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

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Date:   Wed, 26 Jan 2022 20:50:39 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Ariadne Conill <ariadne@...eferenced.org>
Cc:     Kees Cook <keescook@...omium.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

On Wed, Jan 26, 2022 at 7:42 PM Ariadne Conill <ariadne@...eferenced.org> wrote:
> On Wed, 26 Jan 2022, Jann Horn wrote:
> > On Wed, Jan 26, 2022 at 6:58 PM Kees Cook <keescook@...omium.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.
>
> I'm presently compiling a kernel with the patch to see if it works or not.
>
> > 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)?
>
> In the other thread, it was suggested that perhaps we should set up an
> argv of {"", NULL}.  In that case, it seems like it would be safe to claim
> argc == 1.
>
> What do you think?

Sounds good to me, since that's something that could also happen
normally if userspace calls execve(..., {"", NULL}, ...).

(I'd like it even better if we could just bail out with an error code,
but I guess the risk of breakage might be too high with that
approach?)