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. Fixes: ab3e023b1b4c ("drm/cirrus: rewrite and modernize driver.") Assisted-by: Gemini:gemini-3.6-flash Gemini:gemini-3.1-pro-preview syzbot Reported-by: syzbot+2442951a6abb004df963@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=2442951a6abb004df963 Link: https://syzkaller.appspot.com/ai_job?id=ba262a3a-bccf-4ad8-a1b0-583c55d34fd6 Signed-off-by: Slawomir Stepien --- v2: * move the check before aperture_remove_conflicting_pci_devices() as suggested by sashiko. * check if the pci_resource_len() for BAR0 is less than CIRRUS_VRAM_SIZE as suggested by Thomas. The qemu emulates 4, (8 and 16 MB for compatibility) - never less than 4MB. v1: * https://lore.kernel.org/all/b11f8965-5896-4119-98b7-6bd0eb9fa139@mail.kernel.org/T/ --- drivers/gpu/drm/tiny/cirrus-qemu.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/drivers/gpu/drm/tiny/cirrus-qemu.c b/drivers/gpu/drm/tiny/cirrus-qemu.c index 075221b431d3..3bf23fcf6574 100644 --- a/drivers/gpu/drm/tiny/cirrus-qemu.c +++ b/drivers/gpu/drm/tiny/cirrus-qemu.c @@ -582,6 +582,9 @@ static int cirrus_pci_probe(struct pci_dev *pdev, struct cirrus_device *cirrus; int ret; + if (pci_resource_len(pdev, 0) < CIRRUS_VRAM_SIZE) + return -ENODEV; + ret = aperture_remove_conflicting_pci_devices(pdev, cirrus_driver.name); if (ret) return ret; -- 2.55.0