wordexp: handle overflow in positional parameter number (bug 28011)

Use strtoul instead of atoi so that overflow can be detected.

(cherry picked from commit 5adda61f62)

NEWS

@@ -104,6 +104,10 @@ Security related changes:
   potentially resulting in degraded service or Denial of Service on the
   local system.  Reported by Chris Schanzle.
 
+  CVE-2021-35942: The wordexp function may overflow the positional
+  parameter number when processing the expansion resulting in a crash.
+  Reported by Philippe Antoine.
+
 The following bugs are resolved with this release:
 
   [6889] 'PWD' mentioned but not specified
@@ -201,6 +205,7 @@ The following bugs are resolved with this release:
   [26383] bind_textdomain_codeset doesn't accept //TRANSLIT anymore
   [26923] Assertion failure in iconv when converting invalid UCS4 (CVE-2020-29562)
   [27462] nscd: double-free in nscd (CVE-2021-27645)
+  [28011] libc: Wild read in wordexp (parse_param) (CVE-2021-35942)
 
 
 Version 2.27

posix/wordexp-test.c

@@ -200,6 +200,7 @@ struct test_case_struct
     { 0, NULL, "$var", 0, 0, { NULL, }, IFS },
     { 0, NULL, "\"\\n\"", 0, 1, { "\\n", }, IFS },
     { 0, NULL, "", 0, 0, { NULL, }, IFS },
+    { 0, NULL, "${1234567890123456789012}", 0, 0, { NULL, }, IFS },
 
     /* Flags not already covered (testit() has special handling for these) */
     { 0, NULL, "one two", WRDE_DOOFFS, 2, { "one", "two", }, IFS },

posix/wordexp.c

@@ -1407,7 +1407,7 @@ envsubst:
   /* Is it a numeric parameter? */
   else if (isdigit (env[0]))
     {
-      int n = atoi (env);
+      unsigned long n = strtoul (env, NULL, 10);
 
       if (n >= __libc_argc)
 	/* Substitute NULL. */