[PATCH RESEND] x86/pci: Check signature before assigning shadow ROM

[Tomita Moeko <tomitamoeko@gmail.com>]

Modern platforms without VBIOS or UEFI CSM support do not contain
VGA ROM at 0xC0000, this is observed on Intel Ice Lake and later
systems. Check whether the VGA ROM region is a valid PCI option ROM
with 0xAA55 signature before assigning the shadow ROM to device.

Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
---
Note:
This patch was originally sent on 2025/04/06, but did not receive any
feedback. Resending in case it was missed. The commit message is updated
in this version.

 arch/x86/pci/fixup.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/pci/fixup.c b/arch/x86/pci/fixup.c
index e7e71490bd25..362afaaf9874 100644
--- a/arch/x86/pci/fixup.c
+++ b/arch/x86/pci/fixup.c
@@ -317,6 +317,7 @@ static void pci_fixup_video(struct pci_dev *pdev)
 	struct pci_bus *bus;
 	u16 config;
 	struct resource *res;
+	void __iomem *rom;
 
 	/* Is VGA routed to us? */
 	bus = pdev->bus;
@@ -338,9 +339,12 @@ static void pci_fixup_video(struct pci_dev *pdev)
 		}
 		bus = bus->parent;
 	}
-	if (!vga_default_device() || pdev == vga_default_device()) {
+
+	rom = ioremap(0xC0000, 0x20000);
+	if (rom && (!vga_default_device() || pdev == vga_default_device())) {
 		pci_read_config_word(pdev, PCI_COMMAND, &config);
-		if (config & (PCI_COMMAND_IO | PCI_COMMAND_MEMORY)) {
+		if ((config & (PCI_COMMAND_IO | PCI_COMMAND_MEMORY)) &&
+		    (readw(rom) == 0xAA55)) {
 			res = &pdev->resource[PCI_ROM_RESOURCE];
 
 			pci_disable_rom(pdev);
@@ -354,6 +358,7 @@ static void pci_fixup_video(struct pci_dev *pdev)
 			dev_info(&pdev->dev, "Video device with shadowed ROM at %pR\n",
 				 res);
 		}
+		iounmap(rom);
 	}
 }
 DECLARE_PCI_FIXUP_CLASS_HEADER(PCI_ANY_ID, PCI_ANY_ID,
-- 
2.50.1