From dcd46d897adb70d63e025f175a00a89797d31a43 Mon Sep 17 00:00:00 2001
From: Kees Cook <keescook@chromium.org>
Date: Mon, 31 Jan 2022 16:09:47 -0800
Subject: [PATCH] exec: Force single empty string when argv is empty
Git-commit: dcd46d897adb70d63e025f175a00a89797d31a43
Patch-mainline: v5.18-rc1
References: bsc#1200571

[ backport note: quite lots of rewrites from the original patch;
  pr_warn_once() is moved to prepare_arg_pages(), the empty string
  filling is done via copy_strings_kernel() instead of
  copy_string_kernel() (that doesn't exist yet in this kernel),
  and the hunk for kernel_execve() is dropped, too -- tiwai ]

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
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 fs/exec.c |   29 ++++++++++++++++++++++++++++-
 1 file changed, 28 insertions(+), 1 deletion(-)

--- a/fs/exec.c
+++ b/fs/exec.c
@@ -233,8 +233,16 @@ static struct page *get_arg_page(struct
 		 * always add the entire size of the pointers, so that on the
 		 * last call to get_arg_page() we'll actually have the entire
 		 * correct size.
+		 *
+		 * 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 (ptr_size > ULONG_MAX - size)
 			goto fail;
 		size += ptr_size;
@@ -1761,6 +1769,11 @@ static int do_execveat_common(int fd, st
 		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;
 
@@ -1785,6 +1798,20 @@ static int do_execveat_common(int fd, st
 	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) {
+		static const char *dummy_argv0 = "";
+		retval = copy_strings_kernel(1, &dummy_argv0, bprm);
+		if (retval < 0)
+			goto out;
+		bprm->argc = 1;
+	}
+
 	would_dump(bprm, bprm->file);
 
 	retval = exec_binprm(bprm);