CVE-2022-39842: Linux Kernel Buffer Overflow Vulnerability

CVE-2022-39842 Overview

An integer overflow vulnerability exists in the Linux kernel before version 5.19, specifically in the pxa3xx_gcu_write function within drivers/video/fbdev/pxa3xx-gcu.c. The vulnerability arises from a type conflict where the count parameter is handled as size_t in some contexts but as int in others, leading to an integer overflow that bypasses size validation checks. This overflow condition can potentially lead to a heap overflow when the parameter is subsequently passed as the third argument to copy_from_user().

Critical Impact

Local attackers with low privileges could exploit this integer overflow to bypass size checks and potentially trigger a heap overflow, leading to system instability or denial of service conditions.

Affected Products

• Linux Kernel versions prior to 5.19
• Linux Kernel 5.19 RC1, RC2, and RC3
• Debian Linux 10.0 and 11.0

Discovery Timeline

• September 5, 2022 - CVE CVE-2022-39842 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2022-39842

Vulnerability Analysis

The vulnerability resides in the PXA3xx Graphics Controller Unit (GCU) frame buffer driver, a component used for graphics processing on certain ARM-based embedded systems. The root cause is an integer overflow triggered by a type mismatch in the pxa3xx_gcu_write function. When processing write operations, the function divides the count parameter by 4 to calculate the number of words. However, the original code stored this result in an int variable while count itself is of type size_t.

On 64-bit systems, size_t is typically 64 bits while int is 32 bits. This type mismatch means that large values of count could overflow when stored in the int variable, resulting in a much smaller value that passes subsequent size validation checks. The corrupted value is then used in copy_from_user(), potentially allowing more data to be copied than the allocated buffer can hold.

It is worth noting that the original discoverer disputes whether the overflow can actually occur in practical scenarios, as exploiting this would require specific conditions regarding memory allocation and user-controlled input sizes.

Root Cause

The root cause is CWE-190: Integer Overflow or Wraparound. The vulnerable code declares the words variable as int instead of size_t, creating a type conflict when processing the count parameter. This type truncation can cause large input values to wrap around to small positive values, bypassing the intended size validation logic and enabling potential heap corruption.

Attack Vector

The attack requires local access to the system with low privileges. An attacker would need to interact with the PXA3xx GCU device driver, typically through file operations on the associated device node. By crafting a write operation with a specially calculated count value, the attacker could trigger the integer overflow condition. The attack does not require user interaction and affects system availability primarily, with potential for limited integrity impact.

// Security patch in drivers/video/fbdev/pxa3xx-gcu.c
// Source: https://github.com/torvalds/linux/commit/a09d2d00af53b43c6f11e6ab3cb58443c2cac8a7

 	struct pxa3xx_gcu_batch	*buffer;
 	struct pxa3xx_gcu_priv *priv = to_pxa3xx_gcu_priv(file);
 
-	int words = count / 4;
+	size_t words = count / 4;
 
 	/* Does not need to be atomic. There's a lock in user space,
 	 * but anyhow, this is just for statistics. */

The fix changes the type of the words variable from int to size_t, ensuring consistent handling of the count parameter throughout the function and preventing integer overflow during the division operation.

Detection Methods for CVE-2022-39842

Indicators of Compromise

• Unusual write operations to /dev/pxa3xx-gcu device node with abnormally large count values
• Kernel panic or system crashes related to the pxa3xx-gcu driver
• Memory corruption indicators in kernel logs referencing the fbdev subsystem
• Unexpected heap allocation failures or corruption messages

Detection Strategies

• Monitor kernel version to identify systems running vulnerable versions (prior to 5.19)
• Implement audit rules for access to PXA3xx GCU device nodes
• Deploy kernel integrity monitoring to detect exploitation attempts
• Use SentinelOne's kernel-level monitoring capabilities to detect anomalous driver interactions

Monitoring Recommendations

• Enable kernel auditing for device driver operations on embedded ARM systems
• Configure alerts for kernel oops or panic events related to video frame buffer drivers
• Monitor for privilege escalation attempts following driver interaction
• Implement file integrity monitoring on critical kernel modules

How to Mitigate CVE-2022-39842

Immediate Actions Required

• Upgrade Linux kernel to version 5.19 or later where the vulnerability is patched
• Apply security patches from distribution vendors (Debian DSA-5257 and related LTS announcements)
• Restrict access to the PXA3xx GCU device node to authorized users only
• Consider disabling the pxa3xx-gcu driver if not required for system operation

Patch Information

The vulnerability has been addressed in Linux kernel version 5.19. The fix was committed with hash a09d2d00af53b43c6f11e6ab3cb58443c2cac8a7 and changes the variable type from int to size_t to prevent the integer overflow condition. For detailed patch information, refer to the Linux Kernel ChangeLog 5.19 or the GitHub commit.

Debian users should apply updates from DSA-5257 or the relevant LTS announcements for Debian 10 and 11.

Workarounds

• Blacklist the pxa3xx-gcu kernel module if the graphics controller is not required
• Restrict device node permissions using udev rules to limit access
• Implement mandatory access control policies (SELinux/AppArmor) to confine driver access
• Deploy runtime protection solutions like SentinelOne to monitor and block exploitation attempts

# Configuration example - Disable pxa3xx-gcu module loading
echo "blacklist pxa3xx-gcu" | sudo tee /etc/modprobe.d/blacklist-pxa3xx-gcu.conf

# Restrict device node access via udev rule
echo 'KERNEL=="pxa3xx-gcu", MODE="0600", OWNER="root", GROUP="root"' | sudo tee /etc/udev/rules.d/99-pxa3xx-gcu.rules

# Reload udev rules
sudo udevadm control --reload-rules && sudo udevadm trigger