From patchwork Thu May 7 10:32:14 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Christian Brauner X-Patchwork-Id: 226314 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-9.7 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH, MAILING_LIST_MULTI, SIGNED_OFF_BY, SPF_HELO_NONE, SPF_PASS, URIBL_BLOCKED, USER_AGENT_GIT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8B4D3C54E49 for ; Thu, 7 May 2020 10:33:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7519320CC7 for ; Thu, 7 May 2020 10:33:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726555AbgEGKcX (ORCPT ); Thu, 7 May 2020 06:32:23 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:46313 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725939AbgEGKcW (ORCPT ); Thu, 7 May 2020 06:32:22 -0400 Received: from ip5f5af183.dynamic.kabel-deutschland.de ([95.90.241.131] helo=wittgenstein.fritz.box) by youngberry.canonical.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.86_2) (envelope-from ) id 1jWdpC-0007oF-Oi; Thu, 07 May 2020 10:32:18 +0000 From: Christian Brauner To: linux-kernel@vger.kernel.org Cc: Thomas Gleixner , Peter Zijlstra , Ingo Molnar , Eugene Syromiatnikov , Christian Kellner , Aleksa Sarai , "Dmitry V. Levin" , Arnd Bergmann , Serge Hallyn , Tejun Heo , Oleg Nesterov , Joe Perches , Christian Brauner , Jan Stancek , Andreas Schwab , Florian Weimer , libc-alpha@sourceware.org, stable@vger.kernel.org Subject: [PATCH v2] fork: prevent accidental access to clone3 features Date: Thu, 7 May 2020 12:32:14 +0200 Message-Id: <20200507103214.77218-1-christian.brauner@ubuntu.com> X-Mailer: git-send-email 2.26.2 MIME-Version: 1.0 Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org Jan reported an issue where an interaction between sign-extending clone's flag argument on ppc64le and the new CLONE_INTO_CGROUP feature causes clone() to consistently fail with EBADF. The whole story is a little longer. The legacy clone() syscall is odd in a bunch of ways and here two things interact. First, legacy clone's flag argument is word-size dependent, i.e. it's an unsigned long whereas most system calls with flag arguments use int or unsigned int. Second, legacy clone() ignores unknown and deprecated flags. The two of them taken together means that users on 64bit systems can pass garbage for the upper 32bit of the clone() syscall since forever and things would just work fine. Just try this on a 64bit kernel prior to v5.7-rc1 where this will succeed and on v5.7-rc1 where this will fail with EBADF: int main(int argc, char *argv[]) { pid_t pid; /* Note that legacy clone() has different argument ordering on * different architectures so this won't work everywhere. * * Only set the upper 32 bits. */ pid = syscall(__NR_clone, 0xffffffff00000000 | SIGCHLD, NULL, NULL, NULL, NULL); if (pid < 0) exit(EXIT_FAILURE); if (pid == 0) exit(EXIT_SUCCESS); if (wait(NULL) != pid) exit(EXIT_FAILURE); exit(EXIT_SUCCESS); } Since legacy clone() couldn't be extended this was not a problem so far and nobody really noticed or cared since nothing in the kernel ever bothered to look at the upper 32 bits. But once we introduced clone3() and expanded the flag argument in struct clone_args to 64 bit we opened this can of worms. With the first flag-based extension to clone3() making use of the upper 32 bits of the flag argument we've effectively made it possible for the legacy clone() syscall to reach clone3() only flags. The sign extension scenario is just the odd corner-case that we needed to figure this out. The reason we just realized this now and not already when we introduced CLONE_CLEAR_SIGHAND was that CLONE_INTO_CGROUP assumes that a valid cgroup file descriptor has been given. So the sign extension (or the user accidently passing garbage for the upper 32 bits) caused the CLONE_INTO_CGROUP bit to be raised and the kernel to error out when it didn't find a valid cgroup file descriptor. Let's fix this by always capping the upper 32 bits for all codepaths that are not aware of clone3() features. This ensures that we can't reach clone3() only features by accident via legacy clone as with the sign extension case and also that legacy clone() works exactly like before, i.e. ignoring any unknown flags. This solution risks no regressions and is also pretty clean. Reported-by: Jan Stancek Cc: Arnd Bergmann Cc: Dmitry V. Levin Cc: Andreas Schwab Cc: Florian Weimer Cc: libc-alpha@sourceware.org Link: https://sourceware.org/pipermail/libc-alpha/2020-May/113596.html Fixes: 7f192e3cd316 ("fork: add clone3") Fixes: ef2c41cf38a7 ("clone3: allow spawning processes into cgroups") Cc: stable@vger.kernel.org # 5.3+ Signed-off-by: Christian Brauner --- /* v2 */ - Joe Perches : - Use lower_32_bit() macro. - Christian Brauner : - Cap flag argument to 32 bits in every place that is not clone3() feature aware. - Cc stable. --- kernel/fork.c | 13 +++++++------ 1 file changed, 7 insertions(+), 6 deletions(-) base-commit: 0e698dfa282211e414076f9dc7e83c1c288314fd diff --git a/kernel/fork.c b/kernel/fork.c index 8c700f881d92..48ed22774efa 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -2486,11 +2486,11 @@ long do_fork(unsigned long clone_flags, int __user *child_tidptr) { struct kernel_clone_args args = { - .flags = (clone_flags & ~CSIGNAL), + .flags = (lower_32_bits(clone_flags) & ~CSIGNAL), .pidfd = parent_tidptr, .child_tid = child_tidptr, .parent_tid = parent_tidptr, - .exit_signal = (clone_flags & CSIGNAL), + .exit_signal = (lower_32_bits(clone_flags) & CSIGNAL), .stack = stack_start, .stack_size = stack_size, }; @@ -2508,8 +2508,9 @@ long do_fork(unsigned long clone_flags, pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags) { struct kernel_clone_args args = { - .flags = ((flags | CLONE_VM | CLONE_UNTRACED) & ~CSIGNAL), - .exit_signal = (flags & CSIGNAL), + .flags = ((lower_32_bits(flags) | CLONE_VM | + CLONE_UNTRACED) & ~CSIGNAL), + .exit_signal = (lower_32_bits(flags) & CSIGNAL), .stack = (unsigned long)fn, .stack_size = (unsigned long)arg, }; @@ -2570,11 +2571,11 @@ SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp, #endif { struct kernel_clone_args args = { - .flags = (clone_flags & ~CSIGNAL), + .flags = (lower_32_bits(clone_flags) & ~CSIGNAL), .pidfd = parent_tidptr, .child_tid = child_tidptr, .parent_tid = parent_tidptr, - .exit_signal = (clone_flags & CSIGNAL), + .exit_signal = (lower_32_bits(clone_flags) & CSIGNAL), .stack = newsp, .tls = tls, };