lib: Detect and prevent troublesome left shifts in function storeAtts

expat/Changes

@@ -3,9 +3,19 @@ NOTE: We are looking for help with a few things:
       If you can help, please get in touch.  Thanks!
 
 Release x.x.x xxx xxxxxxxx xx xxxx
+        Security fixes:
+       #531 #534  Fix issues with left shifts by >=29 places
+                    for XML documents with >=2^27+1 prefixed attributes
+                    on a single XML tag a la
+                    "<r xmlns:a='[..]' a:a123='[..]' [..] />".
+                    (With xmlwf, it takes argument -n to trigger.)
+
         Other changes:
        #527 #528  Address compiler warnings
 
+        Special thanks to:
+            Tyson Smith
+
 Release 2.4.2 Sun December 19 2021
         Other changes:
        #509 #510  Link againgst libm for function "isnan"

expat/lib/xmlparse.c

@@ -3414,7 +3414,13 @@ storeAtts(XML_Parser parser, const ENCODING *enc, const char *attStr,
   if (nPrefixes) {
     int j; /* hash table index */
     unsigned long version = parser->m_nsAttsVersion;
-    int nsAttsSize = (int)1 << parser->m_nsAttsPower;
+
+    /* Detect and prevent invalid shift */
+    if (parser->m_nsAttsPower >= sizeof(unsigned int) * 8 /* bits per byte */) {
+      return XML_ERROR_NO_MEMORY;
+    }
+
+    unsigned int nsAttsSize = 1u << parser->m_nsAttsPower;
     unsigned char oldNsAttsPower = parser->m_nsAttsPower;
     /* size of hash table must be at least 2 * (# of prefixed attributes) */
     if ((nPrefixes << 1)
@@ -3425,7 +3431,25 @@ storeAtts(XML_Parser parser, const ENCODING *enc, const char *attStr,
         ;
       if (parser->m_nsAttsPower < 3)
         parser->m_nsAttsPower = 3;
-      nsAttsSize = (int)1 << parser->m_nsAttsPower;
+
+      /* Detect and prevent invalid shift */
+      if (parser->m_nsAttsPower >= sizeof(nsAttsSize) * 8 /* bits per byte */) {
+        /* Restore actual size of memory in m_nsAtts */
+        parser->m_nsAttsPower = oldNsAttsPower;
+        return XML_ERROR_NO_MEMORY;
+      }
+
+      nsAttsSize = 1u << parser->m_nsAttsPower;
+
+      /* Detect and prevent integer overflow */
+#if UINT_MAX >= SIZE_MAX // to silence "always false" -Wtype-limits warning
+      if (nsAttsSize > (size_t)(-1) / sizeof(NS_ATT)) {
+        /* Restore actual size of memory in m_nsAtts */
+        parser->m_nsAttsPower = oldNsAttsPower;
+        return XML_ERROR_NO_MEMORY;
+      }
+#endif
+
       temp = (NS_ATT *)REALLOC(parser, parser->m_nsAtts,
                                nsAttsSize * sizeof(NS_ATT));
       if (! temp) {