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TUCoPS :: Games :: c07-2489.htm

Xbox 360 Hypervisor Privilege Escalation Vulnerability



Xbox 360 Hypervisor Privilege Escalation Vulnerability
Xbox 360 Hypervisor Privilege Escalation Vulnerability



Security Advisory

Xbox 360 Hypervisor Privilege Escalation Vulnerability


Release Date:
  February 28, 2007


Author:
Anonymous Hacker  


Timeline:
  Oct 31, 2006 - release of 4532 kernel, which is the first version
                 containing the bug
  Nov 16, 2006 - proof of concept completed; unsigned code running in
                 hypervisor context
  Nov 30, 2006 - release of 4548 kernel, bug still not fixed
  Dec 15, 2006 - first attempt to contact vendor to report bug
  Dec 30, 2006 - public demonstration
  Jan 03, 2007 - vendor contact established, full details disclosed
  Jan 09, 2007 - vendor releases patch
  Feb 28, 2007 - full public release
  Patch Development Time (In Days): 6


Severity:
  Critical (Unsigned Code Execution in Hypervisor Mode)


Vendor:
  Microsoft


Systems Affected:
  All Xbox 360 systems with a kernel version of 4532 (released Oct 31,
  2006) and 4548 (released Nov 30, 2006). Versions prior to 4532 are not
  affected. Bug was fixed in version 4552 (released Jan 09, 2007 - not a
  Patch Tuesday).


Overview:
  We have discovered a vulnerability in the Xbox 360 hypervisor that allows
  privilege escalation into hypervisor mode. Together with a method to
  inject data into non-privileged memory areas, this vulnerability allows
  an attacker with physical access to an Xbox 360 to run arbitrary code
  such as alternative operating systems with full privileges and full
  hardware access.


Technical details:
  The Xbox 360 security system is designed around a hypervisor concept. All
  games and other applications, which must be cryptographically signed with
  Microsoft's private key, run in non-privileged mode, while only a small
  hypervisor runs in privileged ("hypervisor") mode. The hypervisor
  controls access to memory and provides encryption and decryption
  services.

  The policy implemented in the hypervisor forces all executable code to be
  read-only and encrypted. Therefore, unprivileged code cannot change
  executable code. A physical memory attack could modify code; however,
  code memory is encrypted with a unique per-session key, making meaningful
  modification of code memory in a broadly distributable fashion difficult.
  In addition, the stack and heap are always marked as non-executable, and
  therefore data loaded there can never be jumped to by unpriviledged code.

  Unprivileged code interacts with the hypervisor via the "sc" ("syscall")
  instruction, which causes the machine to enter hypervisor mode. The
  vulnerability is a result of incomplete checking of the parameters passed
  to the syscall dispatcher, as illustrated below.

  Preconditions (registers set by unpriviledged code):

  %r0      syscall no.
  %r3-%r12 syscall arguments

  Priviledged code:

  13D8: cmplwi %r0, 0x61
  13DC: bge illegal_syscall
   ...
  13F0: rldicr %r1, %r0, 2, 61
  13F4: lwz %r4, syscall_table(%r1)
  13F8: mtlr %r4
   ...
  1414: blrl

  The problem is that the "cmplwi" instruction compares only the lower 32
  bits of the given syscall number; the upper 32 bits are ignored. The
  "rldicr" instruction, however, operates on the complete 64 bit register
  value.

  The syscall handler address is fetched from the syscall handler offset
  table at 0x00000000.00001F68+%r0*4. Setting the upper 32 bits of %r0 to
  something other than 0 will change the upper 30 bits of the address used
  for the syscall handler offset table lookup. We will now explain how the
  Xbox 360 security architecture interprets and aliases these upper bits.

  When processing the syscall, the processor is running in "hypervisor real
  mode", with the MMU switched off. However, when accessing memory
  locations with the MSB cleared, an additional offset, the Hypervisor Real
  Mode Offset (HRMO), will be applied to all memory addresses.

  Due to the Xbox 360 security architecture, main memory is aliased to
  different addresses with different properties, in order to conditionally
  enable the security features (encryption and hashing). The hypervisor
  sets the value of the HRMO special register so that the hypervisor code,
  including the syscall jump table, resides in memory which is hashed as
  well as encrypted, even when using zero-based addresses.

  When accessing memory locations with the most significant address bit
  set, the HRMOR setting is not applied. Due to the bug in the "cmplwi"
  instruction, setting the corresponding bits in %r0 on syscall entry
  allows setting the MSB, thereby overriding the HRMOR setting and tricking
  the address lookup of the syscall handler to fetch from memory without
  any security features.

  With the syscall handler offset table aliased to unencrypted memory, the
  syscall handler table can now be modified to direct the hypervisor to
  jump to any location in code space that is designated for the hypervisor.
  In the proof of concept implementation, a jump to existing hypervisor
  code is used with a pre-loaded register value as a trampoline to force
  the ultimate execution path to an arbitrary, unencrypted and executable
  location in memory.

Proof of Concept Details:
  As it is not possible to directly overwrite even non-priviledged code,
  existing code needs to be tricked into calling the hypervisor syscall
  with the desired register set. This can be done by setting up a stack
  frame and forcing a context switch to this stack frame. The bug can be
  exploited using the following series of physical memory writes:

  Setup context switch to stack @80130AF0:

  00130390: 00000000 00000000 00000000 FDFFD7FF  MSR mask
  00130360: 00000000 80130AF0 00000000 00000000  New stack pointer

  Setup stack:

  00130BD0: 00000000 80070190 00000000 00000000  NIP to context restore
  00130C90: 00000000 00000000 80070228 80070228  NIP, LR after context
                                                 restore point to syscall
                                                 instruction in kernel
  00130CA0: 00000000 00009030 00000000 00000000  MSR

  00130B40: 20000000 00000046 00000000 80130af0  r0 = syscall nr
                                                 r1 = stack
  00130B60: 80000000 address1                    r4 = address to jump to

  00002080: 00000350                             points to mtctr %r4,
                                                 bctr in hypervisor code

  Code to be executed should be placed at "address1", which can be an
  arbitrary unused memory address.

  Example code to output '!' to the on board serial port:

  1:
    li %r3, '!'
    bl putc
    b 1b

  putc:
    lis %r4, 0x8000
    ori %r4, %r4, 0x200
    rldicr %r4, %r4, 32, 31
    oris %r4, %r4, 0xea00
    slwi %r3, %r3, 24
    stw %r3, 0x1014(%r4)
  1:
    lwz %r3, 0x1018(%r4)
    rlwinm. %r3, %r3, 0, 6, 6
    beq 1b
    blr


Vendor Status:
  Vendor was notified anonymously, and after cordial discussions a patch
  was promptly released.

Recommendation:
  Remove R6T3.


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