Limit special diagnostics size region to 4KiB.

This fixes out-of-bounds access when trying to write the
diagnostics info on platforms where the PAGE_SIZE is larger
than the DumpWriter's 16KiB m_tempBuffer.

src/coreclr/debug/createdump/crashinfo.cpp

@@ -195,7 +195,7 @@ CrashInfo::GatherCrashInfo(DumpType dumpType)
         return false;
     }
     // Add the special (fake) memory region for the special diagnostics info
-    MemoryRegion special(PF_R, SpecialDiagInfoAddress, SpecialDiagInfoAddress + PAGE_SIZE);
+    MemoryRegion special(PF_R, SpecialDiagInfoAddress, SpecialDiagInfoAddress + SpecialDiagInfoSize);
     m_memoryRegions.insert(special);
 #ifdef __APPLE__
     InitializeOtherMappings();

src/coreclr/debug/createdump/specialdiaginfo.h

@@ -24,6 +24,8 @@ const uint64_t SpecialDiagInfoAddress = 0x7fff1000;
 #endif
 #endif
 
+const uint64_t SpecialDiagInfoSize = 0x1000;
+
 struct SpecialDiagInfoHeader
 {
     char Signature[16];