From mboxrd@z Thu Jan 1 00:00:00 1970 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 945A8C433EF for ; Fri, 3 Jun 2022 17:42:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1344880AbiFCRmz (ORCPT ); Fri, 3 Jun 2022 13:42:55 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57720 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1344866AbiFCRmg (ORCPT ); Fri, 3 Jun 2022 13:42:36 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6AE9D544CD; Fri, 3 Jun 2022 10:41:38 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 50BE2B8241E; Fri, 3 Jun 2022 17:41:36 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 718E5C385B8; Fri, 3 Jun 2022 17:41:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1654278095; bh=3ViZWjmJ+Nek5pl0FuNMo3ymqto6EzWkpqpCjbVe+J8=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=q+UhhMdM8ZGiHeU+Wu9gYbJ8SIehtwd30o0Mb9XbmwAJz5OiMJVKS9GD9daCBpXqo hJFeS8uUp/JVmb6pkUfvD+M3RyAaUV6yNfaJephoCJG1Xogha4qx06a4uH0oSEisUB MQKnC30D5e/y1U7RpSAPpl72BnvSj9SS2wpbC9Pc= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Ariadne Conill , Michael Kerrisk , Matthew Wilcox , Christian Brauner , Rich Felker , Eric Biederman , Alexander Viro , linux-fsdevel@vger.kernel.org, Kees Cook , Andy Lutomirski , Vegard Nossum Subject: [PATCH 4.14 13/23] exec: Force single empty string when argv is empty Date: Fri, 3 Jun 2022 19:39:40 +0200 Message-Id: <20220603173814.769065032@linuxfoundation.org> X-Mailer: git-send-email 2.36.1 In-Reply-To: <20220603173814.362515009@linuxfoundation.org> References: <20220603173814.362515009@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org From: Kees Cook commit dcd46d897adb70d63e025f175a00a89797d31a43 upstream. Quoting[1] Ariadne Conill: "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[2]: The argument arg0 should point to a filename string that is associated with the process being started by one of the exec functions. ... Interestingly, Michael Kerrisk opened an issue about this in 2008[3], but there was no consensus to support fixing this issue then. Hopefully now that CVE-2021-4034 shows practical exploitative use[4] of this bug in a shellcode, we can reconsider. This issue is being tracked in the KSPP issue tracker[5]." While the initial code searches[6][7] turned up what appeared to be mostly corner case tests, trying to that just reject argv == NULL (or an immediately terminated pointer list) quickly started tripping[8] existing userspace programs. The next best approach is forcing a single empty string into argv and adjusting argc to match. The number of programs depending on argc == 0 seems a smaller set than those calling execve with a NULL argv. Account for the additional stack space in bprm_stack_limits(). Inject an empty string when argc == 0 (and set argc = 1). Warn about the case so userspace has some notice about the change: process './argc0' launched './argc0' with NULL argv: empty string added Additionally WARN() and reject NULL argv usage for kernel threads. [1] https://lore.kernel.org/lkml/20220127000724.15106-1-ariadne@dereferenced.org/ [2] https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html [3] https://bugzilla.kernel.org/show_bug.cgi?id=8408 [4] https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt [5] https://github.com/KSPP/linux/issues/176 [6] https://codesearch.debian.net/search?q=execve%5C+*%5C%28%5B%5E%2C%5D%2B%2C+*NULL&literal=0 [7] https://codesearch.debian.net/search?q=execlp%3F%5Cs*%5C%28%5B%5E%2C%5D%2B%2C%5Cs*NULL&literal=0 [8] https://lore.kernel.org/lkml/20220131144352.GE16385@xsang-OptiPlex-9020/ Reported-by: Ariadne Conill Reported-by: Michael Kerrisk Cc: Matthew Wilcox Cc: Christian Brauner Cc: Rich Felker Cc: Eric Biederman Cc: Alexander Viro Cc: linux-fsdevel@vger.kernel.org Cc: stable@vger.kernel.org Signed-off-by: Kees Cook Acked-by: Christian Brauner Acked-by: Ariadne Conill Acked-by: Andy Lutomirski Link: https://lore.kernel.org/r/20220201000947.2453721-1-keescook@chromium.org [vegard: fixed conflicts due to missing 886d7de631da71e30909980fdbf318f7caade262^- and 3950e975431bc914f7e81b8f2a2dbdf2064acb0f^- and 655c16a8ce9c15842547f40ce23fd148aeccc074] Signed-off-by: Vegard Nossum Signed-off-by: Greg Kroah-Hartman --- fs/exec.c | 17 +++++++++++++++++ 1 file changed, 17 insertions(+) This has been tested in both argc == 0 and argc >= 1 cases, but I would still appreciate a review given the differences with mainline. If it's considered too risky I'm also fine with dropping it -- just wanted to make sure this didn't fall through the cracks, as it does block a real (albeit old by now) exploit. --- a/fs/exec.c +++ b/fs/exec.c @@ -1788,6 +1788,9 @@ static int do_execveat_common(int fd, st goto out_unmark; bprm->argc = count(argv, MAX_ARG_STRINGS); + if (bprm->argc == 0) + pr_warn_once("process '%s' launched '%s' with NULL argv: empty string added\n", + current->comm, bprm->filename); if ((retval = bprm->argc) < 0) goto out; @@ -1812,6 +1815,20 @@ static int do_execveat_common(int fd, st if (retval < 0) goto out; + /* + * When argv is empty, add an empty string ("") as argv[0] to + * ensure confused userspace programs that start processing + * from argv[1] won't end up walking envp. See also + * bprm_stack_limits(). + */ + if (bprm->argc == 0) { + const char *argv[] = { "", NULL }; + retval = copy_strings_kernel(1, argv, bprm); + if (retval < 0) + goto out; + bprm->argc = 1; + } + retval = exec_binprm(bprm); if (retval < 0) goto out;