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, 53 insertions, 0 deletions

diff --git a/scripts/include/xalloc.h b/scripts/include/xalloc.h
new file mode 100644
index 000000000..cdadb07d0
--- /dev/null
+++ b/scripts/include/xalloc.h
@@ -0,0 +1,53 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef XALLOC_H
+#define XALLOC_H
+
+#include <stdlib.h>
+#include <string.h>
+
+static inline void *xmalloc(size_t size)
+{
+	void *p = malloc(size);
+
+	if (!p)
+		exit(1);
+	return p;
+}
+
+static inline void *xcalloc(size_t nmemb, size_t size)
+{
+	void *p = calloc(nmemb, size);
+
+	if (!p)
+		exit(1);
+	return p;
+}
+
+static inline void *xrealloc(void *p, size_t size)
+{
+	p = realloc(p, size);
+	if (!p)
+		exit(1);
+	return p;
+}
+
+static inline char *xstrdup(const char *s)
+{
+	char *p = strdup(s);
+
+	if (!p)
+		exit(1);
+	return p;
+}
+
+static inline char *xstrndup(const char *s, size_t n)
+{
+	char *p = strndup(s, n);
+
+	if (!p)
+		exit(1);
+	return p;
+}
+
+#endif /* XALLOC_H */