linux-hardening - Re: [PATCH] fs/binfmt_elf: Add padding NULL when argc == 0

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Date:   Wed, 26 Jan 2022 14:10:37 -0600 (CST)
From:   Ariadne Conill <ariadne@...eferenced.org>
To:     Kees Cook <keescook@...omium.org>
cc:     Ariadne Conill <ariadne@...eferenced.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Christian Brauner <brauner@...nel.org>,
        Rich Felker <dalias@...c.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, stable@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] fs/binfmt_elf: Add padding NULL when argc == 0

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@...eferenced.org>
> Reported-by: Michael Kerrisk <mtk.manpages@...il.com>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Christian Brauner <brauner@...nel.org>
> Cc: Rich Felker <dalias@...c.org>
> Cc: Eric Biederman <ebiederm@...ssion.com>
> Cc: Alexander Viro <viro@...iv.linux.org.uk>
> Cc: linux-fsdevel@...r.kernel.org
> Cc: stable@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>

Tested-by: Ariadne Conill <ariadne@...eferenced.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