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

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Message-ID: <202201261210.E0E7EB83@keescook>
Date:   Wed, 26 Jan 2022 12:13:16 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Jann Horn <jannh@...gle.com>,
        Ariadne Conill <ariadne@...eferenced.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        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 08:08:14PM +0000, Matthew Wilcox wrote:
> On Wed, Jan 26, 2022 at 11:58:39AM -0800, Kees Cook wrote:
> > We can't mutate argc; it'll turn at least some userspace into an
> > infinite loop:
> > https://sources.debian.org/src/valgrind/1:3.18.1-1/none/tests/execve.c/?hl=22#L22
> 
> How does that become an infinite loop?  We obviously wouldn't mutate
> argc in the caller, just the callee.

Oh, sorry, I misread. It's using /bin/true, not argv[0] (another bit of
code I found was using argv[0]). Yeah, {"", NULL} could work.

> Also, there's a version of this where we only mutate argc if we're
> executing a setuid program, which would remove the privilege
> escalation part of things.

True; though I'd like to keep the logic as non-specialized as possible.
I don't like making stuff conditional on privilege boundaries if we can
make it always happen.

-- 
Kees Cook