netdev - [PATCH v2] net: fix out-of-bounds access in ops

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Message-Id: <20240501151639.3369988-1-cascardo@igalia.com>
Date: Wed,  1 May 2024 12:16:39 -0300
From: Thadeu Lima de Souza Cascardo <cascardo@...lia.com>
To: netdev@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	kernel-dev@...lia.com,
	Thadeu Lima de Souza Cascardo <cascardo@...lia.com>
Subject: [PATCH v2] net: fix out-of-bounds access in ops_init

net_alloc_generic is called by net_alloc, which is called without any
locking. It reads max_gen_ptrs, which is changed under pernet_ops_rwsem. It
is read twice, first to allocate an array, then to set s.len, which is
later used to limit the bounds of the array access.

It is possible that the array is allocated and another thread is
registering a new pernet ops, increments max_gen_ptrs, which is then used
to set s.len with a larger than allocated length for the variable array.

Fix it by reading max_gen_ptrs only once in net_alloc_generic. If
max_gen_ptrs is later incremented, it will be caught in net_assign_generic.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@...lia.com>
Fixes: 073862ba5d24 ("netns: fix net_alloc_generic()")
Reviewed-by: Eric Dumazet <edumazet@...gle.com>
---
 net/core/net_namespace.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index f0540c557515..4a4f0f87ee36 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -70,11 +70,13 @@ DEFINE_COOKIE(net_cookie);
 static struct net_generic *net_alloc_generic(void)
 {
 	struct net_generic *ng;
-	unsigned int generic_size = offsetof(struct net_generic, ptr[max_gen_ptrs]);
+	unsigned int generic_size;
+	unsigned int gen_ptrs = READ_ONCE(max_gen_ptrs);
+	generic_size = offsetof(struct net_generic, ptr[gen_ptrs]);
 
 	ng = kzalloc(generic_size, GFP_KERNEL);
 	if (ng)
-		ng->s.len = max_gen_ptrs;
+		ng->s.len = gen_ptrs;
 
 	return ng;
 }
@@ -1307,7 +1309,12 @@ static int register_pernet_operations(struct list_head *list,
 		if (error < 0)
 			return error;
 		*ops->id = error;
-		max_gen_ptrs = max(max_gen_ptrs, *ops->id + 1);
+		/*
+		 * This does not require READ_ONCE as writers will take
+		 * pernet_ops_rwsem. But WRITE_ONCE is needed to protect
+		 * net_alloc_generic.
+		 */
+		WRITE_ONCE(max_gen_ptrs, max(max_gen_ptrs, *ops->id + 1));
 	}
 	error = __register_pernet_operations(list, ops);
 	if (error) {
-- 
2.34.1