net: Fix recursive descent in __scm_destroy().

__scm_destroy() walks the list of file descriptors in the scm_fp_list
pointed to by the scm_cookie argument.

Those, in turn, can close sockets and invoke __scm_destroy() again.

There is nothing which limits how deeply this can occur.

The idea for how to fix this is from Linus.  Basically, we do all of
the fput()s at the top level by collecting all of the scm_fp_list
objects hit by an fput().  Inside of the initial __scm_destroy() we
keep running the list until it is empty.
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/sched.h

@@ -1349,6 +1349,8 @@ struct task_struct {
 	 */
 	unsigned long timer_slack_ns;
 	unsigned long default_timer_slack_ns;
+
+	struct list_head	*scm_work_list;
 };
 
 /*

include/net/scm.h

@@ -14,6 +14,7 @@
 
 struct scm_fp_list
 {
+	struct list_head	list;
 	int			count;
 	struct file		*fp[SCM_MAX_FD];
 };

net/core/scm.c

@@ -75,6 +75,7 @@ static int scm_fp_copy(struct cmsghdr *cmsg, struct scm_fp_list **fplp)
 		if (!fpl)
 			return -ENOMEM;
 		*fplp = fpl;
+		INIT_LIST_HEAD(&fpl->list);
 		fpl->count = 0;
 	}
 	fpp = &fpl->fp[fpl->count];
@@ -106,10 +107,26 @@ void __scm_destroy(struct scm_cookie *scm)
 
 	if (fpl) {
 		scm->fp = NULL;
+		if (current->scm_work_list) {
+			list_add_tail(&fpl->list, current->scm_work_list);
+		} else {
+			LIST_HEAD(work_list);
+
+			current->scm_work_list = &work_list;
+
+			list_add(&fpl->list, &work_list);
+			while (!list_empty(&work_list)) {
+				fpl = list_first_entry(&work_list, struct scm_fp_list, list);
+
+				list_del(&fpl->list);
 				for (i=fpl->count-1; i>=0; i--)
 					fput(fpl->fp[i]);
 				kfree(fpl);
 			}
+
+			current->scm_work_list = NULL;
+		}
+	}
 }
 
 int __scm_send(struct socket *sock, struct msghdr *msg, struct scm_cookie *p)
@@ -284,6 +301,7 @@ struct scm_fp_list *scm_fp_dup(struct scm_fp_list *fpl)
 
 	new_fpl = kmalloc(sizeof(*fpl), GFP_KERNEL);
 	if (new_fpl) {
+		INIT_LIST_HEAD(&new_fpl->list);
 		for (i=fpl->count-1; i>=0; i--)
 			get_file(fpl->fp[i]);
 		memcpy(new_fpl, fpl, sizeof(*fpl));