Re: [PATCH v3] Fix writes past the allocated array bounds in execvpe (BZ#20847)

[Adhemerval Zanella <adhemerval dot zanella at linaro dot org>]

Ping.

On 23/11/2016 11:38, Adhemerval Zanella wrote:
> Commit 6c9e1be87a37bf wrongly fixes BZ#20847 by lefting the else branch
> on maybe_script_execute to still being able to invalid write on stack
> allocated buffer.  It happens if execvp{e} is executed with an empty
> arguments list ({ NULL }) and although manual states first argument
> should be the script name itself, by convention, old and current
> implementation allows it.
> 
> This patch fixes the issue by just account for arguments and not the
> final 'NULL' (since the 'argv + 1' will indeed ignored the script name).
> The empty argument list is handled in a special case with a minimum
> allocated size.  The patch also adds extra tests for such case in
> tst-vfork3.
> 
> Tested on x86_64.
> 
> 	[BZ #20847]
> 	* posix/execvpe.c (maybe_script_execute): Remove write past allocated
> 	array bounds for else branch.
> 	(__execvpe): Style fixes.
> 	* posix/tst-vfork3.c (run_script): New function.
> 	(create_script): Likewise.
> 	(do_test): Use run_script internal function.
> 	(do_prepare): Use create_script internal function.
> ---
>  ChangeLog          |  12 ++++
>  posix/execvpe.c    |  19 ++++--
>  posix/tst-vfork3.c | 185 +++++++++++++++++++----------------------------------
>  3 files changed, 91 insertions(+), 125 deletions(-)
> 
> diff --git a/posix/execvpe.c b/posix/execvpe.c
> index 7cdb06a..a2d0145 100644
> --- a/posix/execvpe.c
> +++ b/posix/execvpe.c
> @@ -38,8 +38,8 @@
>  static void
>  maybe_script_execute (const char *file, char *const argv[], char *const envp[])
>  {
> -  ptrdiff_t argc = 0;
> -  while (argv[argc++] != NULL)
> +  ptrdiff_t argc;
> +  for (argc = 0; argv[argc] != NULL; argc++)
>      {
>        if (argc == INT_MAX - 1)
>  	{
> @@ -48,13 +48,18 @@ maybe_script_execute (const char *file, char *const argv[], char *const envp[])
>  	}
>      }
>  
> -  /* Construct an argument list for the shell.  It will contain at minimum 3
> -     arguments (current shell, script, and an ending NULL.  */
> -  char *new_argv[argc + 1];
> +  /* Construct an argument list for the shell based on original arguments:
> +     1. Empty list (argv = { NULL }, argc = 1 }: new argv will contain 3
> +	arguments - default shell, script to execute, and ending NULL.
> +     2. Non empty argument list (argc = { ..., NULL }, argc > 1}: new argv
> +	will contain also the default shell and the script to execute.  It
> +	will also skip the script name in arguments and only copy script
> +	arguments.  */
> +  char *new_argv[argc > 1 ? 2 + argc : 3];
>    new_argv[0] = (char *) _PATH_BSHELL;
>    new_argv[1] = (char *) file;
>    if (argc > 1)
> -    memcpy (new_argv + 2, argv + 1, (argc - 1) * sizeof(char *));
> +    memcpy (new_argv + 2, argv + 1, argc * sizeof(char *));
>    else
>      new_argv[2] = NULL;
>  
> @@ -96,7 +101,7 @@ __execvpe (const char *file, char *const argv[], char *const envp[])
>    size_t path_len = __strnlen (path, PATH_MAX - 1) + 1;
>  
>    /* NAME_MAX does not include the terminating null character.  */
> -  if (((file_len-1) > NAME_MAX)
> +  if ((file_len - 1 > NAME_MAX)
>        || !__libc_alloca_cutoff (path_len + file_len + 1))
>      {
>        errno = ENAMETOOLONG;
> diff --git a/posix/tst-vfork3.c b/posix/tst-vfork3.c
> index 05edc5a..f5fe5c3 100644
> --- a/posix/tst-vfork3.c
> +++ b/posix/tst-vfork3.c
> @@ -33,84 +33,64 @@ char *tmpdirname;
>  #define PREPARE(argc, argv) do_prepare ()
>  #include "../test-skeleton.c"
>  
> -static int
> -do_test (void)
> +static void
> +run_script (const char *script, char *const argv[])
>  {
> -  mtrace ();
> -
> -  const char *path = getenv ("PATH");
> -  if (path == NULL)
> -    path = "/bin";
> -  char pathbuf[strlen (tmpdirname) + 1 + strlen (path) + 1];
> -  strcpy (stpcpy (stpcpy (pathbuf, tmpdirname), ":"), path);
> -  if (setenv ("PATH", pathbuf, 1) < 0)
> -    {
> -      puts ("setenv failed");
> -      return 1;
> -    }
> -
> -  size_t i;
> -  char *argv[3] = { (char *) "script1.sh", (char *) "1", NULL };
> -  for (i = 0; i < 5; i++)
> +  for (size_t i = 0; i < 5; i++)
>      {
>        pid_t pid = vfork ();
>        if (pid < 0)
> -	{
> -	  printf ("vfork failed: %m\n");
> -	  return 1;
> -	}
> +	FAIL_EXIT1 ("vfork failed: %m");
>        else if (pid == 0)
>  	{
> -	  execvp ("script1.sh", argv);
> +	  execvp (script, argv);
>  	  _exit (errno);
>  	}
> +
>        int status;
>        if (TEMP_FAILURE_RETRY (waitpid (pid, &status, 0)) != pid)
> -	{
> -	  puts ("waitpid failed");
> -	  return 1;
> -	}
> +	FAIL_EXIT1 ("waitpid failed");
>        else if (status != 0)
>  	{
>  	  if (WIFEXITED (status))
> -	    printf ("script1.sh failed with status %d\n",
> -		    WEXITSTATUS (status));
> +	    FAIL_EXIT1 ("%s failed with status %d\n", script,
> +			WEXITSTATUS (status));
>  	  else
> -	    printf ("script1.sh kill by signal %d\n",
> -		    WTERMSIG (status));
> -	  return 1;
> +	    FAIL_EXIT1 ("%s killed by signal %d\n", script,
> +			WTERMSIG (status));
>  	}
>      }
> +}
>  
> -  argv[0] = (char *) "script2.sh";
> -  argv[1] = (char *) "2";
> -  for (i = 0; i < 5; i++)
> +static int
> +do_test (void)
> +{
> +  mtrace ();
> +
> +  const char *path = getenv ("PATH");
> +  if (path == NULL)
> +    path = "/bin";
> +  char pathbuf[strlen (tmpdirname) + 1 + strlen (path) + 1];
> +  strcpy (stpcpy (stpcpy (pathbuf, tmpdirname), ":"), path);
> +  if (setenv ("PATH", pathbuf, 1) < 0)
>      {
> -      pid_t pid = vfork ();
> -      if (pid < 0)
> -	{
> -	  printf ("vfork failed: %m\n");
> -	  return 1;
> -	}
> -      else if (pid == 0)
> -	{
> -	  execvp ("script2.sh", argv);
> -	  _exit (errno);
> -	}
> -      int status;
> -      if (TEMP_FAILURE_RETRY (waitpid (pid, &status, 0)) != pid)
> -	{
> -	  puts ("waitpid failed");
> -	  return 1;
> -	}
> -      else if (status != 0)
> -	{
> -	  printf ("script2.sh failed with status %d\n", status);
> -	  return 1;
> -	}
> +      puts ("setenv failed");
> +      return 1;
>      }
>  
> -  for (i = 0; i < 5; i++)
> +  char *argv00[] = { NULL };
> +  run_script ("script0.sh", argv00);
> +  char *argv01[] = { (char*) "script0.sh", NULL };
> +  run_script ("script0.sh", argv01);
> +
> +  char *argv1[] = { (char *) "script1.sh", (char *) "1", NULL };
> +  run_script ("script1.sh", argv1);
> +
> +  char *argv2[] = { (char *) "script2.sh", (char *) "2", NULL };
> +  run_script ("script2.sh", argv2);
> +
> +  /* Same as before but with execlp.  */
> +  for (size_t i = 0; i < 5; i++)
>      {
>        pid_t pid = vfork ();
>        if (pid < 0)
> @@ -137,87 +117,56 @@ do_test (void)
>      }
>  
>    unsetenv ("PATH");
> -  argv[0] = (char *) "echo";
> -  argv[1] = (char *) "script 4";
> -  for (i = 0; i < 5; i++)
> -    {
> -      pid_t pid = vfork ();
> -      if (pid < 0)
> -	{
> -	  printf ("vfork failed: %m\n");
> -	  return 1;
> -	}
> -      else if (pid == 0)
> -	{
> -	  execvp ("echo", argv);
> -	  _exit (errno);
> -	}
> -      int status;
> -      if (TEMP_FAILURE_RETRY (waitpid (pid, &status, 0)) != pid)
> -	{
> -	  puts ("waitpid failed");
> -	  return 1;
> -	}
> -      else if (status != 0)
> -	{
> -	  printf ("echo failed with status %d\n", status);
> -	  return 1;
> -	}
> -    }
> +  char *argv4[] = { (char *) "echo", (char *) "script 4", NULL };
> +  run_script ("echo", argv4);
>  
>    return 0;
>  }
>  
>  static void
> +create_script (const char *script, const char *contents, size_t size)
> +{
> +  int fd = open (script, O_WRONLY | O_CREAT, 0700);
> +  if (fd < 0
> +      || TEMP_FAILURE_RETRY (write (fd, contents, size)) != size
> +      || fchmod (fd, S_IRUSR | S_IXUSR) < 0)
> +    FAIL_EXIT1 ("could not write %s\n", script);
> +  close (fd);
> +}
> +
> +static void
>  do_prepare (void)
>  {
>    size_t len = strlen (test_dir) + sizeof ("/tst-vfork3.XXXXXX");
>    tmpdirname = malloc (len);
> -  char *script1 = malloc (len + sizeof "/script1.sh");
> -  char *script2 = malloc (len + sizeof "/script2.sh");
> -  if (tmpdirname == NULL || script1 == NULL || script2 == NULL)
> -    {
> -      puts ("out of memory");
> -      exit (1);
> -    }
> +  if (tmpdirname == NULL)
> +    FAIL_EXIT1 ("out of memory");
>    strcpy (stpcpy (tmpdirname, test_dir), "/tst-vfork3.XXXXXX");
>  
>    tmpdirname = mkdtemp (tmpdirname);
>    if (tmpdirname == NULL)
> -    {
> -      puts ("could not create temporary directory");
> -      exit (1);
> -    }
> +    FAIL_EXIT1 ("could not create temporary directory");
> +
> +  char script0[len + sizeof "/script0.sh"];
> +  char script1[len + sizeof "/script1.sh"];
> +  char script2[len + sizeof "/script2.sh"];
>  
> +  strcpy (stpcpy (script0, tmpdirname), "/script0.sh");
>    strcpy (stpcpy (script1, tmpdirname), "/script1.sh");
>    strcpy (stpcpy (script2, tmpdirname), "/script2.sh");
>  
> -  /* Need to make sure tmpdirname is at the end of the linked list.  */
> +  add_temp_file (script0);
>    add_temp_file (script1);
> -  add_temp_file (tmpdirname);
>    add_temp_file (script2);
> +  /* Need to make sure tmpdirname is at the end of the linked list.  */
> +  add_temp_file (tmpdirname);
> +
> +  const char content0[] = "#!/bin/sh\necho empty\n";
> +  create_script (script0, content0, sizeof content0);
>  
>    const char content1[] = "#!/bin/sh\necho script $1\n";
> -  int fd = open (script1, O_WRONLY | O_CREAT, 0700);
> -  if (fd < 0
> -      || TEMP_FAILURE_RETRY (write (fd, content1, sizeof content1))
> -	 != sizeof content1
> -      || fchmod (fd, S_IRUSR | S_IXUSR) < 0)
> -    {
> -      printf ("Could not write %s\n", script1);
> -      exit (1);
> -    }
> -  close (fd);
> +  create_script (script1, content1, sizeof content1);
>  
>    const char content2[] = "echo script $1\n";
> -  fd = open (script2, O_WRONLY | O_CREAT, 0700);
> -  if (fd < 0
> -      || TEMP_FAILURE_RETRY (write (fd, content2, sizeof content2))
> -	 != sizeof content2
> -      || fchmod (fd, S_IRUSR | S_IXUSR) < 0)
> -    {
> -      printf ("Could not write %s\n", script2);
> -      exit (1);
> -    }
> -  close (fd);
> +  create_script (script2, content2, sizeof content2);
>  }
>