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

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+*... [7] https://codesearch.debian.net/search?q=execlp%3F%5Cs*%5C%28%5B%5E%2C%5D%2B%2... [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