[PATCH v3] mtd: maps: add INT0800 firmware-flash map driver

[PATCH v3] mtd: maps: add INT0800 firmware-flash map driver

[Stephen Bancroft]

Add a read-only mapping driver that binds the ACPI INT0800 "Intel
82802 firmware hub" device and exposes the system firmware flash as an
MTD ROM device.

On x86 machines the boot flash is decoded into the physical address
space below 4 GB, so a plain ioremap() of the declared resource window
is sufficient for reads - no SPI or LPC controller access is needed.
The window may be larger than the real flash; undecoded holes read as
0xff.

This gives userspace a clean, safe way to read firmware flash contents
without flashrom or relaxed /dev/mem access - for firmware analysis,
and for extracting option ROMs stored inside EFI firmware volumes
(e.g. the NVIDIA VBIOS on EFI-booted Apple machines, which can then be
fed to nouveau via nouveau.config=NvBios=). There is deliberately no
write or erase support.

Tested on a MacBookPro4,1 (ICH8M): /dev/mtd0 reads are byte-identical
to a flashrom dump of the 2 MiB SST25VF016B, except for live NVRAM
variable-store regions.

Signed-off-by: Stephen Bancroft <stevereaver@gmail.com>
---
Changes in v3:
- MODULE_AUTHOR: use full author identity (per Miquèl Raynal's review)

Changes in v2:
- Per-device state via devm_kzalloc instead of globals (fixes shared
  state corruption with multiple INT0800 devices)
- Clean up map probe on mtd_device_register() failure (fixes
  memory/mapping leak)
- Use %pa for resource_size_t in dev_info (fixes format string on
  32-bit)
- Add missing Signed-off-by

 drivers/mtd/maps/Kconfig   |  17 ++++++
 drivers/mtd/maps/Makefile  |   1 +
 drivers/mtd/maps/int0800.c | 108 +++++++++++++++++++++++++++++++++++++
 3 files changed, 126 insertions(+)
 create mode 100644 drivers/mtd/maps/int0800.c

diff --git a/drivers/mtd/maps/Kconfig b/drivers/mtd/maps/Kconfig
index 1bb3dba2631d..649fd145e15d 100644
--- a/drivers/mtd/maps/Kconfig
+++ b/drivers/mtd/maps/Kconfig
@@ -161,6 +161,23 @@ config MTD_AMD76XROM
 
 	  BE VERY CAREFUL.
 
+config MTD_INT0800
+	tristate "Read-only BIOS/firmware flash via ACPI INT0800"
+	depends on X86 && ACPI
+	select MTD_ROM
+	help
+	  Support for reading the system firmware (BIOS/EFI) flash chip
+	  through the memory window described by the ACPI INT0800
+	  "82802 firmware hub" device, present on most x86 machines.
+
+	  The flash is exposed read-only via the ROM chip driver, e.g.
+	  for firmware analysis or extracting option ROMs (such as
+	  video BIOS images embedded in EFI firmware volumes). There is
+	  no write or erase support.
+
+	  To compile this driver as a module, choose M here: the
+	  module will be called int0800.
+
 config MTD_ICHXROM
 	tristate "BIOS flash chip on Intel Controller Hub 2/3/4/5"
 	depends on X86 && MTD_JEDECPROBE
diff --git a/drivers/mtd/maps/Makefile b/drivers/mtd/maps/Makefile
index 01745eca1f73..2f1a7375e13c 100644
--- a/drivers/mtd/maps/Makefile
+++ b/drivers/mtd/maps/Makefile
@@ -14,6 +14,7 @@ obj-$(CONFIG_MTD_L440GX)	+= l440gx.o
 obj-$(CONFIG_MTD_AMD76XROM)	+= amd76xrom.o
 obj-$(CONFIG_MTD_ESB2ROM)	+= esb2rom.o
 obj-$(CONFIG_MTD_ICHXROM)	+= ichxrom.o
+obj-$(CONFIG_MTD_INT0800)	+= int0800.o
 obj-$(CONFIG_MTD_CK804XROM)	+= ck804xrom.o
 obj-$(CONFIG_MTD_TSUNAMI)	+= tsunami_flash.o
 obj-$(CONFIG_MTD_PXA2XX)	+= pxa2xx-flash.o
diff --git a/drivers/mtd/maps/int0800.c b/drivers/mtd/maps/int0800.c
new file mode 100644
index 000000000000..0188d58b168d
--- /dev/null
+++ b/drivers/mtd/maps/int0800.c
@@ -0,0 +1,108 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Read-only MTD access to the system firmware flash behind the ACPI
+ * INT0800 "Intel 82802 firmware hub" device.
+ *
+ * On x86 systems the boot flash is decoded into the physical address
+ * space below 4 GB, so a plain ioremap() is sufficient to read it -
+ * no SPI or LPC controller access is required.  The declared _CRS
+ * window may be larger than the real flash (the whole top-16MiB
+ * decode range is commonly claimed); undecoded holes read as 0xff.
+ *
+ * The device is exposed read-only via the ROM chip driver; there is
+ * deliberately no write or erase support.
+ */
+
+#include <linux/module.h>
+#include <linux/acpi.h>
+#include <linux/io.h>
+#include <linux/mtd/mtd.h>
+#include <linux/mtd/map.h>
+#include <linux/platform_device.h>
+#include <linux/slab.h>
+
+/* top-of-4GB firmware decode, used when _CRS reports no window */
+#define INT0800_DEFAULT_PHYS	0xffe00000UL
+#define INT0800_DEFAULT_SIZE	SZ_2M
+
+struct int0800 {
+	struct map_info		map;
+	struct mtd_info		*mtd;
+};
+
+static int int0800_probe(struct platform_device *pdev)
+{
+	struct resource *res;
+	struct int0800 *fw;
+	resource_size_t end;
+	int ret;
+
+	fw = devm_kzalloc(&pdev->dev, sizeof(*fw), GFP_KERNEL);
+	if (!fw)
+		return -ENOMEM;
+
+	fw->map.name = dev_name(&pdev->dev);
+	fw->map.bankwidth = 1;
+
+	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
+	if (res) {
+		fw->map.phys = res->start;
+		fw->map.size = resource_size(res);
+	} else {
+		fw->map.phys = INT0800_DEFAULT_PHYS;
+		fw->map.size = INT0800_DEFAULT_SIZE;
+	}
+
+	/*
+	 * Plain ioremap on purpose: the window is already claimed by the
+	 * ACPI/pnp resource reservation, so devm_ioremap_resource() would
+	 * fail with -EBUSY.
+	 */
+	fw->map.virt = devm_ioremap(&pdev->dev, fw->map.phys, fw->map.size);
+	if (!fw->map.virt)
+		return -ENOMEM;
+
+	simple_map_init(&fw->map);
+	fw->mtd = do_map_probe("map_rom", &fw->map);
+	if (!fw->mtd)
+		return -ENODEV;
+	fw->mtd->dev.parent = &pdev->dev;
+	platform_set_drvdata(pdev, fw);
+
+	end = fw->map.phys + fw->map.size - 1;
+	dev_info(&pdev->dev, "mapped firmware window %pa-%pa\n",
+		 &fw->map.phys, &end);
+
+	ret = mtd_device_register(fw->mtd, NULL, 0);
+	if (ret)
+		map_destroy(fw->mtd);
+	return ret;
+}
+
+static void int0800_remove(struct platform_device *pdev)
+{
+	struct int0800 *fw = platform_get_drvdata(pdev);
+
+	mtd_device_unregister(fw->mtd);
+	map_destroy(fw->mtd);
+}
+
+static const struct acpi_device_id int0800_ids[] = {
+	{ "INT0800", 0 },
+	{ }
+};
+MODULE_DEVICE_TABLE(acpi, int0800_ids);
+
+static struct platform_driver int0800_driver = {
+	.probe	= int0800_probe,
+	.remove	= int0800_remove,
+	.driver	= {
+		.name		= "int0800",
+		.acpi_match_table = int0800_ids,
+	},
+};
+module_platform_driver(int0800_driver);
+
+MODULE_AUTHOR("Stephen Bancroft <stevereaver@gmail.com>");
+MODULE_DESCRIPTION("Read-only MTD map over the INT0800 firmware flash window");
+MODULE_LICENSE("GPL");
-- 
2.43.0

Re: [PATCH v3] mtd: maps: add INT0800 firmware-flash map driver

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] mtd: maps: Use-After-Free in int0800 via sysfs driver unbind
--

commit 2d904c4a0ae5009245dd1587c804f0ac6e9402b8
Author: Stephen Bancroft <stevereaver@gmail.com>

mtd: maps: add INT0800 firmware-flash map driver

Adds a read-only MTD mapping driver that binds the ACPI INT0800 device and
exposes the system firmware flash as an MTD ROM device.

> diff --git a/drivers/mtd/maps/int0800.c b/drivers/mtd/maps/int0800.c
> new file mode 100644
> index 0000000000000..0188d58b168d9
> --- /dev/null
> +++ b/drivers/mtd/maps/int0800.c

[ ... ]

> +static void int0800_remove(struct platform_device *pdev)
> +{
> +	struct int0800 *fw = platform_get_drvdata(pdev);
> +
> +	mtd_device_unregister(fw->mtd);
> +	map_destroy(fw->mtd);
> +}

[Severity: High]
Can this lead to a use-after-free and unmapped memory dereference if a
user forces a driver unbind via sysfs while the character device is still
open?

If a privileged user opens /dev/mtdX and then writes to
/sys/bus/platform/drivers/int0800/unbind, int0800_remove() is invoked.
This unconditionally calls map_destroy(fw->mtd), freeing the mtd_info
structure and allowing devres to unmap fw->map.virt.

Since mtdchar_open() incrementing the module use-count does not prevent
forced sysfs unbinds, a subsequent read() on the still-open file
descriptor would dereference the freed mtd_info and read from the
unmapped memory.

Should this driver set .suppress_bind_attrs = true in the platform_driver
to prevent forced sysfs unbinds?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260921021459.283023-1-stevereaver@gmail.com?part=1