Linux Kernel Bug Opens Door to Wider Cyberattacks

An information-disclosure security vulnerability has been discovered in the Linux kernel, which can be exploited to expose information in the kernel stack memory of vulnerable devices.

Specifically, the bug (CVE-2020-28588) exists in the /proc/pid/syscall functionality of 32-bit ARM devices running Linux, according to Cisco Talos, which discovered the vulnerability. It arises from an improper conversion of numeric values when reading the file.

With a few commands, attackers can output 24 bytes of uninitialized stack memory, which can be used to bypass kernel address space layout randomization (KASLR). KASLR is an anti-exploit technique that places various objects at random to prevent predictable patterns that are guessable by adversaries.

Attacks also would be “impossible to detect on a network remotely,” the firm explained. And, “if utilized correctly, an attacker could leverage this information leak to successfully exploit additional unpatched Linux vulnerabilities.”

Kernel-Bug Details

Proc is a special, pseudo-filesystem in Unix-like operating systems that is used for dynamically accessing process data held in the kernel. It presents information about processes and other system information in a hierarchical file-like structure. For instance, it contains /proc/[pid] subdirectories, each of which contains files and subdirectories exposing information about specific processes, readable by using the corresponding process ID. In the case of the “syscall” file, it’s a legitimate Linux operating system file that contains logs of system calls used by the kernel.

An attacker could exploit the vulnerability by reading /proc/<pid>/syscall. “We can see the output on any given Linux system whose kernel was configured with CONFIG_HAVE_ARCH_TRACEHOOK,” according to Cisco’s bug report, publicly disclosed on Tuesday..

“This file exposes the system call number and argument registers for the system call currently being executed by the process, followed by the values of the stack pointer and program counter registers,” explained the firm. “The values of all six argument registers are exposed, although most system call use fewer registers.”

The shell commands that trigger the vulnerability are:

# echo 0 > /proc/sys/kernel/randomize_va_space (# only needed for a cleaner output) $ while true; do cat /proc/self/syscall; done | uniq (# waits for changes) $ while true; do free &>/dev/null; done (# triggers changes)

Security Patch Updates Available

Cisco Talos researchers first discovered the issue on an Azure Sphere device (version 20.10), a 32-bit ARM device that runs a patched Linux kernel. It’s been present since v5.1-rc4 of the kernel.

“Users are encouraged to update these affected products as soon as possible: Linux Kernel versions 5.10-rc4, 5.4.66 and 5.9.8,” according to the advisory. “Talos tested and confirmed these versions of the Linux kernel could be exploited by this vulnerability.”

Linux kernel bugs are rare but do happen. For instance, last October Google and Intel warned of the high-severity “BleedingTooth” flaw in BlueZ, the Linux Bluetooth protocol stack that provides support for core Bluetooth layers and protocols to Linux-based internet of things (IoT) devices. It could be exploited in a “zero-click” attack and potentially allow for escalated privileges on affected devices.

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