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, 90 insertions, 0 deletions

diff --git a/drivers/video/fbdev/bt455.h b/drivers/video/fbdev/bt455.h
new file mode 100644
index 000000000..dd1404b40
--- /dev/null
+++ b/drivers/video/fbdev/bt455.h
@@ -0,0 +1,90 @@
+/*
+ *	linux/drivers/video/bt455.h
+ *
+ *	Copyright 2003  Thiemo Seufer <seufer@csv.ica.uni-stuttgart.de>
+ *	Copyright 2016  Maciej W. Rozycki <macro@linux-mips.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/types.h>
+
+/*
+ * Bt455 byte-wide registers, 32-bit aligned.
+ */
+struct bt455_regs {
+	volatile u8 addr_cmap;
+	u8 pad0[3];
+	volatile u8 addr_cmap_data;
+	u8 pad1[3];
+	volatile u8 addr_clr;
+	u8 pad2[3];
+	volatile u8 addr_ovly;
+	u8 pad3[3];
+};
+
+static inline void bt455_select_reg(struct bt455_regs *regs, int ir)
+{
+	mb();
+	regs->addr_cmap = ir & 0x0f;
+}
+
+static inline void bt455_reset_reg(struct bt455_regs *regs)
+{
+	mb();
+	regs->addr_clr = 0;
+}
+
+/*
+ * Read/write to a Bt455 color map register.
+ */
+static inline void bt455_read_cmap_next(struct bt455_regs *regs, u8 *grey)
+{
+	mb();
+	regs->addr_cmap_data;
+	rmb();
+	*grey = regs->addr_cmap_data & 0xf;
+	rmb();
+	regs->addr_cmap_data;
+}
+
+static inline void bt455_write_cmap_next(struct bt455_regs *regs, u8 grey)
+{
+	wmb();
+	regs->addr_cmap_data = 0x0;
+	wmb();
+	regs->addr_cmap_data = grey & 0xf;
+	wmb();
+	regs->addr_cmap_data = 0x0;
+}
+
+static inline void bt455_write_ovly_next(struct bt455_regs *regs, u8 grey)
+{
+	wmb();
+	regs->addr_ovly = 0x0;
+	wmb();
+	regs->addr_ovly = grey & 0xf;
+	wmb();
+	regs->addr_ovly = 0x0;
+}
+
+static inline void bt455_read_cmap_entry(struct bt455_regs *regs,
+					 int cr, u8 *grey)
+{
+	bt455_select_reg(regs, cr);
+	bt455_read_cmap_next(regs, grey);
+}
+
+static inline void bt455_write_cmap_entry(struct bt455_regs *regs,
+					  int cr, u8 grey)
+{
+	bt455_select_reg(regs, cr);
+	bt455_write_cmap_next(regs, grey);
+}
+
+static inline void bt455_write_ovly_entry(struct bt455_regs *regs, u8 grey)
+{
+	bt455_reset_reg(regs);
+	bt455_write_ovly_next(regs, grey);
+}