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.

News-Source: http://www.securityfocus.com