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Date:   Tue, 25 Jan 2022 22:42:41 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Ariadne Conill <ariadne@dereferenced.org>
Cc:     linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Eric Biederman <ebiederm@xmission.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH] fs/exec: require argv[0] presence in do_execveat_common()
Message-ID: <202201252241.7309AE568F@keescook>
References: <20220126043947.10058-1-ariadne@dereferenced.org>
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On Wed, Jan 26, 2022 at 04:39:47AM +0000, Ariadne Conill wrote:
> The first argument to argv when used with execv family of calls is
> required to be the name of the program being executed, per POSIX.
> 
> By validating this in do_execveat_common(), we can prevent execution
> of shellcode which invokes execv(2) family syscalls with argc < 1,
> a scenario which is disallowed by POSIX, thus providing a mitigation
> against CVE-2021-4034 and similar bugs in the future.
> 
> The use of -EFAULT for this case is similar to other systems, such
> as FreeBSD and OpenBSD.
> 
> Interestingly, Michael Kerrisk opened an issue about this in 2008,
> but there was no consensus to support fixing this issue then.
> Hopefully now that CVE-2021-4034 shows practical exploitative use
> of this bug in a shellcode, we can reconsider.
> 
> Signed-off-by: Ariadne Conill <ariadne@dereferenced.org>

Yup. Agreed. For context:
https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt

> ---
>  fs/exec.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 79f2c9483302..de0b832473ed 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1897,8 +1897,10 @@ static int do_execveat_common(int fd, struct filename *filename,
>  	}
>  
>  	retval = count(argv, MAX_ARG_STRINGS);
> -	if (retval < 0)
> +	if (retval < 1) {
> +		retval = -EFAULT;
>  		goto out_free;
> +	}

There shouldn't be anything legitimate actually doing this in userspace.

-Kees

>  	bprm->argc = retval;
>  
>  	retval = count(envp, MAX_ARG_STRINGS);
> -- 
> 2.34.1
> 

-- 
Kees Cook