Land #2467, @wchen-r7's code to allow dynamic size paylods on ropdb

data/ropdb/hxds.xml

@@ -11,7 +11,7 @@
 		<gadget offset="0x0001803c">POP EBP # RETN</gadget>
 		<gadget offset="0x0001803c">skip 4 bytes</gadget>
 		<gadget offset="0x0001750f">POP EBX # RETN</gadget>
-		<gadget value="fffffdff">0x00000201</gadget>
+		<gadget value="safe_negate_size">Safe size to NEG</gadget>
 		<gadget offset="0x00005737">XCHG EAX, EBX # RETN</gadget>
 		<gadget offset="0x0004df88">NEG EAX # RETN</gadget>
 		<gadget offset="0x00005737">XCHG EAX, EBX # RETN</gadget>
@@ -40,7 +40,7 @@
 		<gadget offset="0x0003e4fa">POP EBP # RETN</gadget>
 		<gadget offset="0x0003e4fa">skip 4 bytes</gadget>
 		<gadget offset="0x0006a2b4">POP EBX # RETN</gadget>
-		<gadget value="fffffdff">0x00000201</gadget>
+		<gadget value="safe_negate_size">Safe size to NEG</gadget>
 		<gadget offset="0x00069351">XCHG EAX, EBX # RETN</gadget>
 		<gadget offset="0x00025188">NEG EAX # POP ESI # RETN</gadget>
 		<gadget value="junk">JUNK</gadget>

data/ropdb/java.xml

@@ -9,7 +9,7 @@
 		<gadget offset="0x00024c66">POP EBP # RETN</gadget>
 		<gadget offset="0x00024c66">skip 4 bytes</gadget>
 		<gadget offset="0x00004edc">POP EAX # RETN</gadget>
-		<gadget value="FFFFFBFF">0x00000201</gadget>
+		<gadget value="safe_negate_size">0x00000201</gadget>
 		<gadget offset="0x00011e05">NEG EAX # RETN</gadget>
 		<gadget offset="0x000136e3">POP EBX # RETN</gadget>
 		<gadget value="0xffffffff"></gadget>

data/ropdb/msvcrt.xml

@@ -8,7 +8,7 @@
 
 	<gadgets base="0x77c10000">
 		<gadget offset="0x0002b860">POP EAX # RETN</gadget>
-		<gadget value="0xFFFFFBFF">0xFFFFFBFF -> ebx</gadget>
+		<gadget value="safe_negate_size">0xFFFFFBFF -> ebx</gadget>
 		<gadget offset="0x0000be18">NEG EAX # POP EBP # RETN</gadget>
 		<gadget value="junk">JUNK</gadget>
 		<gadget offset="0x0001362c">POP EBX # RETN</gadget>

lib/rex/exploitation/ropdb.rb

@@ -29,7 +29,7 @@ class RopDb
   #
   # Returns an array of ROP gadgets. Each gadget can either be an offset, or a value (symbol or
   # some integer).  When the value is a symbol, it can be one of these: :nop, :junk, :size,
-  # and :size_negate.
+  # :unsafe_negate_size, and :safe_negate_size
   # Note if no RoP is found, it returns an empry array.
   # Arguments:
   # rop_name - name of the ROP chain.
@@ -90,8 +90,10 @@ class RopDb
         Rex::Text.rand_text(4, badchars).unpack("V")[0].to_i
       elsif e == :size
         payload.length
-      elsif e == :size_negate
-        0xffffffff - payload.length + 1
+      elsif e == :unsafe_negate_size
+        get_unsafe_size(payload.length)
+      elsif e == :safe_negate_size
+        get_safe_size(payload.length)
       else
         e
       end
@@ -105,6 +107,28 @@ class RopDb
   private
 
 
+  #
+  # Returns a size that's safe from null bytes.
+  # This function will keep incrementing the value of "s" until it's safe from null bytes.
+  #
+  def get_safe_size(s)
+    safe_size = get_unsafe_size(s)
+    while (safe_size.to_s(16).rjust(8, '0')).scan(/../).include?("00")
+      safe_size -= 1
+    end
+
+    safe_size
+  end
+
+
+  #
+  # Returns a size that might contain one or more null bytes
+  #
+  def get_unsafe_size(s)
+    0xffffffff - s + 1
+  end
+
+
   #
   # Checks if a ROP chain is compatible
   #
@@ -146,8 +170,10 @@ class RopDb
           gadgets << :junk
         when 'size'
           gadgets << :size
-        when 'size_negate'
-          gadgets << :size_negate
+        when 'unsafe_negate_size'
+          gadgets << :unsafe_negate_size
+        when 'safe_negate_size'
+          gadgets << :safe_negate_size
         else
           gadgets << value.to_i(16)
         end

spec/lib/rex/exploitation/ropdb_spec.rb

@@ -0,0 +1,91 @@
+require 'rex/exploitation/ropdb'
+
+describe Rex::Exploitation::RopDb do
+  context "Class methods" do
+
+    context ".initialize" do
+      it "should initialize with a path of the ROP database ready" do
+        ropdb = Rex::Exploitation::RopDb.new
+        ropdb.instance_variable_get(:@base_path).should =~ /data\/ropdb\/$/
+      end
+    end
+
+    context ".has_rop?" do
+      ropdb = Rex::Exploitation::RopDb.new
+
+      it "should find the msvcrt ROP database" do
+        ropdb.has_rop?("msvcrt").should eq(true)
+      end
+
+      it "should find the java ROP database" do
+        ropdb.has_rop?("java").should eq(true)
+      end
+
+      it "should find the hxds ROP database" do
+        ropdb.has_rop?("hxds").should eq(true)
+      end
+
+      it "should find the flash ROP database" do
+        ropdb.has_rop?("flash").should eq(true)
+      end
+
+      it "should return false when I supply an invalid database" do
+        ropdb.has_rop?("sinn3r").should eq(false)
+      end
+    end
+
+    context ".select_rop" do
+      ropdb = Rex::Exploitation::RopDb.new
+
+      it "should return msvcrt gadgets" do
+        gadgets = ropdb.select_rop('msvcrt')
+        gadgets.length.should > 0
+      end
+
+      it "should return msvcrt gadgets for windows server 2003" do
+        gadgets = ropdb.select_rop('msvcrt', {'target'=>'2003'})
+        gadgets.length.should > 0
+      end
+
+      it "should return msvcrt gadgets with a new base" do
+        gadgets1 = ropdb.select_rop('msvcrt')
+        gadgets2 = ropdb.select_rop('msvcrt', {'base'=>0x10000000})
+
+        gadgets2[0].should_not eq(gadgets1[0])
+      end
+    end
+
+    context ".generate_rop_payload" do
+      ropdb = Rex::Exploitation::RopDb.new
+
+      it "should generate my ROP payload" do
+        ropdb.generate_rop_payload('msvcrt', 'AAAA').should =~ /AAAA$/
+      end
+
+      it "should generate my ROP payload with my stack pivot" do
+        ropdb.generate_rop_payload('msvcrt', 'AAAA', {'pivot'=>'BBBB'}).should =~ /^BBBB/
+      end
+    end
+
+    context ".get_safe_size" do
+      ropdb = Rex::Exploitation::RopDb.new
+
+      it "should return 0xfffffed0 (value does not need to be modified to avoid null bytes)" do
+        ropdb.send(:get_safe_size, 304).should eq(0xfffffed0)
+      end
+
+      it "should return 0xfffffeff (value is modified to avoid null bytes)" do
+        ropdb.send(:get_safe_size, 256).should eq(0xfffffeff)
+      end
+    end
+
+    context ".get_unsafe_size" do
+      ropdb = Rex::Exploitation::RopDb.new
+
+      it "should return 0xfffffc00 (contains a null byte)" do
+        ropdb.send(:get_unsafe_size, 1024).should eq(0xfffffc00)
+      end
+    end
+
+  end
+end