Fun With Thread Local Storage (part 2)

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Fun With Thread Local Storage (part 2)
Peter Ferrie
Senior Anti-virus Researcher
26 June, 2008
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2.

You Can Call Me Al
Thread Local Storage callbacks were discovered in 2000.
However, widespread use didn’t occur until 2004.
Now, it should be the first place to look for code,
since it runs before the main entrypoint.
And that can make all the difference…
Peter Ferrie, Microsoft Corporation
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Empty!
Entry Point
Peter Ferrie, Microsoft Corporation
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4.

Empty!
C3 RET
Peter Ferrie, Microsoft Corporation
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Empty!
So the main file does nothing.
If we assume that the structure is normal,
then we could check the thread local storage table.
Just in case.
Peter Ferrie, Microsoft Corporation
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Empty!
TLS is present
(size doesn’t matter)
Peter Ferrie, Microsoft Corporation
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Empty!
Callback pointer
Peter Ferrie, Microsoft Corporation
Callback array
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Empty!
So the search moves to the callbacks,
of which there is only one... or is there?
Peter Ferrie, Microsoft Corporation
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The One and Only
Peter Ferrie, Microsoft Corporation
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10.

Am I Missing Something?
CODE:00401010
CODE:00401005
CODE:0040100A
push offset LibFileName ; "tls2"
call
j_LoadLibraryA
mov ds:TlsCallbacksEnd, eax
Hmm, LoadLibrary("tls2")
Maybe DllMain contains something interesting?
Peter Ferrie, Microsoft Corporation
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11.

I Am Missing Something
No, that’s not it.
Peter Ferrie, Microsoft Corporation
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12.

Take 2
Let’s revisit the code:
CODE:00401010
CODE:00401005
CODE:0040100A
Peter Ferrie, Microsoft Corporation
push offset LibFileName ; "tls2"
call
j_LoadLibraryA
mov ds:TlsCallbacksEnd, eax
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13.

It’s All About Image
It’s the TlsCallBacks extended array trick again.
Q. What value does the new callback contain?
A. The DLL’s imagebase.
Q. DEP won’t let that run, right?
A. …
Peter Ferrie, Microsoft Corporation
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Surprise!
Peter Ferrie, Microsoft Corporation
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Not OK
Of course it will.
You just have to ask nicely.
Or take a cue from a driver.
Peter Ferrie, Microsoft Corporation
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16.

Chaotic-Evil
When the SectionAlignment value is less than 4kb,
the file header is marked Writable and Executable
(unless the last section characteristics override it).
That turns the ImageBase into code.
Peter Ferrie, Microsoft Corporation
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Before
So we go from this…
Peter Ferrie, Microsoft Corporation
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After
To this…
Peter Ferrie, Microsoft Corporation
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Presto!
Peter Ferrie, Microsoft Corporation
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Really Not OK
Just a little something to add to the workload.
Peter Ferrie, Microsoft Corporation
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