[PATCH v2] 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.

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 <redacted>
---
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/ (local)
---
 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