CVE-2026-64590: dma-buf/udmabuf: skip redundant cpu sync to fix cacheline EEXIST warning

From: Greg Kroah-Hartman <[email protected]>

Description
===========

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.

The Linux kernel CVE team has assigned CVE-2026-64590 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.6 with commit 284562e1f34874e267d4f499362c3816f8f6bc3f and fixed in 6.6.148 with commit 0db56e7eae932f8e2f3eb44ad1a63633d8f504f8
	Issue introduced in 5.6 with commit 284562e1f34874e267d4f499362c3816f8f6bc3f and fixed in 6.12.96 with commit d6552f5cff795d60e629f37513ecf23d88fd2f82
	Issue introduced in 5.6 with commit 284562e1f34874e267d4f499362c3816f8f6bc3f and fixed in 6.18.39 with commit 34696563461c9a23177feb6d8aff43f4c0510278
	Issue introduced in 5.6 with commit 284562e1f34874e267d4f499362c3816f8f6bc3f and fixed in 7.1.4 with commit 0449a6583c0ee76778d314e4e82f166fc97fa9d8
	Issue introduced in 5.6 with commit 284562e1f34874e267d4f499362c3816f8f6bc3f and fixed in 7.2-rc1 with commit 504e2b4ab97a51d56d966cd36d0997ad30b65b2d

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-64590
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/dma-buf/udmabuf.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	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