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: [email protected]
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 <[email protected]>
---
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/[email protected]/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
