Merge bitcoin#15950: Do not construct out-of-bound pointers in SHA2 code

c01c065 Do not construct out-of-bound pointers in SHA512/SHA1/RIPEMD160 code (Pieter Wuille)

Pull request description:

  This looks like an issue in the current SHA256/512 code, where a pointer outside of the area pointed to may be constructed (this is UB in theory, though in practice every supported platform treats pointers as integers).

  I discovered this while investigating bitcoin#14580. Sadly, it does not fix it.

ACKs for commit c01c06:
  practicalswift:
    utACK c01c065

Tree-SHA512: 47660e00f164f38c36a1ab46e52dd91cd33cfda6a6048d67541c2f8e73c050d4d9d81b5c149bfad281212d52f204f57bebf5b19879dc7a6a5f48aa823fbc2c02

src/crypto/ripemd160.cpp

@@ -256,7 +256,7 @@ CRIPEMD160& CRIPEMD160::Write(const unsigned char* data, size_t len)
         ripemd160::Transform(s, buf);
         bufsize = 0;
     }
-    while (end >= data + 64) {
+    while (end - data >= 64) {
         // Process full chunks directly from the source.
         ripemd160::Transform(s, data);
         bytes += 64;

src/crypto/sha1.cpp

@@ -163,7 +163,7 @@ CSHA1& CSHA1::Write(const unsigned char* data, size_t len)
         sha1::Transform(s, buf);
         bufsize = 0;
     }
-    while (end >= data + 64) {
+    while (end - data >= 64) {
         // Process full chunks directly from the source.
         sha1::Transform(s, data);
         bytes += 64;

src/crypto/sha512.cpp

@@ -168,7 +168,7 @@ CSHA512& CSHA512::Write(const unsigned char* data, size_t len)
         sha512::Transform(s, buf);
         bufsize = 0;
     }
-    while (end >= data + 128) {
+    while (end - data >= 128) {
         // Process full chunks directly from the source.
         sha512::Transform(s, data);
         data += 128;