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, 42 insertions, 0 deletions

diff --git a/arch/powerpc/boot/stdlib.c b/arch/powerpc/boot/stdlib.c
new file mode 100644
index 000000000..868b019d6
--- /dev/null
+++ b/arch/powerpc/boot/stdlib.c
@@ -0,0 +1,42 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * stdlib functions
+ *
+ * Author: Scott Wood <scottwood@freescale.com>
+ *
+ * Copyright (c) 2007 Freescale Semiconductor, Inc.
+ */
+
+#include "stdlib.h"
+
+/* Not currently supported: leading whitespace, sign, 0x prefix, zero base */
+unsigned long long int strtoull(const char *ptr, char **end, int base)
+{
+	unsigned long long ret = 0;
+
+	if (base > 36)
+		goto out;
+
+	while (*ptr) {
+		int digit;
+
+		if (*ptr >= '0' && *ptr <= '9' && *ptr < '0' + base)
+			digit = *ptr - '0';
+		else if (*ptr >= 'A' && *ptr < 'A' + base - 10)
+			digit = *ptr - 'A' + 10;
+		else if (*ptr >= 'a' && *ptr < 'a' + base - 10)
+			digit = *ptr - 'a' + 10;
+		else
+			break;
+
+		ret *= base;
+		ret += digit;
+		ptr++;
+	}
+
+out:
+	if (end)
+		*end = (char *)ptr;
+
+	return ret;
+}