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

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

Ping, I think it should be safe to commit this revision since it is what
Dominik has suggested also [1].  I would just like some ack for the test
changes.

[1] https://sourceware.org/ml/libc-alpha/2016-11/msg00887.html

On 28/11/2016 15:56, Adhemerval Zanella wrote:
> 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);
>>  }
>>