Re: [PATCH 4.9.y] exec: Force single empty string when argv is empty

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Jun 01, 2022 at 12:12:26PM +0200, Vegard Nossum wrote:
> From: Kees Cook <keescook@chromium.org>
> 
> commit dcd46d897adb70d63e025f175a00a89797d31a43 upstream.
> 
> 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.
> 
> 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@dereferenced.org>
> Reported-by: Michael Kerrisk <mtk.manpages@gmail.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: Rich Felker <dalias@libc.org>
> Cc: Eric Biederman <ebiederm@xmission.com>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: linux-fsdevel@vger.kernel.org
> Cc: stable@vger.kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.org>
> Acked-by: Christian Brauner <brauner@kernel.org>
> Acked-by: Ariadne Conill <ariadne@dereferenced.org>
> Acked-by: Andy Lutomirski <luto@kernel.org>
> Link: https://lore.kernel.org/r/20220201000947.2453721-1-keescook@chromium.org
> [vegard: fixed conflicts due to missing
>  886d7de631da71e30909980fdbf318f7caade262^- and
>  3950e975431bc914f7e81b8f2a2dbdf2064acb0f^- and
>  655c16a8ce9c15842547f40ce23fd148aeccc074]
> Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
> ---
>  fs/exec.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> This has been tested in both argc == 0 and argc >= 1 cases, but I would
> still appreciate a review given the differences with mainline. If it's
> considered too risky I'm also fine with dropping it -- just wanted to
> make sure this didn't fall through the cracks, as it does block a real
> (albeit old by now) exploit.
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 482a8b4f41a5b..19f8b075d3b6b 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1758,6 +1758,9 @@ static int do_execveat_common(int fd, struct filename *filename,
>  		goto out_unmark;
>  
>  	bprm->argc = count(argv, MAX_ARG_STRINGS);
> +	if (bprm->argc == 0)
> +		pr_warn_once("process '%s' launched '%s' with NULL argv: empty string added\n",
> +			     current->comm, bprm->filename);
>  	if ((retval = bprm->argc) < 0)
>  		goto out;
>  
> @@ -1782,6 +1785,20 @@ static int do_execveat_common(int fd, struct filename *filename,
>  	if (retval < 0)
>  		goto out;
>  
> +	/*
> +	 * When argv is empty, add an empty string ("") as argv[0] to
> +	 * ensure confused userspace programs that start processing
> +	 * from argv[1] won't end up walking envp. See also
> +	 * bprm_stack_limits().
> +	 */
> +	if (bprm->argc == 0) {
> +		const char *argv[] = { "", NULL };
> +		retval = copy_strings_kernel(1, argv, bprm);
> +		if (retval < 0)
> +			goto out;
> +		bprm->argc = 1;
> +	}
> +
>  	retval = exec_binprm(bprm);
>  	if (retval < 0)
>  		goto out;
> -- 
> 2.35.1.46.g38062e73e0
> 

All now queued up, thanks.

greg k-h