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

On 1/31/22 16:09, Kees Cook wrote:

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/

Acked-by: Andy Lutomirski luto@kernel.org

and cc-ing linux-api.

I agree that this should be done regardless of any security context change.

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

---

fs/exec.c | 26 +++++++++++++++++++++++++- 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/fs/exec.c b/fs/exec.c index 79f2c9483302..bbf3aadf7ce1 100644 --- a/fs/exec.c +++ b/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 = (min(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;