eventpoll: Fix semi-unbounded recursion

Ensure that epoll instances can never form a graph deeper than
EP_MAX_NESTS+1 links.

Currently, ep_loop_check_proc() ensures that the graph is loop-free and
does some recursion depth checks, but those recursion depth checks don't
limit the depth of the resulting tree for two reasons:

 - They don't look upwards in the tree.
 - If there are multiple downwards paths of different lengths, only one of
   the paths is actually considered for the depth check since commit
   28d82dc ("epoll: limit paths").

Essentially, the current recursion depth check in ep_loop_check_proc() just
serves to prevent it from recursing too deeply while checking for loops.

A more thorough check is done in reverse_path_check() after the new graph
edge has already been created; this checks, among other things, that no
paths going upwards from any non-epoll file with a length of more than 5
edges exist. However, this check does not apply to non-epoll files.

As a result, it is possible to recurse to a depth of at least roughly 500,
tested on v6.15. (I am unsure if deeper recursion is possible; and this may
have changed with commit 8c44dac ("eventpoll: Fix priority inversion
problem").)

To fix it:

1. In ep_loop_check_proc(), note the subtree depth of each visited node,
and use subtree depths for the total depth calculation even when a subtree
has already been visited.
2. Add ep_get_upwards_depth_proc() for similarly determining the maximum
depth of an upwards walk.
3. In ep_loop_check(), use these values to limit the total path length
between epoll nodes to EP_MAX_NESTS edges.

Fixes: 22bacca ("epoll: prevent creating circular epoll structures")
Cc: stable@vger.kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
Link: https://lore.kernel.org/20250711-epoll-recursion-fix-v1-1-fb2457c33292@google.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: thejh

fs/eventpoll.c

@@ -218,6 +218,7 @@ struct eventpoll {
 	/* used to optimize loop detection check */
 	u64 gen;
 	struct hlist_head refs;
+	u8 loop_check_depth;
 
 	/*
 	 * usage count, used together with epitem->dying to
@@ -2142,37 +2143,36 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
 }
 
 /**
- * ep_loop_check_proc - verify that adding an epoll file inside another
- *                      epoll structure does not violate the constraints, in
- *                      terms of closed loops, or too deep chains (which can
- *                      result in excessive stack usage).
+ * ep_loop_check_proc - verify that adding an epoll file @ep inside another
+ *                      epoll file does not create closed loops, and
+ *                      determine the depth of the subtree starting at @ep
  *
  * @ep: the &struct eventpoll to be currently checked.
  * @depth: Current depth of the path being checked.
  *
- * Return: %zero if adding the epoll @file inside current epoll
- *          structure @ep does not violate the constraints, or %-1 otherwise.
+ * Return: depth of the subtree, or INT_MAX if we found a loop or went too deep.
  */
 static int ep_loop_check_proc(struct eventpoll *ep, int depth)
 {
-	int error = 0;
+	int result = 0;
 	struct rb_node *rbp;
 	struct epitem *epi;
 
+	if (ep->gen == loop_check_gen)
+		return ep->loop_check_depth;
+
 	mutex_lock_nested(&ep->mtx, depth + 1);
 	ep->gen = loop_check_gen;
 	for (rbp = rb_first_cached(&ep->rbr); rbp; rbp = rb_next(rbp)) {
 		epi = rb_entry(rbp, struct epitem, rbn);
 		if (unlikely(is_file_epoll(epi->ffd.file))) {
 			struct eventpoll *ep_tovisit;
 			ep_tovisit = epi->ffd.file->private_data;
-			if (ep_tovisit->gen == loop_check_gen)
-				continue;
 			if (ep_tovisit == inserting_into || depth > EP_MAX_NESTS)
-				error = -1;
+				result = INT_MAX;
 			else
-				error = ep_loop_check_proc(ep_tovisit, depth + 1);
-			if (error != 0)
+				result = max(result, ep_loop_check_proc(ep_tovisit, depth + 1) + 1);
+			if (result > EP_MAX_NESTS)
 				break;
 		} else {
 			/*
@@ -2186,9 +2186,27 @@ static int ep_loop_check_proc(struct eventpoll *ep, int depth)
 			list_file(epi->ffd.file);
 		}
 	}
+	ep->loop_check_depth = result;
 	mutex_unlock(&ep->mtx);
 
-	return error;
+	return result;
+}
+
+/**
+ * ep_get_upwards_depth_proc - determine depth of @ep when traversed upwards
+ */
+static int ep_get_upwards_depth_proc(struct eventpoll *ep, int depth)
+{
+	int result = 0;
+	struct epitem *epi;
+
+	if (ep->gen == loop_check_gen)
+		return ep->loop_check_depth;
+	hlist_for_each_entry_rcu(epi, &ep->refs, fllink)
+		result = max(result, ep_get_upwards_depth_proc(epi->ep, depth + 1) + 1);
+	ep->gen = loop_check_gen;
+	ep->loop_check_depth = result;
+	return result;
 }
 
 /**
@@ -2204,8 +2222,22 @@ static int ep_loop_check_proc(struct eventpoll *ep, int depth)
  */
 static int ep_loop_check(struct eventpoll *ep, struct eventpoll *to)
 {
+	int depth, upwards_depth;
+
 	inserting_into = ep;
-	return ep_loop_check_proc(to, 0);
+	/*
+	 * Check how deep down we can get from @to, and whether it is possible
+	 * to loop up to @ep.
+	 */
+	depth = ep_loop_check_proc(to, 0);
+	if (depth > EP_MAX_NESTS)
+		return -1;
+	/* Check how far up we can go from @ep. */
+	rcu_read_lock();
+	upwards_depth = ep_get_upwards_depth_proc(ep, 0);
+	rcu_read_unlock();
+
+	return (depth+1+upwards_depth > EP_MAX_NESTS) ? -1 : 0;
 }
 
 static void clear_tfile_check_list(void)