A serious flaw was recently resolved in the Linux kernel's AMD GPU (amdgpu) stack, specifically in the way it handles memory migrations via DMA operations. Known as CVE-2024-57897, this vulnerability could produce warnings, lead to potential system instability, and open up exploitable conditions if a malicious process were able to force incorrect DMA synchronization. In this post, I'll break down what happened, what changed, show you the relevant code, and explain how someone might have tried to exploit it.

What Is the Problem?

When the amdgpu driver migrates memory objects (for example, when supporting heterogeneous system architecture, or HSA, where CPU and GPU share memory), it uses Direct Memory Access (DMA) mapping. This migration uses a map *direction* to tell the kernel which way data should go and how to synchronize caches.

If the map and unmap directions are mismatched, the kernel warns and the DMA operation might not be synchronized properly.

Here's the *actual warning* seen in logs

WARNING: CPU: 8 PID: 1812 at kernel/dma/debug.c:1028 check_unmap+x1cc/x930

The Core Bug

The relevant driver code previously set up DMA mapping with a direction (ex: TO_DEVICE, FROM_DEVICE). But then, when unmapping the buffer, it used a potentially misaligned direction. This mismatch was enough to trip kernel warnings and may hint at underlying cache coherence or data integrity problems.

AMD developers discussed whether this memory should be treated as coherent or streamed, and how DMA core cache synchronization is handled (see original discussion). To keep things simple and robust, they set the direction to BIDIRECTIONAL everywhere.

The Fix

The fix is now mainlined: always use DMA_BIDIRECTIONAL for mapping and unmapping SVM objects in migrations.

Relevant Patch Snippet

// Before: (direction mismatch could happen)
dma_map_page(dev, page, offset, size, DMA_TO_DEVICE);
...
dma_unmap_page(dev, dma_addr, size, DMA_FROM_DEVICE);

// After: Always BIDIRECTIONAL to prevent warnings and sync issues
dma_map_page(dev, page, offset, size, DMA_BIDIRECTIONAL);
...
dma_unmap_page(dev, dma_addr, size, DMA_BIDIRECTIONAL);

What Could Go Wrong? (Potential Exploitation)

Before this fix, if a process or attacker could trigger a sequence of GPU memory migration operations, the lack of proper synchronization might:

- Corrupt memory during device/host migration, leading to use-after-free, stale data, or leaks.
- Create a race window where critical system or user data is accessible before being written or after being read.
- Trigger kernel warnings, possibly leading to system instability or even a panic if the warning escalates.

> Practical Exploit Scenario
>
> While a local attacker cannot typically control DMA direction directly, a program using ROCm or GPU programming frameworks might intentionally trigger migratory kernel memory operations, exploiting the incorrect direction to confuse cache state, snoop data, or cause denial of service.

`

WARNING: CPU: X PID: Y at kernel/dma/debug.c: check_unmap+...

References

- Upstream kernel discussion and patch thread
- Linux kernel commit fixing CVE-2024-57897
- AMDGPU kernel driver source code

Takeaway (Simple Language)

- This bug is not a remote code execution hole for unprivileged apps, but it *could* let a cunning user destabilize or corrupt a Linux box running new AMD GPUs.
- *Always* keep your kernel up to date, especially if you work with GPU or heterogeneous memory features.
- The fix was simple: map and unmap memory buffers with the same DMA direction to keep everyone's data safe.

If you want to check your machine (as root)

grep -r "WARNING:.*dma/debug.c: check_unmap" /var/log/

If you see the warning and use affected hardware, it's time to update.

Summary

CVE-2024-57897 is a Linux kernel bug in the AMD GPU driver involving wrong direction flags for memory migration. It could cause subtle data problems, but is fixed by synchronizing map/unmap direction as BIDIRECTIONAL. Keep your systems updated for best security and durability!

Timeline

Published on: 01/15/2025 13:15:14 UTC
Last modified on: 05/04/2025 10:06:08 UTC