Xbox 360 Hypervisor Privilege Escalation Vulnerability

["Anonymous Hacker" <anohacker () googlemail com>]

Security Advisory

Xbox 360 Hypervisor Privilege Escalation Vulnerability


Release Date:
 February 28, 2007


Author:
 Anonymous Hacker <anohacker () gmail com>


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.