From patchwork Thu Jan 27 00:07:24 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Ariadne Conill X-Patchwork-Id: 537584 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 A8319C433F5 for ; Thu, 27 Jan 2022 00:07:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232739AbiA0AHb (ORCPT ); Wed, 26 Jan 2022 19:07:31 -0500 Received: from mx1.mailbun.net ([170.39.20.100]:42890 "EHLO mx1.mailbun.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229839AbiA0AHb (ORCPT ); Wed, 26 Jan 2022 19:07:31 -0500 Received: from localhost.localdomain (unknown [170.39.20.82]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) (Authenticated sender: ariadne@dereferenced.org) by mx1.mailbun.net (Postfix) with ESMTPSA id 1185B11A817; Thu, 27 Jan 2022 00:07:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=dereferenced.org; s=mailbun; t=1643242051; bh=KrviaVq2nktezkC2TngWHh7lS7n6CoRmDUx4MPmAHSo=; h=From:To:Cc:Subject:Date; b=jqFlIp7Lw9yFEc4lYI6x1eZQlXDzyYVr4LfXMHNNIWSsXlrZvTnVdOfzD4QofzCoX L0LKOIALzXCOLL3m36vm1aneR45vxryLTfbBz/1WJV+SVhmpyvRBCkS8bbNbBZxWBX o+4QtnrUZEAxmVRJp2xDdOZYjkNjZXC33YpAqU1y/iiv1wfiUz3czf5TkioPgSGPul 6OxdrZF60z3uBv3sC0uuMCHpO4tRitNzyjFKGBL++xo/h06vVpMiIAq0l2FYx1T+kB u2e0qbo/vhKIBvK7zF/mPMDp/MLP86Vm/bjzj2+NsZKpCzqBhOIFswilovgKMRJ1FP qnEtSkWD840MA== From: Ariadne Conill To: Andrew Morton Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Eric Biederman , Kees Cook , Alexander Viro , Ariadne Conill , Michael Kerrisk , Matthew Wilcox , Christian Brauner , Rich Felker , linux-mm@kvack.org, stable@vger.kernel.org Subject: [PATCH v3] fs/exec: require argv[0] presence in do_execveat_common() Date: Thu, 27 Jan 2022 00:07:24 +0000 Message-Id: <20220127000724.15106-1-ariadne@dereferenced.org> X-Mailer: git-send-email 2.34.1 MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org In several other operating systems, it is a hard requirement that the second argument to execve(2) be the name of a program, thus prohibiting a scenario where argc < 1. POSIX 2017 also recommends this behaviour, but it is not an explicit requirement[0]: The argument arg0 should point to a filename string that is associated with the process being started by one of the exec functions. To ensure that execve(2) with argc < 1 is not a useful tool for shellcode to use, we can validate this in do_execveat_common() and fail for this scenario, effectively blocking successful exploitation of CVE-2021-4034 and similar bugs which depend on execve(2) working with argc < 1. We use -EINVAL for this case, mirroring recent changes to FreeBSD and OpenBSD. -EINVAL is also used by QNX for this, while Solaris uses -EFAULT. In earlier versions of the patch, it was proposed that we create a fake argv for applications to use when argc < 1, but it was concluded that it would be better to just fail the execve(2) in these cases, as launching a process with an empty or NULL argv[0] was likely to just cause more problems. Interestingly, Michael Kerrisk opened an issue about this in 2008[1], but there was no consensus to support fixing this issue then. Hopefully now that CVE-2021-4034 shows practical exploitative use[2] of this bug in a shellcode, we can reconsider. This issue is being tracked in the KSPP issue tracker[3]. There are a few[4][5] minor edge cases (primarily in test suites) that are caught by this, but we plan to work with the projects to fix those edge cases. [0]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html [1]: https://bugzilla.kernel.org/show_bug.cgi?id=8408 [2]: https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt [3]: https://github.com/KSPP/linux/issues/176 [4]: https://codesearch.debian.net/search?q=execve%5C+*%5C%28%5B%5E%2C%5D%2B%2C+*NULL&literal=0 [5]: https://codesearch.debian.net/search?q=execlp%3F%5Cs*%5C%28%5B%5E%2C%5D%2B%2C%5Cs*NULL&literal=0 Changes from v2: - Switch to using -EINVAL as the error code for this. - Use pr_warn_once() to warn when an execve(2) is rejected due to NULL argv. Changes from v1: - Rework commit message significantly. - Make the argv[0] check explicit rather than hijacking the error-check for count(). Reported-by: Michael Kerrisk To: Andrew Morton Cc: Matthew Wilcox Cc: Christian Brauner Cc: Rich Felker Cc: Eric Biederman Cc: Alexander Viro Cc: Kees Cook Cc: linux-fsdevel@vger.kernel.org Cc: linux-mm@kvack.org Cc: stable@vger.kernel.org Signed-off-by: Ariadne Conill Acked-by: "Eric W. Biederman" Reported-by: kernel test robot --- fs/exec.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/fs/exec.c b/fs/exec.c index 79f2c9483302..982730cfe3b8 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1897,6 +1897,10 @@ static int do_execveat_common(int fd, struct filename *filename, } retval = count(argv, MAX_ARG_STRINGS); + if (retval == 0) { + pr_warn_once("Attempted to run process '%s' with NULL argv\n", bprm->filename); + retval = -EINVAL; + } if (retval < 0) goto out_free; bprm->argc = retval;