From: Ariadne Conill <ariadne@dereferenced.org>
To: Kees Cook <keescook@chromium.org>
Cc: Ariadne Conill <ariadne@dereferenced.org>,
Michael Kerrisk <mtk.manpages@gmail.com>,
Matthew Wilcox <willy@infradead.org>,
Christian Brauner <brauner@kernel.org>,
Rich Felker <dalias@libc.org>,
Eric Biederman <ebiederm@xmission.com>,
Alexander Viro <viro@zeniv.linux.org.uk>,
linux-fsdevel@vger.kernel.org, stable@vger.kernel.org,
linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
Subject: Re: [PATCH] fs/binfmt_elf: Add padding NULL when argc == 0
Date: Wed, 26 Jan 2022 14:10:37 -0600 (CST) [thread overview]
Message-ID: <af7838bd-4cc1-fc2c-6cb9-8ddd65d5f96@dereferenced.org> (raw)
In-Reply-To: <20220126175747.3270945-1-keescook@chromium.org>
Hi,
On Wed, 26 Jan 2022, Kees Cook wrote:
> Quoting Ariadne Conill:
>
> "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[1]:
>
> 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[2],
> but there was no consensus to support fixing this issue then.
> Hopefully now that CVE-2021-4034 shows practical exploitative use[3]
> of this bug in a shellcode, we can reconsider."
>
> An examination of existing[4] users of execve(..., NULL, NULL) shows
> mostly test code, or example rootkit code. While rejecting a NULL argv
> would be preferred, it looks like the main cause of userspace confusion
> is an assumption that argc >= 1, and buggy programs may skip argv[0]
> when iterating. To protect against userspace bugs of this nature, insert
> an extra NULL pointer in argv when argc == 0, so that argv[1] != envp[0].
>
> Note that this is only done in the argc == 0 case because some userspace
> programs expect to find envp at exactly argv[argc]. The overlap of these
> two misguided assumptions is believed to be zero.
>
> [1] https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html
> [2] https://bugzilla.kernel.org/show_bug.cgi?id=8408
> [3] https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt
> [4] https://codesearch.debian.net/search?q=execve%5C+*%5C%28%5B%5E%2C%5D%2B%2C+*NULL&literal=0
>
> Reported-by: Ariadne Conill <ariadne@dereferenced.org>
> Reported-by: Michael Kerrisk <mtk.manpages@gmail.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: Rich Felker <dalias@libc.org>
> Cc: Eric Biederman <ebiederm@xmission.com>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: linux-fsdevel@vger.kernel.org
> Cc: stable@vger.kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.org>
Tested-by: Ariadne Conill <ariadne@dereferenced.org>
It seems to work, but I still think bailing early with -EINVAL is a more
reasonable position to take. For example, the following code, when used
with BusyBox applets results in a segfault, as the multicall stub does not
support scenarios where argc < 1:
#include <stdio.h>
#include <unistd.h>
#include <sys/syscall.h>
int main(int argc, const char **argv) {
if (syscall(SYS_execve, "/bin/date", NULL, NULL) < 0)
perror("execve");
return 0;
}
Ariadne
next prev parent reply other threads:[~2022-01-26 20:10 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-26 17:57 [PATCH] fs/binfmt_elf: Add padding NULL when argc == 0 Kees Cook
2022-01-26 18:07 ` Jann Horn
2022-01-26 18:42 ` Ariadne Conill
2022-01-26 19:50 ` Jann Horn
2022-01-26 19:58 ` Kees Cook
2022-01-26 20:08 ` Matthew Wilcox
2022-01-26 20:13 ` Kees Cook
2022-01-26 20:31 ` Eric W. Biederman
2022-01-26 19:56 ` Kees Cook
2022-01-26 20:10 ` Ariadne Conill [this message]
2022-01-26 20:46 ` Ariadne Conill
2022-01-26 20:52 ` Rich Felker
2022-01-29 7:41 ` [fs/binfmt_elf] 4736b95ed2: kernel-selftests.x86.make_fail kernel test robot
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