posix_spawnp stack overflow/corruption by child when PATH is large?

[Will Dietz <w@wdtz.org>]

Hi,

I believe there is a bug in posix_spawn/execvpe, please take a look and confirm
or kindly let me know if I'm mistaken and accept my apologies :).

It looks like __posix_spawnx calls clone() with a 1024-byte stack buffer
(allocated from its own stack), which is insufficient to handle stack
allocations performed
in execvpe which are something around a few bytes more than NAME_MAX+PATH_MAX.

This path is taken when using posix_spawnp, and the problem exists on
1.1.16 and latest git.

For what it's worth I tracked this down from a crash in 'bison' when
invoking m4,
but I've had success reproducing it with the following demo program
and driver script:

-------------------------------------------
#include <spawn.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>

extern char **environ;

int main() {

  pid_t p;
  char *argv[] = {"sh", "-c", "echo Hello", NULL};
  int s, status;
  s = posix_spawnp(&p, "sh", NULL, NULL, argv, environ);
  if (s) {
    perror("posix_spawn");
    exit(1);
  }

  s = waitpid(p, &status, 0);

  printf("pid: %d, s: %d, status: %d\n", p, s, status);

  return 0;
}
--------------

And little shell script to create a suitably large PATH (mostly to
demonstrate what I mean, not for unmodified use):
---------------
#!/bin/sh

SLASH_100_As="/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
SUFFIX="/123456789012345678901234567" #1234567890" #1234567890"

VAR="/bin:$SUFFIX"
for x in `seq 10`; do
  VAR="${SLASH_100_As}:$VAR"
done

echo $VAR
echo $VAR|wc -c

# Works fine with normal PATH
~/cur/musl-spawn/test
~/cur/musl-spawn/test

# Crashes when PATH is ~1050 characters
PATH=$VAR \
~/cur/musl-spawn/test
--------------

Where "~/cur/musl-spawn/test" is the test program compiled against musl.

I cannot speak regarding any security implications, but since this may
grant some measure of stack-scribbling-powers it seems to warrant
being given brief attention in this context.

An easy fix is to bump the size of the 'char stack[1024]' in
src/process/posix_spawn.c to a suitable value-- 8096 is overkill but
does the trick, for example.

Please let me know if I'm missing something or if details are not clear.

Thanks!

~Will