From 0adcb34c49bee5b19bd29b16a578c510c23597ea Mon Sep 17 00:00:00 2001
From: Sebastian Pipping <sebastian@pipping.org>
Date: Mon, 27 Dec 2021 20:15:02 +0100
Subject: [PATCH] lib: Detect and prevent troublesome left shifts in function
 storeAtts (CVE-2021-45960)

---
 expat/lib/xmlparse.c | 31 +++++++++++++++++++++++++++++--
 1 file changed, 29 insertions(+), 2 deletions(-)

diff --git a/lib/xmlparse.c b/lib/xmlparse.c
index d730f41c3..b47c31b05 100644
--- a/lib/xmlparse.c
+++ b/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 +3431,28 @@ 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.
+       * The preprocessor guard addresses the "always false" warning
+       * from -Wtype-limits on platforms where
+       * sizeof(unsigned int) < sizeof(size_t), e.g. on x86_64. */
+#if UINT_MAX >= SIZE_MAX
+      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) {