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 7AD05C433F5 for ; Sat, 2 Apr 2022 12:43:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234698AbiDBMpA (ORCPT ); Sat, 2 Apr 2022 08:45:00 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57622 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233007AbiDBMo7 (ORCPT ); Sat, 2 Apr 2022 08:44:59 -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 272F03BF97 for ; Sat, 2 Apr 2022 05:43:07 -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 DB4DAB80022 for ; Sat, 2 Apr 2022 12:43:05 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 36850C340EC; Sat, 2 Apr 2022 12:43:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1648903384; bh=yftO47bDke+ZbZaBREb7p3xqXHL9t1uZglnM5Kk2V+I=; h=Subject:To:Cc:From:Date:From; b=pVWPlyPEht84oWbGuOKlY1SJZf/E/wWWreKbLIzKjDA1RtgnDe3L81CTFuTTgRJ4J cnby3RupToUitz03XbViKXh2YejBVtWrooiH+OWzKes/05ojn1WKotkj/zYj4tk4uK 3dnEPO4CMRWnRZ6wVuKtffvtkDVcnPMiykJbLjiY= Subject: FAILED: patch "[PATCH] exec: Force single empty string when argv is empty" failed to apply to 4.9-stable tree To: keescook@chromium.org, ariadne@dereferenced.org, brauner@kernel.org, dalias@libc.org, ebiederm@xmission.com, luto@kernel.org, mtk.manpages@gmail.com, viro@zeniv.linux.org.uk, willy@infradead.org Cc: From: Date: Sat, 02 Apr 2022 14:43:02 +0200 Message-ID: <164890338266171@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=ANSI_X3.4-1968 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org The patch below does not apply to the 4.9-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to . thanks, greg k-h ------------------ original commit in Linus's tree ------------------ >From dcd46d897adb70d63e025f175a00a89797d31a43 Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Mon, 31 Jan 2022 16:09:47 -0800 Subject: [PATCH] exec: Force single empty string when argv is empty 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 diff --git a/fs/exec.c b/fs/exec.c index 79f2c9483302..40b1008fb0f7 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -495,8 +495,14 @@ static int bprm_stack_limits(struct linux_binprm *bprm) * the stack. They aren't stored until much later when we can't * signal to the parent that the child has run out of stack space. * Instead, calculate it here so it's possible to fail gracefully. + * + * In the case of argc = 0, make sure there is space for adding a + * empty string (which will bump argc to 1), to ensure confused + * userspace programs don't start processing from argv[1], thinking + * argc can never be 0, to keep them from walking envp by accident. + * See do_execveat_common(). */ - ptr_size = (bprm->argc + bprm->envc) * sizeof(void *); + ptr_size = (max(bprm->argc, 1) + bprm->envc) * sizeof(void *); if (limit <= ptr_size) return -E2BIG; limit -= ptr_size; @@ -1897,6 +1903,9 @@ static int do_execveat_common(int fd, struct filename *filename, } retval = count(argv, MAX_ARG_STRINGS); + if (retval == 0) + pr_warn_once("process '%s' launched '%s' with NULL argv: empty string added\n", + current->comm, bprm->filename); if (retval < 0) goto out_free; bprm->argc = retval; @@ -1923,6 +1932,19 @@ static int do_execveat_common(int fd, struct filename *filename, if (retval < 0) goto out_free; + /* + * 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) { + retval = copy_string_kernel("", bprm); + if (retval < 0) + goto out_free; + bprm->argc = 1; + } + retval = bprm_execve(bprm, fd, filename, flags); out_free: free_bprm(bprm); @@ -1951,6 +1973,8 @@ int kernel_execve(const char *kernel_filename, } retval = count_strings_kernel(argv); + if (WARN_ON_ONCE(retval == 0)) + retval = -EINVAL; if (retval < 0) goto out_free; bprm->argc = retval;