Re: [PATCH v2] drm/cirrus-qemu: Validate BAR0 size during probe

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi

Am 25.08.26 um 14:07 schrieb Slawomir Stepien:
> 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 <sst@poczta.fm>

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>

Thanks for fixing this issue.

Best regards
Thomas

> ---
> 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;

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Jochen Jaser, Andrew McDonald, (HRB 36809, AG Nürnberg)