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, 151 insertions, 0 deletions

diff --git a/drivers/video/fbdev/q40fb.c b/drivers/video/fbdev/q40fb.c
new file mode 100644
index 000000000..1ff8fa176
--- /dev/null
+++ b/drivers/video/fbdev/q40fb.c
@@ -0,0 +1,151 @@
+/*
+ * linux/drivers/video/q40fb.c -- Q40 frame buffer device
+ *
+ * Copyright (C) 2001
+ *
+ *      Richard Zidlicky <rz@linux-m68k.org>
+ *
+ *  This file is subject to the terms and conditions of the GNU General Public
+ *  License. See the file COPYING in the main directory of this archive for
+ *  more details.
+ */
+
+#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/string.h>
+#include <linux/mm.h>
+#include <linux/delay.h>
+#include <linux/interrupt.h>
+#include <linux/platform_device.h>
+
+#include <linux/uaccess.h>
+#include <asm/setup.h>
+#include <asm/q40_master.h>
+#include <linux/fb.h>
+#include <linux/module.h>
+
+#define Q40_PHYS_SCREEN_ADDR 0xFE800000
+
+static struct fb_fix_screeninfo q40fb_fix = {
+	.id		= "Q40",
+	.smem_len	= 1024*1024,
+	.type		= FB_TYPE_PACKED_PIXELS,
+	.visual		= FB_VISUAL_TRUECOLOR,
+	.line_length	= 1024*2,
+	.accel		= FB_ACCEL_NONE,
+};
+
+static const struct fb_var_screeninfo q40fb_var = {
+	.xres		= 1024,
+	.yres		= 512,
+	.xres_virtual	= 1024,
+	.yres_virtual	= 512,
+	.bits_per_pixel	= 16,
+    	.red		= {6, 5, 0},
+	.green		= {11, 5, 0},
+	.blue		= {0, 6, 0},
+	.activate	= FB_ACTIVATE_NOW,
+	.height		= 230,
+	.width		= 300,
+	.vmode		= FB_VMODE_NONINTERLACED,
+};
+
+static int q40fb_setcolreg(unsigned regno, unsigned red, unsigned green,
+			   unsigned blue, unsigned transp,
+			   struct fb_info *info)
+{
+    /*
+     *  Set a single color register. The values supplied have a 16 bit
+     *  magnitude.
+     *  Return != 0 for invalid regno.
+     */
+
+    if (regno > 255)
+	    return 1;
+    red>>=11;
+    green>>=11;
+    blue>>=10;
+
+    if (regno < 16) {
+	((u32 *)info->pseudo_palette)[regno] = ((red & 31) <<6) |
+					       ((green & 31) << 11) |
+					       (blue & 63);
+    }
+    return 0;
+}
+
+static const struct fb_ops q40fb_ops = {
+	.owner		= THIS_MODULE,
+	FB_DEFAULT_IOMEM_OPS,
+	.fb_setcolreg	= q40fb_setcolreg,
+};
+
+static int q40fb_probe(struct platform_device *dev)
+{
+	struct fb_info *info;
+
+	if (!MACH_IS_Q40)
+		return -ENXIO;
+
+	/* mapped in q40/config.c */
+	q40fb_fix.smem_start = Q40_PHYS_SCREEN_ADDR;
+
+	info = framebuffer_alloc(sizeof(u32) * 16, &dev->dev);
+	if (!info)
+		return -ENOMEM;
+
+	info->var = q40fb_var;
+	info->fix = q40fb_fix;
+	info->fbops = &q40fb_ops;
+	info->pseudo_palette = info->par;
+	info->par = NULL;
+	info->screen_base = (char *) q40fb_fix.smem_start;
+
+	if (fb_alloc_cmap(&info->cmap, 256, 0) < 0) {
+		framebuffer_release(info);
+		return -ENOMEM;
+	}
+
+	master_outb(3, DISPLAY_CONTROL_REG);
+
+	if (register_framebuffer(info) < 0) {
+		printk(KERN_ERR "Unable to register Q40 frame buffer\n");
+		fb_dealloc_cmap(&info->cmap);
+		framebuffer_release(info);
+		return -EINVAL;
+	}
+
+	fb_info(info, "Q40 frame buffer alive and kicking !\n");
+	return 0;
+}
+
+static struct platform_driver q40fb_driver = {
+	.probe	= q40fb_probe,
+	.driver	= {
+		.name	= "q40fb",
+	},
+};
+
+static struct platform_device q40fb_device = {
+	.name	= "q40fb",
+};
+
+static int __init q40fb_init(void)
+{
+	int ret = 0;
+
+	if (fb_get_options("q40fb", NULL))
+		return -ENODEV;
+
+	ret = platform_driver_register(&q40fb_driver);
+
+	if (!ret) {
+		ret = platform_device_register(&q40fb_device);
+		if (ret)
+			platform_driver_unregister(&q40fb_driver);
+	}
+	return ret;
+}
+
+module_init(q40fb_init);
+MODULE_LICENSE("GPL");