mm, hwpoison: fix possible use-after-free in mf_dax_kill_procs()

After kill_procs(), tk will be freed without being removed from the
to_kill list.  In the next iteration, the freed list entry in the to_kill
list will be accessed, thus leading to use-after-free issue.  Adding
list_del() in kill_procs() to fix the issue.

Link: https://lkml.kernel.org/r/20220823032346.4260-5-linmiaohe@huawei.com
Fixes: c36e2024 ("mm: introduce mf_dax_kill_procs() for fsdax case")
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Acked-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memory-failure.c

@@ -413,7 +413,7 @@ static void kill_procs(struct list_head *to_kill, int forcekill, bool fail,
 {
 	struct to_kill *tk, *next;
 
-	list_for_each_entry_safe (tk, next, to_kill, nd) {
+	list_for_each_entry_safe(tk, next, to_kill, nd) {
 		if (forcekill) {
 			/*
 			 * In case something went wrong with munmapping
@@ -437,6 +437,7 @@ static void kill_procs(struct list_head *to_kill, int forcekill, bool fail,
 				pr_err("%#lx: Cannot send advisory machine check signal to %s:%d\n",
 				       pfn, tk->tsk->comm, tk->tsk->pid);
 		}
+		list_del(&tk->nd);
 		put_task_struct(tk->tsk);
 		kfree(tk);
 	}