apparmor: avoid per-cpu hold underflow in aa_get_buffer

When aa_get_buffer() pulls from the per-cpu list it unconditionally
decrements cache->hold. If hold reaches 0 while count is still non-zero,
the unsigned decrement wraps to UINT_MAX. This keeps hold non-zero for a
very long time, so aa_put_buffer() never returns buffers to the global
list, which can starve other CPUs and force repeated kmalloc(aa_g_path_max)
allocations.

Guard the decrement so hold never underflows.
Fixes: ea9bae1 ("apparmor: cache buffers on percpu list if there is lock contention")

Signed-off-by: Zhengmian Hu <huzhengmian@gmail.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>

Author: xiaoniu-578fa6bff964d005

security/apparmor/lsm.c

@@ -2137,7 +2137,8 @@ char *aa_get_buffer(bool in_atomic)
 	if (!list_empty(&cache->head)) {
 		aa_buf = list_first_entry(&cache->head, union aa_buffer, list);
 		list_del(&aa_buf->list);
-		cache->hold--;
+		if (cache->hold)
+			cache->hold--;
 		cache->count--;
 		put_cpu_ptr(&aa_local_buffers);
 		return &aa_buf->buffer[0];