linux-kernel - [PATCH] x86/pci: Check signature before assigning shadow ROM

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Message-ID: <20250406090835.7721-1-tomitamoeko@gmail.com>
Date: Sun,  6 Apr 2025 17:08:34 +0800
From: Tomita Moeko <tomitamoeko@...il.com>
To: Bjorn Helgaas <bhelgaas@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>
Cc: Tomita Moeko <tomitamoeko@...il.com>,
	linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] x86/pci: Check signature before assigning shadow ROM

Recent IGD platforms without VBIOS or UEFI CSM support do not contain
VGA ROM at 0xC0000. Check whether the VGA ROM region is a valid PCI
option ROM with 0xAA55 signature before assigning the shadow ROM.

Signed-off-by: Tomita Moeko <tomitamoeko@...il.com>
---
 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 efefeb82ab61..da9fb86c2ea0 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,
-- 
2.47.2