Linux: drm/cirrus-qemu: Validate BAR0 size during probe

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: <TASK> 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.