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 9831EC433EF for ; Tue, 1 Feb 2022 02:01:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232695AbiBACBB (ORCPT ); Mon, 31 Jan 2022 21:01:01 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54930 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232745AbiBACAR (ORCPT ); Mon, 31 Jan 2022 21:00:17 -0500 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id BC931C06177A; Mon, 31 Jan 2022 18:00:04 -0800 (PST) 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 dfw.source.kernel.org (Postfix) with ESMTPS id 57F606126B; Tue, 1 Feb 2022 02:00:04 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id B874AC3410F; Tue, 1 Feb 2022 02:00:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1643680803; bh=3PdNMUFDL2c1SaDDgYYpUoeisb94BFr3OOxDIdhXXC0=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=P3hFMTo6e331awHrxQmjo08WyXCAMcYy6uoJONB3oRzw+Cta+lFj5utaQOeElGZL4 nWMl+08Ia3mHljCGJCNLNzXR3ohGkxK6tfQBxWuV3lBr4uUEmRnD5AdJThagnF/gsk 62B2XJAW/mT9Tlp5tHxgAmXOCkJGOt9QvRyUotZJ4EiaMaelFIN6uZDuyOnQCvBm2J D7Aq7kuP2c8Aa618tkQNIgyO/TVux4kFia6T+x5omkuhZj94Yyh70q5z4CNJibFRM4 dgrRSA0Ebi7amcF3/d3Z5g7X5rsKn8t5G9wRW3ord76iZgoXddkoxIRTs0KjobvN9D /yMEk9KeajqVA== Message-ID: <234f8cb0-8f9c-0caf-c169-cf9355b33075@kernel.org> Date: Mon, 31 Jan 2022 18:00:01 -0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.5.0 Subject: Re: [PATCH] exec: Force single empty string when argv is empty Content-Language: en-US To: Kees Cook , Andrew Morton Cc: Ariadne Conill , Michael Kerrisk , Matthew Wilcox , Christian Brauner , Rich Felker , Eric Biederman , Alexander Viro , linux-fsdevel@vger.kernel.org, stable@vger.kernel.org, linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org, Linux API References: <20220201000947.2453721-1-keescook@chromium.org> From: Andy Lutomirski In-Reply-To: <20220201000947.2453721-1-keescook@chromium.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On 1/31/22 16:09, Kees Cook wrote: > 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/ Acked-by: Andy Lutomirski and cc-ing linux-api. I agree that this should be done regardless of any security context change. > > 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 > --- > fs/exec.c | 26 +++++++++++++++++++++++++- > 1 file changed, 25 insertions(+), 1 deletion(-) > > diff --git a/fs/exec.c b/fs/exec.c > index 79f2c9483302..bbf3aadf7ce1 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 = (min(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;