CVE-2026-64590

Summary

In the Linux kernel, the following vulnerability has been resolved:

dma-buf/udmabuf: skip redundant cpu sync to fix cacheline EEXIST warning

When CONFIG_DMA_API_DEBUG_SG is enabled, importing a udmabuf into a DRM driver (e.g. amdgpu for video playback in GNOME Videos / Showtime) triggers a spurious warning:

DMA-API: amdgpu 0000:03:00.0: cacheline tracking EEXIST,
overlapping mappings aren't supported WARNING: kernel/dma/debug.c:619 at add_dma_entry+0x473/0x5f0

The call chain is:

amdgpu_cs_ioctl -> amdgpu_ttm_backend_bind -> dma_buf_map_attachment -> [udmabuf] map_udmabuf -> get_sg_table -> dma_map_sgtable(dev, sg, direction, 0) // attrs=0 -> debug_dma_map_sg -> add_dma_entry -> EEXIST

This happens because udmabuf builds a per-page scatter-gather list via sg_set_folio(). When begin_cpu_udmabuf() has already created an sg table mapped for the misc device, and an importer such as amdgpu maps the same pages for its own device via map_udmabuf(), the DMA debug infrastructure sees two active mappings whose physical addresses share cacheline boundaries and warns about the overlap.

The DMA_ATTR_SKIP_CPU_SYNC flag suppresses this check in add_dma_entry() because it signals that no CPU cache maintenance is performed at map/unmap time, making the cacheline overlap harmless.

All other major dma-buf exporters already pass this flag:

• drm_gem_map_dma_buf() passes DMA_ATTR_SKIP_CPU_SYNC
• amdgpu_dma_buf_map() passes DMA_ATTR_SKIP_CPU_SYNC

The CPU sync at map/unmap time is also redundant for udmabuf: begin_cpu_udmabuf() and end_cpu_udmabuf() already perform explicit cache synchronization via dma_sync_sgtable_for_cpu/device() when CPU access is requested through the dma-buf interface.

Pass DMA_ATTR_SKIP_CPU_SYNC to dma_map_sgtable() and dma_unmap_sgtable() in udmabuf to suppress the spurious warning and skip the redundant sync.

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/0db56e7eae932f8e2f3eb44ad1a63633d8f504f8
• https://git.kernel.org/stable/c/d6552f5cff795d60e629f37513ecf23d88fd2f82
• https://git.kernel.org/stable/c/34696563461c9a23177feb6d8aff43f4c0510278
• https://git.kernel.org/stable/c/0449a6583c0ee76778d314e4e82f166fc97fa9d8
• https://git.kernel.org/stable/c/504e2b4ab97a51d56d966cd36d0997ad30b65b2d

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