Fix multiple heap-based buffer overflows in CmtkLoader::load()

Changes in src/mtk.cpp for loading files:
* Fail early if the (decompressed) size is too small to hold
  mtkdata minus patterns. That avoids attempts to copy data
  from beyond allocated memory.
* In the data decompression section, there are multiple cases
  where the code actually has checks for available space before
  copying data, but the size of the copy is increased after
  the check, so a buffer overflow is still possible (issue #90).
  Fix that by moving the check after the size computation,
  and also check for a valid source offset where applicable.
* Also add several checks whether source data is exhausted
  during decompession, so
* When copying the patterns, don't copy more data than the
  "pattern" array can hold.

In src/mtk.h, method getinstrument(), check for valid instrument
number to avoid accessing the array with an invalid index.

This commit fixes CVE-2019-14734.

Fixes: #90

src/mtk.cpp

@@ -39,6 +39,8 @@ bool CmtkLoader::load(const std::string &filename, const CFileProvider &fp)
   struct mtkdata {
     char songname[34],composername[34],instname[0x80][34];
     unsigned char insts[0x80][12],order[0x80],dummy,patterns[0x32][0x40][9];
+    // HSC pattern has different type and size from patterns, but that
+    // doesn't matter much since we memcpy() the data. Still confusing.
   } *data;
   unsigned char *cmp,*org;
   unsigned int i;
@@ -51,8 +53,10 @@ bool CmtkLoader::load(const std::string &filename, const CFileProvider &fp)
   header.size = f->readInt(2);
 
   // file validation section
-  if(strncmp(header.id,"mpu401tr\x92kk\xeer@data",18))
-    { fp.close(f); return false; }
+  if (memcmp(header.id, "mpu401tr\x92kk\xeer@data", 18) ||
+      header.size < sizeof(*data) - sizeof(data->patterns)) {
+    fp.close(f); return false;
+  }
 
   // load section
   cmpsize = fp.filesize(f) - 22;
@@ -64,6 +68,7 @@ bool CmtkLoader::load(const std::string &filename, const CFileProvider &fp)
   while(cmpptr < cmpsize) {	// decompress
     ctrlmask >>= 1;
     if(!ctrlmask) {
+      if (cmpptr + 2 > cmpsize) goto err;
       ctrlbits = cmp[cmpptr] + (cmp[cmpptr + 1] << 8);
       cmpptr += 2;
       ctrlmask = 0x8000;
@@ -81,37 +86,40 @@ bool CmtkLoader::load(const std::string &filename, const CFileProvider &fp)
     cmd = (cmp[cmpptr] >> 4) & 0x0f;
     cnt = cmp[cmpptr] & 0x0f;
     cmpptr++;
+    if (cmpptr >= cmpsize) goto err;
     switch(cmd) {
     case 0:
-      if(orgptr + cnt > header.size) goto err;
       cnt += 3;
+      if (orgptr + cnt > header.size) goto err;
       memset(&org[orgptr],cmp[cmpptr],cnt);
       cmpptr++; orgptr += cnt;
       break;
 
     case 1:
-      if(orgptr + cnt > header.size) goto err;
       cnt += (cmp[cmpptr] << 4) + 19;
+      if (orgptr + cnt > header.size || cmpptr >= cmpsize) goto err;
       memset(&org[orgptr],cmp[++cmpptr],cnt);
       cmpptr++; orgptr += cnt;
       break;
 
     case 2:
-      if(orgptr + cnt > header.size) goto err;
+      if (cmpptr + 2 > cmpsize) goto err;
       offs = (cnt+3) + (cmp[cmpptr] << 4);
       cnt = cmp[++cmpptr] + 16; cmpptr++;
+      if (orgptr + cnt > header.size || offs > orgptr) goto err;
       memcpy(&org[orgptr],&org[orgptr - offs],cnt);
       orgptr += cnt;
       break;
 
     default:
-      if(orgptr + cmd > header.size) goto err;
       offs = (cnt+3) + (cmp[cmpptr++] << 4);
+      if (orgptr + cmd > header.size || offs > orgptr) goto err;
       memcpy(&org[orgptr],&org[orgptr-offs],cmd);
       orgptr += cmd;
       break;
     }
   }
+  // orgptr should match header.size; add a check?
   delete [] cmp;
   data = (struct mtkdata *) org;
 
@@ -123,12 +131,14 @@ bool CmtkLoader::load(const std::string &filename, const CFileProvider &fp)
     strncpy(instname[i],data->instname[i]+1,33);
   memcpy(instr,data->insts,0x80 * 12);
   memcpy(song,data->order,0x80);
-  memcpy(patterns,data->patterns,header.size-6084);
   for (i=0;i<128;i++) {				// correct instruments
     instr[i][2] ^= (instr[i][2] & 0x40) << 1;
     instr[i][3] ^= (instr[i][3] & 0x40) << 1;
     instr[i][11] >>= 4;		// make unsigned
   }
+  cnt = header.size - (sizeof(*data) - sizeof(data->patterns)); // was off by 1
+  if (cnt > sizeof(patterns)) cnt = sizeof(patterns); // fail?
+  memcpy(patterns, data->patterns, cnt);
 
   delete [] org;
   rewind(0);

src/mtk.h

@@ -43,7 +43,7 @@ class CmtkLoader: public ChscPlayer
   unsigned int getinstruments()
     { return 128; };
   std::string getinstrument(unsigned int n)
-    { return std::string(instname[n]); };
+    { return n < 128 ? std::string(instname[n]) : std::string(); };
 
  private:
   char title[34],composer[34],instname[0x80][34];