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August 30, 2010

Theories of how PS Jailbreak works

Filed under: Embedded,Hacking,Reverse engineering,Security — Nate Lawson @ 5:59 pm

A company recently announced a modchip for the PS3 and claims they will be shipping them soon. It plugs into the USB port and allows running backup games. Inside the device is an ATmega8U2 USB microcontroller, something I’ve worked with before. While I didn’t have access to this device and don’t know PS3 internals, I spent a few minutes looking at a hex dump from a USB trace.

An article on Gamefreax (German, alternate in English)¬†claimed to have reverse-engineered this device. They believe that a stack overflow in USB config descriptor processing allows the device to execute code on the PS3 host. However, their page shows a useless set of packets from a USB trace and some hex code obscured with a logo so it’s not possible to verify their claims from the info given.

In a later comment, a user named Descrambler posted a more complete dump of hex data from the USB trace. It is still not fully complete, but it’s enough to look into more of the details.

The first trace starts with a standard configuration descriptor. It is followed by a relatively standard interface descriptor, except it is class 254, subclass 1, and protocol 2. Since this is in the reserved class range, perhaps this is related to an internal Sony test tool. Following this descriptor is some data and PowerPC code.

The second trace starts with a config descriptor that is a bit weird. It claims the total length is 77 bytes (68 bytes of interface descriptors after the 9-byte config descriptor). It also claims to support 10 interfaces. With the standard interface descriptor length of 9 bytes, the total length should be 99 bytes.

There are multiple ways this might affect the PS3. If it believes the total length field for sizing the buffer, the first bytes after the initial 77 could overflow a buffer (in this case, “00 00 fe 01 02 00 09 04″ are what follows). Or it might simply copy it into a static buffer (if 256 bytes long, the first bytes to overflow would be “fe 01 02 00 09 04 00 00″).

If it’s not an overflow, it could be related to how the PS3 parses the first 10 interface descriptors. The sequence is not regular. Taken in 9-byte chunks, it diverges after the first 6 interface descriptors, giving a next descriptor of “09 00 09 04 00 00 00 fe 01″. This is not a valid descriptor. Or, if the PS3 parses lengths of descriptors, it will end up with a few very short ones (“02 00″).

These are all just theories. It’s quite possible the second trace is just a decoy, meant to slow down reversing. The behavior described by Gamefreax cannot be validated from the USB traces posted by Descrambler. It appears Gamefreax may have misread the trace (77 bytes in the second trace is 0x4d, but they claim a descriptor length is 0xAD). Also, Descrambler’s hex dumps are incomplete and don’t show the various phases described in the Gamefreax post.

It’s definitely too early to claim that the PS Jailbreak exploit has been reverse-engineered. However, it should be quite easy to clone since all the data needed to do so is present in a USB trace. Just paste the data into example code for an AT90USB and replay the same descriptors. You might have to add in a bus disconnect in the right place but it should be relatively simple.

7 Comments

  1. The device presents a 6 port hub, and various devices with variously corrupted descriptors are plugged in and removed in a particular order, with a few transfers thrown in. It’s far from “relatively simple” but it is possible to get most AT90USB series chips to behave enough like a USB hub for it to be cloned.

    Comment by Jim — August 31, 2010 @ 12:55 am

    • The easiest way is to just lift the firmware wholesale and drop it into another ATmega8U2. The price on the original has already dropped from $170 to $120.

      Comment by Nate Lawson — August 31, 2010 @ 9:06 am

  2. The big question now is how quickly will Sony find and patch the bug?

    Comment by Mike Hearn — August 31, 2010 @ 2:08 am

    • I guess less than 1 month. The bigger question is whether there are other similar holes lurking in this area that they’ll miss. If so, it could be easy for PS Jailbreak to ship a firmware update to their microcontroller to exploit those too.

      Comment by Nate Lawson — August 31, 2010 @ 9:08 am

      • Well, it could be easy to ship, right, but only if you got anything to ship :)

        Comment by Break — August 31, 2010 @ 10:35 am

  3. http://github.com/psgroove/psgroove

    Comment by Rui Paulo — September 1, 2010 @ 5:13 am

    • Thanks, Rui. I also saw this on the Reverse Engineering reddit.

      Comment by Nate Lawson — September 1, 2010 @ 2:16 pm


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