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

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

[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>
---
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

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

[Thomas Zimmermann]

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)

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

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] The driver maps PCI BAR1 (MMIO) without verifying that its length is sufficient to cover the register offsets accessed by the driver (up to 0x15 for CRT_DATA). If a malicious or malformed PCI device provides a very small BAR1 (e.g., 16 bytes), accesses to higher offsets could cross page boundaries into unmapped memory, potentially causing a kernel panic.
--

--- Patch [1]: [PATCH v2] drm/cirrus-qemu: Validate BAR0 size during probe ---
commit b04af41f1fca891d59ccb1049a9b053cf626fb58
Author: Slawomir Stepien <sst@poczta.fm>
drm/cirrus-qemu: Validate BAR0 size during probe

This commit adds a check during probe to validate that the PCI BAR0 size is at
least CIRRUS_VRAM_SIZE. This prevents out-of-bounds memory accesses when
drm_fb_memcpy() copies framebuffers to VRAM mapped from a smaller BAR0.

Link: https://syzkaller.appspot.com/bug?extid=2442951a6abb004df963
Link: https://syzkaller.appspot.com/ai_job?id=ba262a3a-bccf-4ad8-a1b0-583c55d34fd6

> diff --git a/drivers/gpu/drm/tiny/cirrus-qemu.c b/drivers/gpu/drm/tiny/cirrus-qemu.c
> index 075221b431d37..3bf23fcf65749 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;
> +

[Severity: High]
This is a pre-existing issue, but since we are validating the length of BAR0
(VRAM) here to prevent out-of-bounds accesses, should we also validate the
length of BAR1 (MMIO)?

Later in cirrus_pci_probe(), BAR1 is mapped using its raw length:

    cirrus->mmio = devm_ioremap(&pdev->dev, pci_resource_start(pdev, 1),
                                pci_resource_len(pdev, 1));

The driver subsequently accesses MMIO offsets up to 0x15:

drivers/gpu/drm/tiny/cirrus-qemu.c:wreg_crt() {
    ...
    iowrite8(reg, cirrus->mmio + CRT_INDEX);
    iowrite8(val, cirrus->mmio + CRT_DATA);
}

If a malformed PCI device presents a very small BAR1 (e.g., 16 bytes) and is
allocated such that its physical address is near the end of a page (e.g.,
ending at 0xFFF), devm_ioremap() will map only that single page.

Could accessing offset 0x15 cross the page boundary into unmapped virtual
memory and trigger a supervisor page fault?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260825120729.493611-1-sst@poczta.fm?part=1

[syzbot ci] Re: drm/cirrus-qemu: Validate BAR0 size during probe

[syzbot ci]

syzbot ci has tested the following series

[v2] drm/cirrus-qemu: Validate BAR0 size during probe
https://lore.kernel.org/all/20260825120729.493611-1-sst@poczta.fm
* [PATCH v2] drm/cirrus-qemu: Validate BAR0 size during probe

and found no issues.

Full report is available here:
https://ci.syzbot.org/series/125adc4a-cb5c-4dbe-a3e7-05381b5113e9

***

If these findings have caused you to resend the series or submit a
separate fix, please add the following tag to your commit message:
  Tested-by: syzbot@syzkaller.appspotmail.com

---
This report is generated by a bot. It may contain errors.
syzbot ci engineers can be reached at syzkaller@googlegroups.com.

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

[Thomas Zimmermann]

Merged into drm-misc-fixes

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>
> ---
> 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)