VirtualAlloc trouble

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Teehee

Joined: 05 Aug 2009
Posts: 570
Location: Brazil

Teehee 20 Dec 2009, 11:00

what's wrong in code below? VirtualAlloc seems does not work...

Code:

format PE GUI 4.0
entry _start
include 'win32ax.inc'
section '.data' data readable writeable
        stA dd ?

section '.text' code readable executable
_start:
        invoke VirtualAlloc,0,8192,MEM_COMMIT,PAGE_READWRITE ; <-- I think it doesn't allocating...
        mov    [stA], eax

        mov    ecx, 0
    @@: mov    [stA+ecx*4], 'abcd'
        inc    ecx
        cmp    ecx, 1023        ; <--- crashes if more than 1024!
        jne    @b
        mov    [stA+ecx*4],0

        invoke VirtualFree,stA,8192,MEM_RELEASE ; <-- also it's not releasing

        invoke  MessageBox,NULL,stA,NULL,MB_OK

  exit:
        invoke  ExitProcess,0

section '.idata' import data readable writeable
    library kernel32,'KERNEL32.DLL', user32,'USER32.DLL'
    include 'API\KERNEL32.INC'
    include 'API\USER32.INC'

_________________
Sorry if bad english.

20 Dec 2009, 11:00

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20298
Location: In your JS exploiting you and your system

revolution 20 Dec 2009, 11:04

the problem is here:

Code:

    @@: mov    [stA+ecx*4], 'abcd'
...
    @@: mov    [stA+ecx*4], 'abcd'

Your stA buffer is too small.

Perhaps you meant:

Code:

    @@: mov    [eax+ecx*4], 'abcd'
...
    @@: mov    [eax+ecx*4], 'abcd'
        invoke VirtualFree,[stA],8192,MEM_RELEASE ; <-- also it's not releasing

20 Dec 2009, 11:04

Teehee

Joined: 05 Aug 2009
Posts: 570
Location: Brazil

Teehee 20 Dec 2009, 11:13

Quote:

Your stA buffer is too small

But I alloc 8192 bytes...

It still doesn't working... Sad

20 Dec 2009, 11:13

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20298
Location: In your JS exploiting you and your system

revolution 20 Dec 2009, 11:14

No, you allocate just 4 bytes.

The 8192 byte allocation is never used in your original code.

20 Dec 2009, 11:14

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20298
Location: In your JS exploiting you and your system

revolution 20 Dec 2009, 11:17

Also check the documentation for VirtualFree:

Quote:

If the dwFreeType parameter includes the MEM_RELEASE flag, this parameter (dwSize) must be zero.

Code:

        invoke VirtualFree,[stA],0,MEM_RELEASE

20 Dec 2009, 11:17

Teehee

Joined: 05 Aug 2009
Posts: 570
Location: Brazil

Teehee 20 Dec 2009, 11:18

So how can I use the 8192 bytes?

_________________
Sorry if bad english.

20 Dec 2009, 11:18

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20298
Location: In your JS exploiting you and your system

revolution 20 Dec 2009, 11:18

Teehee wrote:

So how can I use the 8192 bytes?

In my first reply I show where to change 'stA' to 'eax'

20 Dec 2009, 11:18

Teehee

Joined: 05 Aug 2009
Posts: 570
Location: Brazil

Teehee 20 Dec 2009, 11:21

but and this line:

Code:

mov    [stA], eax

?
stA doenst points to eax?

20 Dec 2009, 11:21

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20298
Location: In your JS exploiting you and your system

revolution 20 Dec 2009, 11:22

But then you overwrite it with this:

Code:

    @@: mov    [stA+ecx*4], 'abcd'

When ecx=0 the value at [stA] is overwritten with 'abcd'.

20 Dec 2009, 11:22

Teehee

Joined: 05 Aug 2009
Posts: 570
Location: Brazil

Teehee 20 Dec 2009, 11:27

oh.. you right! thanks Smile

now it works Very Happy

20 Dec 2009, 11:27

Teehee

Joined: 05 Aug 2009
Posts: 570
Location: Brazil

Teehee 20 Dec 2009, 11:41

here:

Code:

        invoke VirtualAlloc,0,8000,MEM_COMMIT,PAGE_READWRITE
        mov    [stA], eax

        mov    ecx, 0
    @@: mov    [eax+ecx], byte 'a'
        inc    ecx
        cmp    ecx,8191        
        jne    @b
        mov    [eax+ecx], dword 0

        invoke  MessageBox,NULL,[stA],NULL,MB_OK
        invoke VirtualFree,[stA],0,MEM_RELEASE

I alloced 8000 bytes, but I can go till 8191 bytes. Why?

20 Dec 2009, 11:41

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20298
Location: In your JS exploiting you and your system

revolution 20 Dec 2009, 11:43

Teehee wrote:

I alloced 8000 bytes, but I can go till 8191 bytes. Why?

Minimum allocation granularity in Windows is 4k. All allocations are in steps of 4096 bytes.

20 Dec 2009, 11:43

Teehee

Joined: 05 Aug 2009
Posts: 570
Location: Brazil

Teehee 20 Dec 2009, 11:43

bc I need to alloc pow 2 space?

20 Dec 2009, 11:43

Teehee

Joined: 05 Aug 2009
Posts: 570
Location: Brazil

Teehee 20 Dec 2009, 11:44

ohh ok

20 Dec 2009, 11:44

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20298
Location: In your JS exploiting you and your system

revolution 20 Dec 2009, 11:45

It is related to the hardware paging in the CPU. You can't page less than 4k in the x86 CPU.

20 Dec 2009, 11:45

Teehee

Joined: 05 Aug 2009
Posts: 570
Location: Brazil

Teehee 20 Dec 2009, 11:52

now I understand Smile

THANK YOU VERY MUCH!

20 Dec 2009, 11:52

Teehee

Joined: 05 Aug 2009
Posts: 570
Location: Brazil

Teehee 20 Dec 2009, 12:03

One more question. How can I reallocate space?

ex:
1. untill 4096 bytes you fill with 'a'.
2. realloc to 8192.
3. 4096 untill 8192 byte you fill with 'b'.

20 Dec 2009, 12:03

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20298
Location: In your JS exploiting you and your system

revolution 20 Dec 2009, 12:09

You can't realloc virtual memory in any automatic way.

You have to allocate a new piece of memory. Copy in existing data. And then free the original memory.

20 Dec 2009, 12:09

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20298
Location: In your JS exploiting you and your system

revolution 20 Dec 2009, 12:13

Consider using VirtualAlloc to reserve space and just commit when you need it.

Or, consider using the LocalAlloc functions instead of the Virtual* functions.

20 Dec 2009, 12:13

Teehee

Joined: 05 Aug 2009
Posts: 570
Location: Brazil

Teehee 20 Dec 2009, 12:17

so i need to copy all bytes to the new buffer, then fill the rest with 'b' and release the last buffer? that's seems so slow Confused

20 Dec 2009, 12:17

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