[PATCH v2] fs/exec: require argv[0] presence in do_execveat_common()

[Ariadne Conill <ariadne@dereferenced.org>]

In several other operating systems, it is a hard requirement that the
first 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 gadget 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 this gadget.

The use of -EFAULT for this case is similar to other systems, such
as FreeBSD, OpenBSD and Solaris.  QNX uses -EINVAL for this case.

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
of this bug in a shellcode, we can reconsider.

[0]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html
[1]: https://bugzilla.kernel.org/show_bug.cgi?id=8408

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

Signed-off-by: Ariadne Conill <ariadne@dereferenced.org>
---
 fs/exec.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/exec.c b/fs/exec.c
index 79f2c9483302..e52c41991aab 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1899,6 +1899,10 @@ static int do_execveat_common(int fd, struct filename *filename,
 	retval = count(argv, MAX_ARG_STRINGS);
 	if (retval < 0)
 		goto out_free;
+	if (retval == 0) {
+		retval = -EFAULT;
+		goto out_free;
+	}
 	bprm->argc = retval;
 
 	retval = count(envp, MAX_ARG_STRINGS);
-- 
2.34.1