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

From: Kees Cook <keescook@chromium.org>
To: Jann Horn <jannh@google.com>
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 11:56:30 -0800	[thread overview]
Message-ID: <202201261155.7CC0A992@keescook> (raw)
In-Reply-To: <CAG48ez3hN8+zNCmLVP0yU0A5op6BAS+A-rs05aiLm4RQvzzBpg@mail.gmail.com>

On Wed, Jan 26, 2022 at 07:07:20PM +0100, Jann Horn wrote:
> 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)?

Hrm, yeah, I guess it's libc providing the envp pointer; it's not passes
separately. Hrm.

-- 
Kees Cook