CVE-2021-45095: Linux Kernel Information Disclosure Flaw

CVE-2021-45095 Overview

CVE-2021-45095 is a memory leak vulnerability in the Linux kernel's Phonet protocol implementation. The flaw exists in the pep_sock_accept function within net/phonet/pep.c, where a reference count leak occurs during socket acceptance operations. This vulnerability affects Linux kernel versions through 5.15.8 and allows local attackers to potentially leak sensitive information from kernel memory.

Critical Impact
Local attackers with low privileges can exploit this reference count leak to potentially access sensitive information from kernel memory, compromising system confidentiality.

Affected Products

Linux Kernel (versions through 5.15.8)
Debian Linux 9.0
Debian Linux 10.0
Debian Linux 11.0

Discovery Timeline

December 16, 2021 - CVE-2021-45095 published to NVD
November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-45095

Vulnerability Analysis

This vulnerability is classified as an information exposure issue (CWE-200) stemming from improper reference counting in the Linux kernel's Phonet protocol stack. The Phonet protocol is a packet-oriented network protocol used primarily in Nokia cellular modems for communication between the modem and application processor.

The flaw occurs in the pep_sock_accept function, which handles socket acceptance for Phonet Pipe End Point (PEP) connections. When the pep_accept_conn function fails and returns an error, the code path fails to properly decrement the reference count on the parent socket before proceeding to error handling. This creates a reference count leak that can be exploited by local attackers.

Reference count leaks are particularly dangerous in kernel code because they can lead to memory disclosure, denial of service through resource exhaustion, or in some cases, use-after-free conditions when reference counts underflow in subsequent operations.

Root Cause

The root cause is a missing __sock_put(sk) call in the error handling path of pep_sock_accept. When pep_accept_conn() returns an error, the function correctly puts the new socket reference but fails to release the reference on the parent socket (sk) that was acquired earlier in the function. This asymmetric reference counting leads to the leak.

Attack Vector

The attack requires local access to the system with low privileges. An attacker would need to:

Create Phonet protocol sockets on the target system
Trigger the error condition in pep_sock_accept by manipulating connection acceptance
Repeatedly trigger the vulnerability to leak kernel memory references
Potentially extract sensitive information from the leaked memory

The following patch addresses the reference count leak by adding the missing socket put operation:

 
 	err = pep_accept_conn(newsk, skb);
 	if (err) {
+		__sock_put(sk);
 		sock_put(newsk);
 		newsk = NULL;
 		goto drop;

Source: GitHub Linux Commit

Detection Methods for CVE-2021-45095

Indicators of Compromise

Unusual Phonet protocol socket activity from unprivileged processes
Increasing kernel memory allocation without corresponding deallocation in the Phonet subsystem
Abnormal patterns of socket accept failures on Phonet endpoints
Memory leak warnings or OOM (Out of Memory) conditions related to network subsystems

Detection Strategies

Monitor for processes attempting to create Phonet protocol sockets, especially from non-system users
Implement kernel memory tracking to detect reference count anomalies in network subsystems
Use kernel debugging tools like kmemleak to identify memory leaks in the net/phonet/ code path
Deploy endpoint detection solutions capable of monitoring kernel-level socket operations

Monitoring Recommendations

Enable kernel auditing for socket syscalls, particularly those involving the Phonet protocol family (AF_PHONET)
Monitor /proc/slabinfo for abnormal growth in socket-related slab caches
Implement alerting on repeated socket accept failures that may indicate exploitation attempts
Review system logs for Phonet-related kernel warnings or errors

How to Mitigate CVE-2021-45095

Immediate Actions Required

Update the Linux kernel to a patched version that includes commit bcd0f93353326954817a4f9fa55ec57fb38acbb0
Apply distribution-specific security patches from Debian or your Linux vendor
If patching is not immediately possible, consider disabling the Phonet protocol module if not required
Restrict local user access and review user privileges to limit attack surface

Patch Information

The vulnerability has been addressed through official kernel commits and distribution security advisories:

Linux Kernel Commit - Official kernel fix
GitHub Linux Commit - Mirror of the patch
Debian Security Advisory DSA-5050 - Debian stable security update
Debian Security Advisory DSA-5096 - Additional Debian security update

Workarounds

Disable the Phonet protocol kernel module if not required for system operation
Restrict access to create Phonet sockets using Linux Security Modules (SELinux/AppArmor)
Implement mandatory access controls to limit which processes can interact with the Phonet subsystem
Monitor and limit local user access to reduce the attack surface for this local vulnerability

bash

# Disable Phonet kernel module if not needed
echo "blacklist phonet" >> /etc/modprobe.d/blacklist-phonet.conf
echo "blacklist pn_pep" >> /etc/modprobe.d/blacklist-phonet.conf

# Remove the module if currently loaded
modprobe -r pn_pep
modprobe -r phonet

# Verify module is not loaded
lsmod | grep phonet