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From:   Jann Horn <jannh@google.com>
Date:   Wed, 26 Jan 2022 19:07:20 +0100
Message-ID: <CAG48ez3hN8+zNCmLVP0yU0A5op6BAS+A-rs05aiLm4RQvzzBpg@mail.gmail.com>
Subject: Re: [PATCH] fs/binfmt_elf: Add padding NULL when argc == 0
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
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On Wed, Jan 26, 2022 at 6:58 PM Kees Cook <keescook@chromium.org> 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.

Will this result in the executed program being told that argc==0 but
having an extra NULL pointer on the stack?
If so, I believe this breaks the x86-64 ABI documented at
https://refspecs.linuxbase.org/elf/x86_64-abi-0.99.pdf - page 29,
figure 3.9 describes the layout of the initial process stack.

Actually, does this even work? Can a program still properly access its
environment variables when invoked with argc==0 with this patch
applied? AFAIU the way userspace locates envv on x86-64 is by
calculating 8*(argc+1)?