Fix CVE-2016-2381 (ambiguous handling of duplicated environment variables).

Copied from upstream:
http://perl5.git.perl.org/perl.git/commit/ae37b791a73a9e78dedb89fb2429d2628cf58076

References:
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-2381
http://www.nntp.perl.org/group/perl.perl5.porters/2016/03/msg234747.html
https://security-tracker.debian.org/tracker/CVE-2016-2381

---

From 1237ea93fb2475a5ae576d5ee1358a5bb4ebe426 Mon Sep 17 00:00:00 2001
From: Tony Cook <tony@develop-help.com>
Date: Wed, 27 Jan 2016 11:52:15 +1100
Subject: remove duplicate environment variables from environ

If we see duplicate environment variables while iterating over
environ[]:

a) make sure we use the same value in %ENV that getenv() returns.

Previously on a duplicate, %ENV would have the last entry for the name
from environ[], but a typical getenv() would return the first entry.

Rather than assuming all getenv() implementations return the first entry
explicitly call getenv() to ensure they agree.

b) remove duplicate entries from environ

Previously if there was a duplicate definition for a name in environ[]
setting that name in %ENV could result in an unsafe value being passed
to a child process, so ensure environ[] has no duplicates.

Patch-Name: fixes/CVE-2016-2381_duplicate_env.diff
---
 perl.c | 51 +++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 49 insertions(+), 2 deletions(-)

diff --git a/perl.c b/perl.c
index 67d32ce..26aeb91 100644
--- a/perl.c
+++ b/perl.c
@@ -4277,23 +4277,70 @@ S_init_postdump_symbols(pTHX_ int argc, char **argv, char **env)
 	}
 	if (env) {
 	  char *s, *old_var;
+          STRLEN nlen;
 	  SV *sv;
+          HV *dups = newHV();
+
 	  for (; *env; env++) {
 	    old_var = *env;
 
 	    if (!(s = strchr(old_var,'=')) || s == old_var)
 		continue;
+            nlen = s - old_var;
 
 #if defined(MSDOS) && !defined(DJGPP)
 	    *s = '\0';
 	    (void)strupr(old_var);
 	    *s = '=';
 #endif
-	    sv = newSVpv(s+1, 0);
-	    (void)hv_store(hv, old_var, s - old_var, sv, 0);
+            if (hv_exists(hv, old_var, nlen)) {
+                const char *name = savepvn(old_var, nlen);
+
+                /* make sure we use the same value as getenv(), otherwise code that
+                   uses getenv() (like setlocale()) might see a different value to %ENV
+                 */
+                sv = newSVpv(PerlEnv_getenv(name), 0);
+
+                /* keep a count of the dups of this name so we can de-dup environ later */
+                if (hv_exists(dups, name, nlen))
+                    ++SvIVX(*hv_fetch(dups, name, nlen, 0));
+                else
+                    (void)hv_store(dups, name, nlen, newSViv(1), 0);
+
+                Safefree(name);
+            }
+            else {
+                sv = newSVpv(s+1, 0);
+            }
+	    (void)hv_store(hv, old_var, nlen, sv, 0);
 	    if (env_is_not_environ)
 	        mg_set(sv);
 	  }
+          if (HvKEYS(dups)) {
+              /* environ has some duplicate definitions, remove them */
+              HE *entry;
+              hv_iterinit(dups);
+              while ((entry = hv_iternext_flags(dups, 0))) {
+                  STRLEN nlen;
+                  const char *name = HePV(entry, nlen);
+                  IV count = SvIV(HeVAL(entry));
+                  IV i;
+                  SV **valp = hv_fetch(hv, name, nlen, 0);
+
+                  assert(valp);
+
+                  /* try to remove any duplicate names, depending on the
+                   * implementation used in my_setenv() the iteration might
+                   * not be necessary, but let's be safe.
+                   */
+                  for (i = 0; i < count; ++i)
+                      my_setenv(name, 0);
+
+                  /* and set it back to the value we set $ENV{name} to */
+                  my_setenv(name, SvPV_nolen(*valp));
+              }
+          }
+          SvREFCNT_dec_NN(dups);
       }
 #endif /* USE_ENVIRON_ARRAY */
 #endif /* !PERL_MICRO */