fs/exec: require argv[0] presence in do_execveat_common()

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[0]:

    The argument arg0 should point to a filename string that is
    associated with the process being started by one of the exec
    functions.

To ensure that execve(2) with argc < 1 is not a useful tool for
shellcode to use, we can validate this in do_execveat_common() and
fail for this scenario, effectively blocking successful exploitation
of CVE-2021-4034 and similar bugs which depend on execve(2) working
with argc < 1.

We use -EINVAL for this case, mirroring recent changes to FreeBSD and
OpenBSD.  -EINVAL is also used by QNX for this, while Solaris uses
-EFAULT.

In earlier versions of the patch, it was proposed that we create a
fake argv for applications to use when argc < 1, but it was concluded
that it would be better to just fail the execve(2) in these cases, as
launching a process with an empty or NULL argv[0] was likely to just
cause more problems.

Interestingly, Michael Kerrisk opened an issue about this in 2008[1],
but there was no consensus to support fixing this issue then.
Hopefully now that CVE-2021-4034 shows practical exploitative use[2]
of this bug in a shellcode, we can reconsider.

This issue is being tracked in the KSPP issue tracker[3].

There are a few[4][5] minor edge cases (primarily in test suites) that
are caught by this, but we plan to work with the projects to fix those
edge cases.

[0]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html
[1]: https://bugzilla.kernel.org/show_bug.cgi?id=8408
[2]: https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt
[3]: KSPP#176
[4]: https://codesearch.debian.net/search?q=execve%5C+*%5C%28%5B%5E%2C%5D%2B%2C+*NULL&literal=0
[5]: https://codesearch.debian.net/search?q=execlp%3F%5Cs*%5C%28%5B%5E%2C%5D%2B%2C%5Cs*NULL&literal=0

Changes from v2:
- Switch to using -EINVAL as the error code for this.
- Use pr_warn_once() to warn when an execve(2) is rejected due to NULL
  argv.

Changes from v1:
- Rework commit message significantly.
- Make the argv[0] check explicit rather than hijacking the error-check
  for count().

Reported-by: Michael Kerrisk <mtk.manpages@gmail.com>
To: Andrew Morton <akpm@linux-foundation.org>
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: Kees Cook <keescook@chromium.org>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: stable@vger.kernel.org
Signed-off-by: Ariadne Conill <ariadne@dereferenced.org>

fs/exec.c

@@ -1896,6 +1896,10 @@ static int do_execveat_common(int fd, struct filename *filename,
 	}
 
 	retval = count(argv, MAX_ARG_STRINGS);
+	if (retval == 0) {
+		pr_warn_once("Attempted to run process '%s' with NULL argv\n", bprm->filename);
+		retval = -EINVAL;
+	}
 	if (retval < 0)
 		goto out_free;
 	bprm->argc = retval;