nscd: Fix double free in netgroupcache [BZ #27462]

In commit 745664b a use-after-free
was fixed, but this led to an occasional double-free.  This patch
tracks the "live" allocation better.

Tested manually by a third party.

Related: RHBZ 1927877

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
(cherry picked from commit dca5658)

NEWS

@@ -99,6 +99,11 @@ Security related changes:
   CVE-2020-29562: An assertion failure has been fixed in the iconv function
   when invoked with UCS4 input containing an invalid character.
 
+  CVE-2021-27645: The nameserver caching daemon (nscd), when processing
+  a request for netgroup lookup, may crash due to a double-free,
+  potentially resulting in degraded service or Denial of Service on the
+  local system.  Reported by Chris Schanzle.
+
 The following bugs are resolved with this release:
 
   [6889] 'PWD' mentioned but not specified
@@ -195,6 +200,7 @@ The following bugs are resolved with this release:
     character sets (CVE-2020-27618)
   [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)
 
 
 Version 2.27

nscd/netgroupcache.c

@@ -248,7 +248,7 @@ addgetnetgrentX (struct database_dyn *db, int fd, request_header *req,
 					     : NULL);
 				    ndomain = (ndomain ? newbuf + ndomaindiff
 					       : NULL);
-				    buffer = newbuf;
+				    *tofreep = buffer = newbuf;
 				  }
 
 				nhost = memcpy (buffer + bufused,
@@ -319,7 +319,7 @@ addgetnetgrentX (struct database_dyn *db, int fd, request_header *req,
 		    else if (status == NSS_STATUS_TRYAGAIN && e == ERANGE)
 		      {
 			buflen *= 2;
-			buffer = xrealloc (buffer, buflen);
+			*tofreep = buffer = xrealloc (buffer, buflen);
 		      }
 		    else if (status == NSS_STATUS_RETURN
 			     || status == NSS_STATUS_NOTFOUND