linux-hardening - Re: [PATCH] exec: Force single empty string when argv is empty

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Date:   Mon, 31 Jan 2022 19:00:46 -0600 (CST)
From:   Ariadne Conill <ariadne@...eferenced.org>
To:     Kees Cook <keescook@...omium.org>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        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] exec: Force single empty string when argv is empty

Hi,

On Mon, 31 Jan 2022, Kees Cook wrote:

> Quoting[1] Ariadne Conill:
>
> "In several other operating systems, it is a hard requirement that the
> second 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[2]:
>
>    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[3],
> but there was no consensus to support fixing this issue then.
> Hopefully now that CVE-2021-4034 shows practical exploitative use[4]
> of this bug in a shellcode, we can reconsider.
>
> This issue is being tracked in the KSPP issue tracker[5]."
>
> While the initial code searches[6][7] turned up what appeared to be
> mostly corner case tests, trying to that just reject argv == NULL
> (or an immediately terminated pointer list) quickly started tripping[8]
> existing userspace programs.

Yes, it's a shame this is the case, but we do what we have to do, I guess 
:)

>
> The next best approach is forcing a single empty string into argv and
> adjusting argc to match. The number of programs depending on argc == 0
> seems a smaller set than those calling execve with a NULL argv.
>
> Account for the additional stack space in bprm_stack_limits(). Inject an
> empty string when argc == 0 (and set argc = 1). Warn about the case so
> userspace has some notice about the change:
>
>    process './argc0' launched './argc0' with NULL argv: empty string added
>
> Additionally WARN() and reject NULL argv usage for kernel threads.
>
> [1] https://lore.kernel.org/lkml/20220127000724.15106-1-ariadne@dereferenced.org/
> [2] https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html
> [3] https://bugzilla.kernel.org/show_bug.cgi?id=8408
> [4] https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt
> [5] https://github.com/KSPP/linux/issues/176
> [6] https://codesearch.debian.net/search?q=execve%5C+*%5C%28%5B%5E%2C%5D%2B%2C+*NULL&literal=0
> [7] https://codesearch.debian.net/search?q=execlp%3F%5Cs*%5C%28%5B%5E%2C%5D%2B%2C%5Cs*NULL&literal=0
> [8] https://lore.kernel.org/lkml/20220131144352.GE16385@xsang-OptiPlex-9020/
>
> 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>

In terms of going with this approach as an alternative verses my original 
patch,

Acked-by: Ariadne Conill <ariadne@...eferenced.org>

Ariadne