspf X-HE-Tag: 1720582060-560796 X-HE-Meta: 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 m016hyE3 Kvg2EcYyn+YeADh7E0YGLZi9QrRV4PbqsHC34W6bKEOXQlPXsFD0ITasnZ+5buxk6HxBV6WP7t0H6wFvK1enMC1wiYFSzuBoZ4r8yU5RZlrOmUo6f8HIxREpuytOX0IcQibQHYXN8kF+4V3WjSe7fQX2Ao2h0hrm0zf9JAGeIbkqeyIYKvXYogXP6ojCz4os3Xh/In84s3B6VIyYieANpDqRb5LKzZDQUDJbV9gUTLfsgDTeHkQb7+BlqYQvIbUgfMzIn2J0TmricC2XtcJdURY7BjkHAnjfWhP25IPeVWd9sOLA7ZreMVB89X/RxTq+ilRHmLzT4u1M1dVoOxydkzFPY1Huf+z7a4ET66CFB+MDLmrsaAIbkAt73t+7V9I6E0d9hy2LjtLZUXXtfV9xblNNCJU3k7mipw8XE 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: List-Subscribe: List-Unsubscribe: On 09.07.24 15:05, Jason A. Donenfeld wrote: > The vDSO getrandom() implementation works with a buffer allocated with a > new system call that has certain requirements: > > - It shouldn't be written to core dumps. > * Easy: VM_DONTDUMP. > - It should be zeroed on fork. > * Easy: VM_WIPEONFORK. > > - It shouldn't be written to swap. > * Uh-oh: mlock is rlimited. > * Uh-oh: mlock isn't inherited by forks. > > It turns out that the vDSO getrandom() function has three really nice > characteristics that we can exploit to solve this problem: > > 1) Due to being wiped during fork(), the vDSO code is already robust to > having the contents of the pages it reads zeroed out midway through > the function's execution. > > 2) In the absolute worst case of whatever contingency we're coding for, > we have the option to fallback to the getrandom() syscall, and > everything is fine. > > 3) The buffers the function uses are only ever useful for a maximum of > 60 seconds -- a sort of cache, rather than a long term allocation. > > These characteristics mean that we can introduce VM_DROPPABLE, which > has the following semantics: > > a) It never is written out to swap. > b) Under memory pressure, mm can just drop the pages (so that they're > zero when read back again). > c) It is inherited by fork. > d) It doesn't count against the mlock budget, since nothing is locked. > > This is fairly simple to implement, with the one snag that we have to > use 64-bit VM_* flags, but this shouldn't be a problem, since the only > consumers will probably be 64-bit anyway. > > This way, allocations used by vDSO getrandom() can use: > > VM_DROPPABLE | VM_DONTDUMP | VM_WIPEONFORK | VM_NORESERVE > > And there will be no problem with using memory when not in use, not > wiping on fork(), coredumps, or writing out to swap. > > In order to let vDSO getrandom() use this, expose these via mmap(2) as > MAP_DROPPABLE. > > Finally, the provided self test ensures that this is working as desired. Acked-by: David Hildenbrand I'll try to think of some corner cases we might be missing. As raised, I think we could do better at naming, such as "MAP_FREEABLE" to match MADV_FREE, MAP_VOLATILE, ... but if nobody else care, I shall not care :) -- Cheers, David / dhildenb