net: Fix recursive descent in __scm_destroy().

commit f8d570a4 and
3b53fbf4 upstream (because once wasn't
good enough...)

__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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

include/linux/sched.h

@@ -1288,6 +1288,8 @@ struct task_struct {
 	atomic_t fs_excl;	/* holding fs exclusive resources */
 	struct rcu_head rcu;
 
+	struct list_head	*scm_work_list;
+
 	/*
 	 * cache last used pipe for splice
 	 */

include/net/scm.h

@@ -14,8 +14,9 @@
 
 struct scm_fp_list
 {
-	int		count;
-	struct file	*fp[SCM_MAX_FD];
+	struct list_head	list;
+	int			count;
+	struct file		*fp[SCM_MAX_FD];
 };
 
 struct scm_cookie

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];
@@ -106,9 +107,25 @@ void __scm_destroy(struct scm_cookie *scm)
 
 	if (fpl) {
 		scm->fp = NULL;
-		for (i=fpl->count-1; i>=0; i--)
-			fput(fpl->fp[i]);
-		kfree(fpl);
+		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;
+		}
 	}
 }
 
@@ -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));