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 7D65FC4332F for ; Wed, 1 Jun 2022 10:11:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1350815AbiFAKLA (ORCPT ); Wed, 1 Jun 2022 06:11:00 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54332 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1352044AbiFAKKA (ORCPT ); Wed, 1 Jun 2022 06:10:00 -0400 Received: from mx0b-00069f02.pphosted.com (mx0b-00069f02.pphosted.com [205.220.177.32]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id F33B85AEF4; Wed, 1 Jun 2022 03:07:21 -0700 (PDT) Received: from pps.filterd (m0246632.ppops.net [127.0.0.1]) by mx0b-00069f02.pphosted.com (8.17.1.5/8.17.1.5) with ESMTP id 2519YQih004560; Wed, 1 Jun 2022 10:06:58 GMT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=oracle.com; h=from : to : cc : subject : date : message-id : in-reply-to : references : mime-version : content-transfer-encoding; s=corp-2021-07-09; bh=KPTpxwpefYVv5ysqWKaz5Z+/bKkSteAjdrAheGXnVO0=; b=LD5d7cUySrho8ClrlmQHHhe2IoOFTwAWRFYrkUXWybu4CzHTfMsFU/C0MhbCA6f2fHPk fUKIuMGsvJ/1ff6MP2B0VaWBnmU1MWarCCZpqbJGVogSMzg1R8+ebTEole5QtpihOJ8M AfrrNJi10J2Vyf3j0+E+9z9BAmAI8bZzFB2bC8XlwwUXnisJNGtsYlI8dvK7BniqO7V6 4JrG30lNDXNl9KAWB/evUZypWmlSgl0j/4YbTgPYxZz7qWP7T+JZcCxjdQ8D2eq0m+/T BotsSC55EzeNwRi7h3a+tchaIW3UeP1wTNBGj6Iry2OErCdM9+7FioKmJOt2McbEwC5E 7A== Received: from iadpaimrmta03.imrmtpd1.prodappiadaev1.oraclevcn.com (iadpaimrmta03.appoci.oracle.com [130.35.103.27]) by mx0b-00069f02.pphosted.com (PPS) with ESMTPS id 3gbc6x7d4e-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Wed, 01 Jun 2022 10:06:58 +0000 Received: from pps.filterd (iadpaimrmta03.imrmtpd1.prodappiadaev1.oraclevcn.com [127.0.0.1]) by iadpaimrmta03.imrmtpd1.prodappiadaev1.oraclevcn.com (8.16.1.2/8.16.1.2) with SMTP id 251A5g4k004893; Wed, 1 Jun 2022 10:06:57 GMT Received: from pps.reinject (localhost [127.0.0.1]) by iadpaimrmta03.imrmtpd1.prodappiadaev1.oraclevcn.com with ESMTP id 3gc8kgwvwt-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Wed, 01 Jun 2022 10:06:57 +0000 Received: from iadpaimrmta03.imrmtpd1.prodappiadaev1.oraclevcn.com (iadpaimrmta03.imrmtpd1.prodappiadaev1.oraclevcn.com [127.0.0.1]) by pps.reinject (8.16.0.36/8.16.0.36) with SMTP id 251A6v0O008065; Wed, 1 Jun 2022 10:06:57 GMT Received: from t460.home (dhcp-10-175-24-90.vpn.oracle.com [10.175.24.90]) by iadpaimrmta03.imrmtpd1.prodappiadaev1.oraclevcn.com with ESMTP id 3gc8kgwvv9-1; Wed, 01 Jun 2022 10:06:56 +0000 From: Vegard Nossum To: stable@vger.kernel.org Cc: Kees Cook , Ariadne Conill , Michael Kerrisk , Matthew Wilcox , Christian Brauner , Rich Felker , Eric Biederman , Alexander Viro , linux-fsdevel@vger.kernel.org, Andy Lutomirski , Vegard Nossum Subject: [PATCH 5.4.y] exec: Force single empty string when argv is empty Date: Wed, 1 Jun 2022 12:02:20 +0200 Message-Id: <20220601100220.653-1-vegard.nossum@oracle.com> X-Mailer: git-send-email 2.35.1.46.g38062e73e0 In-Reply-To: <164890338512664@kroah.com> References: <164890338512664@kroah.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Proofpoint-GUID: xoSF8dkxbA69HzVOoDEXM9KGRDQkvw95 X-Proofpoint-ORIG-GUID: xoSF8dkxbA69HzVOoDEXM9KGRDQkvw95 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^-] Signed-off-by: Vegard Nossum --- fs/exec.c | 25 ++++++++++++++++++++++++- 1 file changed, 24 insertions(+), 1 deletion(-) 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. diff --git a/fs/exec.c b/fs/exec.c index 098de820abcc9..a7d78241082a2 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -454,6 +454,9 @@ static int prepare_arg_pages(struct linux_binprm *bprm, unsigned long limit, ptr_size; 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 (bprm->argc < 0) return bprm->argc; @@ -482,8 +485,14 @@ static int prepare_arg_pages(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; @@ -1848,6 +1857,20 @@ static int __do_execve_file(int fd, struct filename *filename, 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; -- 2.35.1.46.g38062e73e0