[PATCH v2] Fix writes past the allocated array bounds in execvpe (BZ#20847)

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

Changes from previous version:

 - Fix memcpy write out the bound in maybe_script_execute.

--

This patch fixes an invalid write out or stack allocated buffer in
2 places at execvpe implementation:

  1. On 'maybe_script_execute' function where it allocates the new
     argument list and it does not account that a minimum of argc
     plus 3 elements (default shell path, script name, arguments,
     and ending null pointer) should be considered.  The straightforward
     fix is just to take account of the correct list size.

  2. On '__execvpe' where the executable file name lenght may not
     account for ending '\0' and thus subsequent path creation may
     write past array bounds because it requires to add the terminating
     null.  The fix is to change how to calculate the executable name
     size to add the final '\0' and adjust the rest of the code
     accordingly.

As described in GCC bug report 78433 [1], these issues were masked off by
GCC because it allocated several bytes more than necessary so that many
off-by-one bugs went unnoticed.

Checked on x86_64 with a latest GCC (7.0.0 20161121) with -O3 on CFLAGS.

	[BZ #20847]
	* posix/execvpe.c (maybe_script_execute): Remove write past allocated
	array bounds.
	(__execvpe): Likewise.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78433
---
 ChangeLog       |  7 +++++++
 posix/execvpe.c | 19 ++++++++++++-------
 2 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/posix/execvpe.c b/posix/execvpe.c
index d933f9c..96a12bf5 100644
--- a/posix/execvpe.c
+++ b/posix/execvpe.c
@@ -41,19 +41,20 @@ maybe_script_execute (const char *file, char *const argv[], char *const envp[])
   ptrdiff_t argc = 0;
   while (argv[argc++] != NULL)
     {
-      if (argc == INT_MAX - 1)
+      if (argc == INT_MAX - 2)
 	{
 	  errno = E2BIG;
 	  return;
 	}
     }
 
-  /* Construct an argument list for the shell.  */
-  char *new_argv[argc + 1];
+  /* 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 + 2];
   new_argv[0] = (char *) _PATH_BSHELL;
   new_argv[1] = (char *) file;
   if (argc > 1)
-    memcpy (new_argv + 2, argv + 1, argc * sizeof(char *));
+    memcpy (new_argv + 2, argv + 1, (argc - 1) * sizeof(char *));
   else
     new_argv[2] = NULL;
 
@@ -91,10 +92,11 @@ __execvpe (const char *file, char *const argv[], char *const envp[])
   /* Although GLIBC does not enforce NAME_MAX, we set it as the maximum
      size to avoid unbounded stack allocation.  Same applies for
      PATH_MAX.  */
-  size_t file_len = __strnlen (file, NAME_MAX + 1);
+  size_t file_len = __strnlen (file, NAME_MAX) + 1;
   size_t path_len = __strnlen (path, PATH_MAX - 1) + 1;
 
-  if ((file_len > NAME_MAX)
+  /* NAME_MAX does not include the terminating null character.  */
+  if (((file_len-1) > NAME_MAX)
       || !__libc_alloca_cutoff (path_len + file_len + 1))
     {
       errno = ENAMETOOLONG;
@@ -103,6 +105,9 @@ __execvpe (const char *file, char *const argv[], char *const envp[])
 
   const char *subp;
   bool got_eacces = false;
+  /* The resulting string maximum size would be potentially a entry
+     in PATH plus '/' (path_len + 1) and then the the resulting file name
+     plus '\0' (file_len since it already accounts for the '\0').  */
   char buffer[path_len + file_len + 1];
   for (const char *p = path; ; p = subp)
     {
@@ -123,7 +128,7 @@ __execvpe (const char *file, char *const argv[], char *const envp[])
          execute.  */
       char *pend = mempcpy (buffer, p, subp - p);
       *pend = '/';
-      memcpy (pend + (p < subp), file, file_len + 1);
+      memcpy (pend + (p < subp), file, file_len);
 
       __execve (buffer, argv, envp);
 
-- 
2.7.4