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 8DAACC2BA4C for ; Wed, 26 Jan 2022 20:24:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230302AbiAZUYN (ORCPT ); Wed, 26 Jan 2022 15:24:13 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57282 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229519AbiAZUYJ (ORCPT ); Wed, 26 Jan 2022 15:24:09 -0500 Received: from mx1.mailbun.net (unknown [IPv6:2602:fd37:1::100]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 37E10C06161C; Wed, 26 Jan 2022 12:24:07 -0800 (PST) Received: from [2607:fb90:d98b:8818:5079:94eb:24d5:e5c3] (unknown [172.58.104.31]) (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 EBDBBE0E1E; Wed, 26 Jan 2022 20:24:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=dereferenced.org; s=mailbun; t=1643228646; bh=unMLJ93l5bXRQagJHUJJZiZSzDz3UCGTstIa9PckNN4=; h=Date:From:To:cc:Subject:In-Reply-To:References; b=NhCGMOcJ+ycxrhEJSkJLQw5eOWe9M99S29/PjGA5+P0eDmgDSbZ1mwaYW5HA+kiGT e72+Plp/owfKiaVVOTKHXoIyS0FxSDLG4oy8Ov1SQkuuvAJwfnFda0AoTHMEQ6y+vA o4ADMShg9PhqH+tvUXoTyyyzflxr4Ucf2qg9IjM7f8k201cSu69+/xGUI6toP3amwp oQn9v/ccMY38XgLQMNfAdfDyK5dJCY7sGVKRrOUwV6/uspdFjfdOgldv0DV+8EEnTM Fcovc2cAhcKmFrWXg3uAnMsnL6XRCPc4lfxj2MJhU8MSU8YRo1Up9hKLHX8BkAaRyR BeS5vIIN5Zokw== Date: Wed, 26 Jan 2022 14:23:59 -0600 (CST) From: Ariadne Conill To: Kees Cook cc: Ariadne Conill , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Eric Biederman , Alexander Viro Subject: Re: [PATCH v2] fs/exec: require argv[0] presence in do_execveat_common() In-Reply-To: <202201261202.EC027EB@keescook> Message-ID: References: <20220126114447.25776-1-ariadne@dereferenced.org> <202201261202.EC027EB@keescook> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On Wed, 26 Jan 2022, Kees Cook wrote: > On Wed, Jan 26, 2022 at 11:44:47AM +0000, Ariadne Conill wrote: >> In several other operating systems, it is a hard requirement that the >> first 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 gadget 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 this gadget. >> >> The use of -EFAULT for this case is similar to other systems, such >> as FreeBSD, OpenBSD and Solaris. QNX uses -EINVAL for this case. >> >> 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 >> of this bug in a shellcode, we can reconsider. >> >> [0]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html >> [1]: https://bugzilla.kernel.org/show_bug.cgi?id=8408 >> >> Changes from v1: >> - Rework commit message significantly. >> - Make the argv[0] check explicit rather than hijacking the error-check >> for count(). >> >> Signed-off-by: Ariadne Conill >> --- >> fs/exec.c | 4 ++++ >> 1 file changed, 4 insertions(+) >> >> diff --git a/fs/exec.c b/fs/exec.c >> index 79f2c9483302..e52c41991aab 100644 >> --- a/fs/exec.c >> +++ b/fs/exec.c >> @@ -1899,6 +1899,10 @@ static int do_execveat_common(int fd, struct filename *filename, >> retval = count(argv, MAX_ARG_STRINGS); >> if (retval < 0) >> goto out_free; >> + if (retval == 0) { >> + retval = -EFAULT; >> + goto out_free; >> + } >> bprm->argc = retval; >> >> retval = count(envp, MAX_ARG_STRINGS); >> -- >> 2.34.1 > > Okay, so, the dangerous condition is userspace iterating through envp > when it thinks it's iterating argv. > > Assuming it is not okay to break valgrind's test suite: > https://sources.debian.org/src/valgrind/1:3.18.1-1/none/tests/execve.c/?hl=22#L22 > we cannot reject a NULL argv (test will fail), and we cannot mutate > argc=0 into argc=1 (test will enter infinite loop). > > Perhaps we need to reject argv=NULL when envp!=NULL, and add a > pr_warn_once() about using a NULL argv? Sure, I can rework the patch to do it for only the envp != NULL case. I think we should combine it with the {NULL, NULL} padding patch in this case though, since it appears to work, that way the execve(..., NULL, NULL) case gets some protection. > I note that glibc already warns about NULL argv: > argc0.c:7:3: warning: null argument where non-null required (argument 2) > [-Wnonnull] > 7 | execve(argv[0], NULL, envp); > | ^~~~~~ > > in the future we could expand this to only looking at argv=NULL? I don't think musl's headers generate a diagnostic for this, but main(0, {NULL}) is not a supported use-case at least as far as Alpine is concerned. I am sure it is the same with the other musl distributions. Will send a v3 patch with this logic change and move to EINVAL shortly. Ariadne