Kernel Memory Corruption in Qualcomm Display Driver via Unvalidated Array Index in EnableTestMode

The Qualcomm Display driver serves as the interface between the operating system kernel and the display hardware subsystem. To support diagnostics and factory testing, the driver exposes a set of 'Escape' calls—vendor-specific extensions to the standard graphics driver API (such as D3DKMTEscape on Windows or specific ioctl commands on Linux/Android). These interfaces allow user-mode clients to send opaque command buffers directly to the kernel-mode driver.

The vulnerability, identified as CVE-2025-21423, is located specifically within the handler for the EnableTestMode command. This function is designed to activate specific diagnostic configurations based on user input. However, the mechanism used to select these configurations lacks robust input validation, creating a direct path for memory corruption.

Because graphics drivers typically have a wide attack surface and are accessible to low-privileged applications (to support rendering), this vulnerability presents a significant risk. It allows a compromised low-privileged process—such as a malicious app or a browser renderer process that has achieved a sandbox escape—to attack the kernel directly.

The root cause of this vulnerability is an Improper Validation of Array Index (CWE-129). The EnableTestMode function retrieves an integer index from the user-supplied input buffer and uses it to reference an internal array of test mode configuration structures or function pointers.

In the vulnerable code path, the driver fails to verify that the user-supplied index is strictly less than the maximum number of defined test modes. The driver logic assumes the client will provide a valid index. This is a classic 'Time-of-Check to Time-of-Use' (TOCTOU) or missing bounds check scenario where untrusted data controls control flow or memory access.

When an attacker provides an index that exceeds the array's bounds, the driver calculates a memory address outside the intended data structure. If this operation is a read, it may leak adjacent kernel memory (information disclosure). If it is a write—for example, setting a 'mode active' flag—it results in memory corruption. Given the context of 'Enabling' a mode, the operation likely involves writing state data to the calculated address, making this a kernel memory corruption primitive.

While the exact proprietary source code is not public, the vulnerability follows a standard pattern often seen in driver dispatch routines. Below is a reconstruction of the vulnerable logic versus the remediated code path.

Vulnerable Logic

In the vulnerable version, the driver extracts the TestModeIndex and immediately uses it for pointer arithmetic.

NTSTATUS QclDisplayDispatchEscape(PVOID InputBuffer, ULONG InputSize) {
    // Cast input to expected structure
    PTEST_MODE_CMD cmd = (PTEST_MODE_CMD)InputBuffer;
 
    // VULNERABILITY: No check against MAX_TEST_MODES
    // The attacker controls cmd->TestModeIndex (e.g., 0xFFFFFFFF or large positive int)
    
    PTEST_CONFIG config = &GlobalTestModeArray[cmd->TestModeIndex];
    
    // Write to kernel memory (Corruption)
    config->IsEnabled = TRUE;
    config->Flags = cmd->Flags;
    
    return STATUS_SUCCESS;
}

Remediated Logic

The patch introduces a strict boundary check before the index is used. This ensures that the pointer arithmetic yields a valid address within the GlobalTestModeArray.

NTSTATUS QclDisplayDispatchEscape(PVOID InputBuffer, ULONG InputSize) {
    PTEST_MODE_CMD cmd = (PTEST_MODE_CMD)InputBuffer;
 
    // FIX: Validate index against the array size
    if (cmd->TestModeIndex >= MAX_TEST_MODES) {
        return STATUS_INVALID_PARAMETER;
    }
 
    // Safe access
    PTEST_CONFIG config = &GlobalTestModeArray[cmd->TestModeIndex];
    
    config->IsEnabled = TRUE;
    config->Flags = cmd->Flags;
    
    return STATUS_SUCCESS;
}

This simple validation effectively neutralizes the attack vector by rejecting malformed requests before any memory operation occurs.

Exploitation of CVE-2025-21423 requires local code execution on the target device. The attack chain typically proceeds as follows:

1. Interface Enumeration: The attacker identifies the handle to the display device. On Windows, this involves opening the GDI adapter; on Android, it involves opening the framebuffer or DRM device nodes (/dev/dri/card0).
2. Escape Call Crafting: The attacker constructs a specifically formatted buffer matching the structure expected by the EnableTestMode handler. The critical payload is the TestModeIndex integer.
3. Heap Grooming (Optional but likely): To maximize reliability, the attacker might perform heap spraying or grooming to ensure that the memory immediately following the GlobalTestModeArray contains sensitive data structures (like a process token or a function pointer table).
4. Triggering the Corruption: The attacker invokes the Escape system call. The driver uses the OOB index to calculate a target address relative to the array base.
5. Privilege Escalation:
   • Data-Only Attack: The attacker targets a structure like _SEP_TOKEN_PRIVILEGES (on Windows) or cred struct (on Linux/Android) to grant the current process elevated capabilities (e.g., SeDebugPrivilege or CAP_SYS_ADMIN).
   • Control Flow Hijacking: If the corruption overwrites a function pointer, the attacker can redirect execution to a ROP chain or shellcode (though Supervisor Mode Execution Prevention (SMEP) makes this harder, necessitating data-only attacks).

The impact of this vulnerability is rated High (CVSS 7.8).

Confidentiality & Integrity: Successful exploitation allows arbitrary modification of kernel memory. This is sufficient to bypass OS security controls, disable code signing enforcement, or extract encryption keys stored in kernel memory.

Availability: Improper usage of the OOB index can access unmapped memory pages, triggering a page fault in kernel mode. This results in an immediate system crash (Blue Screen of Death on Windows or Kernel Panic on Linux), causing a Denial of Service.

Scope: The vulnerability affects both mobile devices (Android smartphones using Snapdragon chips) and Always-Connected PCs (Windows on Snapdragon). This makes the blast radius significant, covering millions of consumer and enterprise devices.

There are no viable workarounds for this vulnerability that do not involve patching, as disabling the display driver is not feasible for device operation. Remediation relies entirely on vendor updates.

For End Users:

• Windows-on-ARM: Check Windows Update for firmware/driver updates. Look for 'Qualcomm System' or 'Display' driver updates released after April 2025.
• Android: Install the Android Security Patch Level (SPL) matching the OEM's release timeline (typically included in the monthly security bulletin updates).

For Enterprise Administrators:

• Deploy forced compliance policies to ensure devices (especially Snapdragon-based laptops like Surface Pro X or ThinkPad X13s) are running the latest driver versions.
• Monitor EDR logs for abnormal D3DKMTEscape calls, although detection at this granularity is difficult without specific driver instrumentation.