ARM: 9485/1: mm: acquire mmap write lock around show_pte() for user faultsgrafted

When CONFIG_DEBUG_USER=y, and cmdline "user_debug=31" is set,
a user fault may trigger show_pte() without any lock.
If another thread in the same process concurrently calls munmap(),
the page table pages may be freed while show_pte() is still traversing
them, causing a use-after-free in show_pte().

If CONFIG_ARM_LPAE=y, this may cause a kernel panic if the pages table
of PMD are freed when show_pte() is running.

Acquire mmap_write_lock() around show_pte() for user faults to fix the
contention.

For user faults, additionally restrict that show_pte() is called only
when the addr is a user-space address (addr < TASK_SIZE). This is because
the lock of tsk->mm only protects the virtual memory of user address space,
furthermore, dumping the page tables of a kernel-space address for user
faults is unnecessary and may have security implications.

Keep everything unchanged for kernel faults, because the kernel is
already in the "oops" state, acquiring a lock may risk a deadlock.

Co-developed-by: Qi Xi <xiqi2@huawei.com>

Fixes: 6d021b724481 ("ARM: dump pgd, pmd and pte states on unhandled data abort faults")
Link: https://lore.kernel.org/20260716014022.2823-1-xieyuanbin1@huawei.com
Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Signed-off-by: Qi Xi <xiqi2@huawei.com>
Signed-off-by: Xie Yuanbin <xieyuanbin1@huawei.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

1 files changed, 233 insertions, 0 deletions

diff --git a/samples/vfio-mdev/mdpy-fb.c b/samples/vfio-mdev/mdpy-fb.c
new file mode 100644
index 000000000..149af7f59
--- /dev/null
+++ b/samples/vfio-mdev/mdpy-fb.c
@@ -0,0 +1,233 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Framebuffer driver for mdpy (mediated virtual pci display device).
+ *
+ * See mdpy-defs.h for device specs
+ *
+ *   (c) Gerd Hoffmann <kraxel@redhat.com>
+ *
+ * Using some code snippets from simplefb and cirrusfb.
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ */
+#include <linux/errno.h>
+#include <linux/fb.h>
+#include <linux/io.h>
+#include <linux/pci.h>
+#include <linux/module.h>
+#include <drm/drm_fourcc.h>
+#include "mdpy-defs.h"
+
+static const struct fb_fix_screeninfo mdpy_fb_fix = {
+	.id		= "mdpy-fb",
+	.type		= FB_TYPE_PACKED_PIXELS,
+	.visual		= FB_VISUAL_TRUECOLOR,
+	.accel		= FB_ACCEL_NONE,
+};
+
+static const struct fb_var_screeninfo mdpy_fb_var = {
+	.height		= -1,
+	.width		= -1,
+	.activate	= FB_ACTIVATE_NOW,
+	.vmode		= FB_VMODE_NONINTERLACED,
+
+	.bits_per_pixel = 32,
+	.transp.offset	= 24,
+	.red.offset	= 16,
+	.green.offset	= 8,
+	.blue.offset	= 0,
+	.transp.length	= 8,
+	.red.length	= 8,
+	.green.length	= 8,
+	.blue.length	= 8,
+};
+
+#define PSEUDO_PALETTE_SIZE 16
+
+struct mdpy_fb_par {
+	u32 palette[PSEUDO_PALETTE_SIZE];
+};
+
+static int mdpy_fb_setcolreg(u_int regno, u_int red, u_int green, u_int blue,
+			      u_int transp, struct fb_info *info)
+{
+	u32 *pal = info->pseudo_palette;
+	u32 cr = red >> (16 - info->var.red.length);
+	u32 cg = green >> (16 - info->var.green.length);
+	u32 cb = blue >> (16 - info->var.blue.length);
+	u32 value, mask;
+
+	if (regno >= PSEUDO_PALETTE_SIZE)
+		return -EINVAL;
+
+	value = (cr << info->var.red.offset) |
+		(cg << info->var.green.offset) |
+		(cb << info->var.blue.offset);
+	if (info->var.transp.length > 0) {
+		mask = (1 << info->var.transp.length) - 1;
+		mask <<= info->var.transp.offset;
+		value |= mask;
+	}
+	pal[regno] = value;
+
+	return 0;
+}
+
+static void mdpy_fb_destroy(struct fb_info *info)
+{
+	if (info->screen_base)
+		iounmap(info->screen_base);
+}
+
+static const struct fb_ops mdpy_fb_ops = {
+	.owner		= THIS_MODULE,
+	FB_DEFAULT_IOMEM_OPS,
+	.fb_destroy	= mdpy_fb_destroy,
+	.fb_setcolreg	= mdpy_fb_setcolreg,
+};
+
+static int mdpy_fb_probe(struct pci_dev *pdev,
+			 const struct pci_device_id *ent)
+{
+	struct fb_info *info;
+	struct mdpy_fb_par *par;
+	u32 format, width, height;
+	int ret;
+
+	ret = pci_enable_device(pdev);
+	if (ret < 0)
+		return ret;
+
+	ret = pci_request_regions(pdev, "mdpy-fb");
+	if (ret < 0)
+		goto err_disable_dev;
+
+	pci_read_config_dword(pdev, MDPY_FORMAT_OFFSET, &format);
+	pci_read_config_dword(pdev, MDPY_WIDTH_OFFSET,	&width);
+	pci_read_config_dword(pdev, MDPY_HEIGHT_OFFSET, &height);
+	if (format != DRM_FORMAT_XRGB8888) {
+		pci_err(pdev, "format mismatch (0x%x != 0x%x)\n",
+			format, DRM_FORMAT_XRGB8888);
+		ret = -EINVAL;
+		goto err_release_regions;
+	}
+	if (width < 100	 || width > 10000) {
+		pci_err(pdev, "width (%d) out of range\n", width);
+		ret = -EINVAL;
+		goto err_release_regions;
+	}
+	if (height < 100 || height > 10000) {
+		pci_err(pdev, "height (%d) out of range\n", height);
+		ret = -EINVAL;
+		goto err_release_regions;
+	}
+	pci_info(pdev, "mdpy found: %dx%d framebuffer\n",
+		 width, height);
+
+	info = framebuffer_alloc(sizeof(struct mdpy_fb_par), &pdev->dev);
+	if (!info) {
+		ret = -ENOMEM;
+		goto err_release_regions;
+	}
+	pci_set_drvdata(pdev, info);
+	par = info->par;
+
+	info->fix = mdpy_fb_fix;
+	info->fix.smem_start = pci_resource_start(pdev, 0);
+	info->fix.smem_len = pci_resource_len(pdev, 0);
+	info->fix.line_length = width * 4;
+
+	info->var = mdpy_fb_var;
+	info->var.xres = width;
+	info->var.yres = height;
+	info->var.xres_virtual = width;
+	info->var.yres_virtual = height;
+
+	info->screen_size = info->fix.smem_len;
+	info->screen_base = ioremap(info->fix.smem_start,
+				    info->screen_size);
+	if (!info->screen_base) {
+		pci_err(pdev, "ioremap(pcibar) failed\n");
+		ret = -EIO;
+		goto err_release_fb;
+	}
+
+	info->fbops = &mdpy_fb_ops;
+	info->pseudo_palette = par->palette;
+
+	ret = register_framebuffer(info);
+	if (ret < 0) {
+		pci_err(pdev, "mdpy-fb device register failed: %d\n", ret);
+		goto err_unmap;
+	}
+
+	pci_info(pdev, "fb%d registered\n", info->node);
+	return 0;
+
+err_unmap:
+	iounmap(info->screen_base);
+
+err_release_fb:
+	framebuffer_release(info);
+
+err_release_regions:
+	pci_release_regions(pdev);
+
+err_disable_dev:
+	pci_disable_device(pdev);
+
+	return ret;
+}
+
+static void mdpy_fb_remove(struct pci_dev *pdev)
+{
+	struct fb_info *info = pci_get_drvdata(pdev);
+
+	unregister_framebuffer(info);
+	iounmap(info->screen_base);
+	framebuffer_release(info);
+	pci_release_regions(pdev);
+	pci_disable_device(pdev);
+}
+
+static struct pci_device_id mdpy_fb_pci_table[] = {
+	{
+		.vendor	   = MDPY_PCI_VENDOR_ID,
+		.device	   = MDPY_PCI_DEVICE_ID,
+		.subvendor = MDPY_PCI_SUBVENDOR_ID,
+		.subdevice = MDPY_PCI_SUBDEVICE_ID,
+	}, {
+		/* end of list */
+	}
+};
+
+static struct pci_driver mdpy_fb_pci_driver = {
+	.name		= "mdpy-fb",
+	.id_table	= mdpy_fb_pci_table,
+	.probe		= mdpy_fb_probe,
+	.remove		= mdpy_fb_remove,
+};
+
+static int __init mdpy_fb_init(void)
+{
+	int ret;
+
+	ret = pci_register_driver(&mdpy_fb_pci_driver);
+	if (ret)
+		return ret;
+
+	return 0;
+}
+
+module_init(mdpy_fb_init);
+
+MODULE_DEVICE_TABLE(pci, mdpy_fb_pci_table);
+MODULE_DESCRIPTION("Framebuffer driver for mdpy (mediated virtual pci display device)");
+MODULE_LICENSE("GPL v2");