From 83cd53c9b1c52498d62fb6d501396ff782037c70 Mon Sep 17 00:00:00 2001
From: "alexey.lysiuk" <alexey.lysiuk@gmail.com>
Date: Sat, 6 Jan 2018 13:26:51 +0200
Subject: [PATCH] Added partial bounds check for instruction pointer in ACS VM

This change adds verification of instruction pointer for indirect usages only, i.e. via NEXT...() macros
---
 src/p_acs.cpp | 45 ++++++++++++++++++++++++++++++---------------
 1 file changed, 30 insertions(+), 15 deletions(-)

diff --git a/src/p_acs.cpp b/src/p_acs.cpp
index 7ef6f56839..19f9de486d 100644
--- a/src/p_acs.cpp
+++ b/src/p_acs.cpp
@@ -761,6 +761,35 @@ protected:
 private:
 	DLevelScript();
 
+	int getbyte(int *&pc)
+	{
+		CheckInstructionPointer(pc);
+
+		int res = *(uint8_t *)pc;
+		pc = (int *)((uint8_t *)pc+1);
+		return res;
+	}
+
+	int getshort(int *&pc)
+	{
+		CheckInstructionPointer(pc);
+
+		int res = LittleShort( *(int16_t *)pc);
+		pc = (int *)((uint8_t *)pc+2);
+		return res;
+	}
+
+	void CheckInstructionPointer(int *pc) const
+	{
+		const uint32_t offset = activeBehavior->PC2Ofs(pc);
+		const uint32_t size = activeBehavior->GetDataSize();
+
+		if (offset >= size)
+		{
+			I_Error("Out of bounds instruction pointer in ACS VM");
+		}
+	}
+
 	friend class DACSThinker;
 };
 
@@ -6896,7 +6925,7 @@ enum
 };
 
 
-#define NEXTWORD	(LittleLong(*pc++))
+#define NEXTWORD	(CheckInstructionPointer(pc), LittleLong(*pc++))
 #define NEXTBYTE	(fmt==ACS_LittleEnhanced?getbyte(pc):NEXTWORD)
 #define NEXTSHORT	(fmt==ACS_LittleEnhanced?getshort(pc):NEXTWORD)
 #define STACK(a)	(Stack[sp - (a)])
@@ -6904,20 +6933,6 @@ enum
 // Direct instructions that take strings need to have the tag applied.
 #define TAGSTR(a)	(a|activeBehavior->GetLibraryID())
 
-inline int getbyte (int *&pc)
-{
-	int res = *(uint8_t *)pc;
-	pc = (int *)((uint8_t *)pc+1);
-	return res;
-}
-
-inline int getshort (int *&pc)
-{
-	int res = LittleShort( *(int16_t *)pc);
-	pc = (int *)((uint8_t *)pc+2);
-	return res;
-}
-
 static bool CharArrayParms(int &capacity, int &offset, int &a, FACSStackMemory& Stack, int &sp, bool ranged)
 {
 	if (ranged)