CVE-2026-64590 in Linux
Summary
by MITRE • 08/06/2026
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.
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Analysis
by VulDB Data Team • 08/06/2026
The vulnerability under discussion involves a spurious warning generated by the Linux kernel's DMA debugging infrastructure when udmabuf buffers are imported into DRM drivers such as amdgpu. This issue manifests specifically when CONFIG_DMA_API_DEBUG_SG is enabled, creating a false positive condition that incorrectly flags cacheline overlaps between mappings. The problem occurs during video playback operations in GNOME Videos or Showtime applications where the amdgpu driver attempts to map udmabuf buffers for GPU processing while maintaining separate CPU mappings through the dma-buf interface.
The technical flaw stems from the interaction between multiple kernel subsystems in the DMA buffer management pipeline. When a udmabuf is imported into a DRM driver like amdgpu, the call chain executes through several functions including amdgpu_cs_ioctl, amdgpu_ttm_backend_bind, and dma_buf_map_attachment before reaching the udmabuf-specific map_udmabuf function. The udmabuf implementation constructs per-page scatter-gather lists using sg_set_folio(), but when begin_cpu_udmabuf() has already created an SG table mapped for the misc device, subsequent mapping operations by drivers like amdgpu create overlapping physical address mappings that share cacheline boundaries.
The DMA debug infrastructure in kernel/dma/debug.c at line 619 triggers the EEXIST warning through add_dma_entry() function when it detects overlapping mappings with shared cacheline boundaries. This occurs because udmabuf creates multiple mapping contexts for the same physical pages without proper coordination between the CPU and device access patterns. The issue is classified under CWE-691 as an Insufficient Control Flow Management problem, where the kernel's DMA debugging mechanism fails to properly handle legitimate overlapping mappings that are intentionally created for different access domains.
The operational impact of this vulnerability primarily manifests as noisy warning messages in system logs during normal video playback operations, which can obscure genuine DMA-related issues and create unnecessary administrative overhead. The spurious warnings also indicate a potential performance degradation due to redundant CPU cache synchronization operations that should be skipped. This affects user experience through increased logging verbosity and may complicate debugging efforts when real DMA errors need to be identified among the false positives.
The recommended mitigation involves implementing the DMA_ATTR_SKIP_CPU_SYNC flag in udmabuf's dma_map_sgtable() and dma_unmap_sgtable() functions, which signals to the DMA debug infrastructure that no CPU cache maintenance is performed at map/unmap time. This approach aligns with existing practices in other major dma-buf exporters such as drm_gem_map_dma_buf() and amdgpu_dma_buf_map() that already utilize this flag. The solution addresses the root cause by acknowledging that udmabuf already performs explicit cache synchronization through begin_cpu_udmabuf() and end_cpu_udmabuf() functions when CPU access is required, making additional redundant sync operations unnecessary. This approach follows ATT&CK technique T1547.006 for privilege escalation prevention through proper resource management and adheres to the principle of least privilege by avoiding unnecessary system calls that could impact performance or create false security alerts.