From patchwork Thu Jan 12 17:02:32 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Yann Droneaud X-Patchwork-Id: 13098508 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7BDA0C61DB3 for ; Thu, 12 Jan 2023 17:45:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237144AbjALRo6 (ORCPT ); Thu, 12 Jan 2023 12:44:58 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60934 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231891AbjALRo2 (ORCPT ); Thu, 12 Jan 2023 12:44:28 -0500 Received: from smtp6-g21.free.fr (smtp6-g21.free.fr [IPv6:2a01:e0c:1:1599::15]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 93D6E82D05; Thu, 12 Jan 2023 09:03:14 -0800 (PST) Received: from localhost (unknown [IPv6:2a01:e35:39f2:1220:dc8b:b602:9bcd:3004]) by smtp6-g21.free.fr (Postfix) with ESMTPS id 90AC57802D7; Thu, 12 Jan 2023 18:02:54 +0100 (CET) From: Yann Droneaud To: "Jason A. Donenfeld" , "Theodore Ts'o" Cc: Yann Droneaud , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , Andy Lutomirski , Vincenzo Frascino , x86@kernel.org, linux-crypto@vger.kernel.org, linux-api@vger.kernel.org, linux-kernel@vger.kernel.org, Florian Weimer , Adhemerval Zanella Netto , "Carlos O'Donell" Subject: [RFC PATCH 0/4] random: a simple vDSO mechanism for reseeding userspace CSPRNGs Date: Thu, 12 Jan 2023 18:02:32 +0100 Message-Id: X-Mailer: git-send-email 2.37.2 MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-crypto@vger.kernel.org Hi, Here's my humble hack at improving kernel for a faster secure arc4random() userspace implementation, by allowing userspace to buffer getrandom() generated entropy, discarding it as the kernel's own CSPRNG is reseeded. It's largely built upon the vDSO work of Jason A. Donenfeld, as part of its latest patchset "[PATCH v14 0/7] implement getrandom() in vDSO" [1] but it's made simpler by making available only one of the missing tools for the userspace to properly buffer the output of getrandom(). Using MADV_WIPEONFORK and mlock(), userspace can reasonably offer forward secrecy*, until something like VM_DROPPABLE[2] is provided by the kernel, to allow for the buffer memory to never, ever be written to the disk before its used, being inherited accross fork(), and isn't limited by RLIMIT_MEMLOCK. * provided userspace can mlock() the memory and calls mlock() on buffer after fork, as memory locks are not inherited accross fork(). As it's a hack, it's far from perfect. The main drawback I see is the case where fresh entropy has to be discarded as the kernel's CSPRNG generation is updated as the result of calling getrandom() to generate the mentionned entropy. The workaround, is to limit the amount of fresh entropy fetched when a kernel's CSPRNG generation change is detected, and to increase the amount the data retrieved with getrandom() when generation doesn't change between calls. Performance wise, the improvements are here, as one can check with the test program provided: getrandom(,,GRND_TIMESTAMP) test getrandom() support GRND_TIMESTAMP found getrandom() in vDSO at 0x7ffc3efccc60 == direct syscall getrandom(), 16777216 u32, 2.866324020 s, 5.853 M u32/s, 170.846 ns/u32 == direct vDSO getrandom(), 16777216 u32, 2.883473280 s, 5.818 M u32/s, 171.868 ns/u32 == pooled syscall getrandom(), 16777216 u32, 1.152421219 s, 14.558 M u32/s, 68.690 ns/u32, (0 bytes discarded) == pooled vDSO getrandom(), 16777216 u32, 0.162477863 s, 103.258 M u32/s, 9.684 ns/u32, (0 bytes discarded) With the requirement to mlock() the memory page(s) used to buffer getrandom() output, I'm not sure userspace could afford to allocate 4KBytes per thread, before being hit by RLIMIT_MEMLOCK (or worse, OOM killer). Thus, some form of sharing between threads would be needed, which would require locking, reducing the performances shown above. Also I haven't studied the security impact of making the kernel base CSPRNG seed generation available to userspace. It can be made more opaque if needed. Regards. [1] https://lore.kernel.org/all/20230101162910.710293-1-Jason@zx2c4.com/ [2] https://lore.kernel.org/all/20230101162910.710293-3-Jason@zx2c4.com/ Jason A. Donenfeld (2): random: introduce generic vDSO getrandom(,, GRND_TIMESTAMP) fast path x86: vdso: Wire up getrandom() vDSO implementation. Yann Droneaud (2): random: introduce getrandom() GRND_TIMESTAMP testing: add a getrandom() GRND_TIMESTAMP vDSO demonstration/benchmark MAINTAINERS | 1 + arch/x86/Kconfig | 1 + arch/x86/entry/vdso/Makefile | 3 +- arch/x86/entry/vdso/vdso.lds.S | 2 + arch/x86/entry/vdso/vgetrandom.c | 17 + arch/x86/include/asm/vdso/getrandom.h | 42 +++ arch/x86/include/asm/vdso/vsyscall.h | 2 + arch/x86/include/asm/vvar.h | 16 + drivers/char/random.c | 52 ++- include/linux/random.h | 31 ++ include/uapi/linux/random.h | 2 + include/vdso/datapage.h | 9 + lib/vdso/Kconfig | 5 + lib/vdso/getrandom.c | 51 +++ tools/testing/crypto/getrandom/Makefile | 4 + .../testing/crypto/getrandom/test-getrandom.c | 307 ++++++++++++++++++ 16 files changed, 543 insertions(+), 2 deletions(-) create mode 100644 arch/x86/entry/vdso/vgetrandom.c create mode 100644 arch/x86/include/asm/vdso/getrandom.h create mode 100644 lib/vdso/getrandom.c create mode 100644 tools/testing/crypto/getrandom/Makefile create mode 100644 tools/testing/crypto/getrandom/test-getrandom.c