exec: Force single empty string when argv is empty

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/[email protected]/
[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] KSPP/linux#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 <[email protected]>
Reported-by: Michael Kerrisk <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Rich Felker <[email protected]>
Cc: Eric Biederman <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
Acked-by: Christian Brauner <[email protected]>
Acked-by: Ariadne Conill <[email protected]>
Acked-by: Andy Lutomirski <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: kees

fs/exec.c

@@ -495,8 +495,14 @@ static int bprm_stack_limits(struct linux_binprm *bprm)
 	 * the stack. They aren't stored until much later when we can't
 	 * signal to the parent that the child has run out of stack space.
 	 * Instead, calculate it here so it's possible to fail gracefully.
+	 *
+	 * In the case of argc = 0, make sure there is space for adding a
+	 * empty string (which will bump argc to 1), to ensure confused
+	 * userspace programs don't start processing from argv[1], thinking
+	 * argc can never be 0, to keep them from walking envp by accident.
+	 * See do_execveat_common().
 	 */
-	ptr_size = (bprm->argc + bprm->envc) * sizeof(void *);
+	ptr_size = (max(bprm->argc, 1) + bprm->envc) * sizeof(void *);
 	if (limit <= ptr_size)
 		return -E2BIG;
 	limit -= ptr_size;
@@ -1897,6 +1903,9 @@ static int do_execveat_common(int fd, struct filename *filename,
 	}
 
 	retval = count(argv, MAX_ARG_STRINGS);
+	if (retval == 0)
+		pr_warn_once("process '%s' launched '%s' with NULL argv: empty string added\n",
+			     current->comm, bprm->filename);
 	if (retval < 0)
 		goto out_free;
 	bprm->argc = retval;
@@ -1923,6 +1932,19 @@ static int do_execveat_common(int fd, struct filename *filename,
 	if (retval < 0)
 		goto out_free;
 
+	/*
+	 * 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) {
+		retval = copy_string_kernel("", bprm);
+		if (retval < 0)
+			goto out_free;
+		bprm->argc = 1;
+	}
+
 	retval = bprm_execve(bprm, fd, filename, flags);
 out_free:
 	free_bprm(bprm);
@@ -1951,6 +1973,8 @@ int kernel_execve(const char *kernel_filename,
 	}
 
 	retval = count_strings_kernel(argv);
+	if (WARN_ON_ONCE(retval == 0))
+		retval = -EINVAL;
 	if (retval < 0)
 		goto out_free;
 	bprm->argc = retval;