In the Linux kernel, the following vulnerability has been resolved:
drm/cirrus-qemu: Validate BAR0 size during probe
The `cirrus-qemu` driver relies on `CIRRUS_VRAM_SIZE` (4 MB) to validate
framebuffer sizes. However, during PCI probe, the driver mapped BAR0
without verifying that its size matches `CIRRUS_VRAM_SIZE`.
If a PCI device with a BAR0 smaller than 4 MB is bound to the driver, the
mapped VRAM will be smaller than expected. Because validation checks assume
4 MB VRAM, framebuffers larger than the mapped memory can be created.
When the display plane is updated (e.g. during release),
`cirrus_primary_plane_helper_atomic_update()` copies the framebuffer to
VRAM using `drm_fb_memcpy()`. Writing past the end of the mapped I/O memory
causes a supervisor write page fault:
BUG: unable to handle page fault for address: ffffc9000389c000
…
RIP: 0010:memcpy_toio+0x7c/0xe0 arch/x86/lib/iomem.c:110
…
Call Trace:
iosys_map_memcpy_to include/linux/iosys-map.h:285 [inline]
drm_fb_memcpy+0x325/0x5d0 drivers/gpu/drm/drm_format_helper.c:442
cirrus_primary_plane_helper_atomic_update+0x98a/0xb00
drivers/gpu/drm/tiny/cirrus-qemu.c:358
drm_atomic_helper_commit_planes+0x626/0xea0
drivers/gpu/drm/drm_atomic_helper.c:3038
drm_atomic_helper_commit_tail+0x60/0x510
drivers/gpu/drm/drm_atomic_helper.c:1989
commit_tail+0x2b1/0x3c0 drivers/gpu/drm/drm_atomic_helper.c:2074
drm_atomic_helper_commit+0xa77/0xb10
drivers/gpu/drm/drm_atomic_helper.c:2312
Fix this by validating in `cirrus_pci_probe()` that the PCI BAR0 resource
is not less than `CIRRUS_VRAM_SIZE`, returning `-ENODEV` if it is less.
