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, 44 insertions, 0 deletions

diff --git a/io_uring/alloc_cache.c b/io_uring/alloc_cache.c
new file mode 100644
index 000000000..58423888b
--- /dev/null
+++ b/io_uring/alloc_cache.c
@@ -0,0 +1,44 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include "alloc_cache.h"
+
+void io_alloc_cache_free(struct io_alloc_cache *cache,
+			 void (*free)(const void *))
+{
+	void *entry;
+
+	if (!cache->entries)
+		return;
+
+	while ((entry = io_alloc_cache_get(cache)) != NULL)
+		free(entry);
+
+	kvfree(cache->entries);
+	cache->entries = NULL;
+}
+
+/* returns false if the cache was initialized properly */
+bool io_alloc_cache_init(struct io_alloc_cache *cache,
+			 unsigned max_nr, unsigned int size,
+			 unsigned int init_bytes)
+{
+	cache->entries = kvmalloc_array(max_nr, sizeof(void *), GFP_KERNEL);
+	if (!cache->entries)
+		return true;
+
+	cache->nr_cached = 0;
+	cache->max_cached = max_nr;
+	cache->elem_size = size;
+	cache->init_clear = init_bytes;
+	return false;
+}
+
+void *io_cache_alloc_new(struct io_alloc_cache *cache, gfp_t gfp)
+{
+	void *obj;
+
+	obj = kmalloc(cache->elem_size, gfp);
+	if (obj && cache->init_clear)
+		memset(obj, 0, cache->init_clear);
+	return obj;
+}