[PATCH RFC 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
#PF: supervisor write access in kernel mode
#PF: error_code(0x0002) - not-present page
...
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
length equals `CIRRUS_VRAM_SIZE`, returning `-ENODEV` if it does not match.

Fixes: ab3e023b1b4c ("drm/cirrus: rewrite and modernize driver.")
Assisted-by: Gemini:gemini-3.6-flash Gemini:gemini-3.1-pro-preview syzbot
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=2442951a6abb004df963
Link: https://syzkaller.appspot.com/ai_job?id=ba262a3a-bccf-4ad8-a1b0-583c55d34fd6
To: "David Airlie" <[email protected]>
To: "Dave Airlie" <[email protected]>
To: <[email protected]>
To: "Gerd Hoffmann" <[email protected]>
To: "Maarten Lankhorst" <[email protected]>
To: "Maxime Ripard" <[email protected]>
To: "Simona Vetter" <[email protected]>
To: "Thomas Zimmermann" <[email protected]>
To: <[email protected]>
Cc: "Jani Nikula" <[email protected]>
Cc: <[email protected]>

---
v2:
- Reject device in probe if PCI BAR0 resource length does not equal CIRRUS_VRAM_SIZE instead of dynamically tracking VRAM size.
- Retain CIRRUS_VRAM_SIZE and keep atomic check and mode validation hooks unchanged.

v1:
https://lore.kernel.org/all/[email protected]/T/
---
diff --git a/drivers/gpu/drm/tiny/cirrus-qemu.c b/drivers/gpu/drm/tiny/cirrus-qemu.c
index 075221b43..70c995025 100644
--- a/drivers/gpu/drm/tiny/cirrus-qemu.c
+++ b/drivers/gpu/drm/tiny/cirrus-qemu.c
@@ -602,6 +602,9 @@ static int cirrus_pci_probe(struct pci_dev *pdev,
 
 	dev = &cirrus->dev;
 
+	if (pci_resource_len(pdev, 0) != CIRRUS_VRAM_SIZE)
+		return -ENODEV;
+
 	cirrus->vram = devm_ioremap(&pdev->dev, pci_resource_start(pdev, 0),
 				    pci_resource_len(pdev, 0));
 	if (cirrus->vram == NULL)


base-commit: db2ddb87143519e20a95aa36c60b36107b736a58
-- 
This is an AI-generated patch subject to moderation.
Reply with '#syz upstream' to Sign-off the patch as a human author
and send it to the upstream kernel mailing lists.
Reply with '#syz reject' to reject it ('#syz unreject' to undo).

See https://goo.gle/syzbot-ai-patches for information about AI-generated patches.
You can comment on the patch as usual, syzbot will try to address
the comments and send a new version of the patch if necessary.
syzbot engineers can be reached at [email protected].