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, 106 insertions, 0 deletions

diff --git a/net/ceph/string_table.c b/net/ceph/string_table.c
new file mode 100644
index 000000000..3191d9d16
--- /dev/null
+++ b/net/ceph/string_table.c
@@ -0,0 +1,106 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/slab.h>
+#include <linux/gfp.h>
+#include <linux/string.h>
+#include <linux/spinlock.h>
+#include <linux/ceph/string_table.h>
+
+static DEFINE_SPINLOCK(string_tree_lock);
+static struct rb_root string_tree = RB_ROOT;
+
+struct ceph_string *ceph_find_or_create_string(const char* str, size_t len)
+{
+	struct ceph_string *cs, *exist;
+	struct rb_node **p, *parent;
+	int ret;
+
+	exist = NULL;
+	spin_lock(&string_tree_lock);
+	p = &string_tree.rb_node;
+	while (*p) {
+		exist = rb_entry(*p, struct ceph_string, node);
+		ret = ceph_compare_string(exist, str, len);
+		if (ret > 0)
+			p = &(*p)->rb_left;
+		else if (ret < 0)
+			p = &(*p)->rb_right;
+		else
+			break;
+		exist = NULL;
+	}
+	if (exist && !kref_get_unless_zero(&exist->kref)) {
+		rb_erase(&exist->node, &string_tree);
+		RB_CLEAR_NODE(&exist->node);
+		exist = NULL;
+	}
+	spin_unlock(&string_tree_lock);
+	if (exist)
+		return exist;
+
+	cs = kmalloc(sizeof(*cs) + len + 1, GFP_NOFS);
+	if (!cs)
+		return NULL;
+
+	kref_init(&cs->kref);
+	cs->len = len;
+	memcpy(cs->str, str, len);
+	cs->str[len] = 0;
+
+retry:
+	exist = NULL;
+	parent = NULL;
+	p = &string_tree.rb_node;
+	spin_lock(&string_tree_lock);
+	while (*p) {
+		parent = *p;
+		exist = rb_entry(*p, struct ceph_string, node);
+		ret = ceph_compare_string(exist, str, len);
+		if (ret > 0)
+			p = &(*p)->rb_left;
+		else if (ret < 0)
+			p = &(*p)->rb_right;
+		else
+			break;
+		exist = NULL;
+	}
+	ret = 0;
+	if (!exist) {
+		rb_link_node(&cs->node, parent, p);
+		rb_insert_color(&cs->node, &string_tree);
+	} else if (!kref_get_unless_zero(&exist->kref)) {
+		rb_erase(&exist->node, &string_tree);
+		RB_CLEAR_NODE(&exist->node);
+		ret = -EAGAIN;
+	}
+	spin_unlock(&string_tree_lock);
+	if (ret == -EAGAIN)
+		goto retry;
+
+	if (exist) {
+		kfree(cs);
+		cs = exist;
+	}
+
+	return cs;
+}
+EXPORT_SYMBOL(ceph_find_or_create_string);
+
+void ceph_release_string(struct kref *ref)
+{
+	struct ceph_string *cs = container_of(ref, struct ceph_string, kref);
+
+	spin_lock(&string_tree_lock);
+	if (!RB_EMPTY_NODE(&cs->node)) {
+		rb_erase(&cs->node, &string_tree);
+		RB_CLEAR_NODE(&cs->node);
+	}
+	spin_unlock(&string_tree_lock);
+
+	kfree_rcu(cs, rcu);
+}
+EXPORT_SYMBOL(ceph_release_string);
+
+bool ceph_strings_empty(void)
+{
+	return RB_EMPTY_ROOT(&string_tree);
+}