Patch "net/l2tp: fix warning in l2tp_exit_net found by syzbot" has been added to the 6.12-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    net/l2tp: fix warning in l2tp_exit_net found by syzbot

to the 6.12-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     net-l2tp-fix-warning-in-l2tp_exit_net-found-by-syzbo.patch
and it can be found in the queue-6.12 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 12fa7625dd6f9c5cc5c8f0306e7e4bff33c3dcf6
Author: James Chapman <jchapman@xxxxxxxxxxx>
Date:   Mon Nov 18 14:04:11 2024 +0000

    net/l2tp: fix warning in l2tp_exit_net found by syzbot
    
    [ Upstream commit 5d066766c5f1252f98ff859265bcd1a5b52ac46c ]
    
    In l2tp's net exit handler, we check that an IDR is empty before
    destroying it:
    
            WARN_ON_ONCE(!idr_is_empty(&pn->l2tp_tunnel_idr));
            idr_destroy(&pn->l2tp_tunnel_idr);
    
    By forcing memory allocation failures in idr_alloc_32, syzbot is able
    to provoke a condition where idr_is_empty returns false despite there
    being no items in the IDR. This turns out to be because the radix tree
    of the IDR contains only internal radix-tree nodes and it is this that
    causes idr_is_empty to return false. The internal nodes are cleaned by
    idr_destroy.
    
    Use idr_for_each to check that the IDR is empty instead of
    idr_is_empty to avoid the problem.
    
    Reported-by: syzbot+332fe1e67018625f63c9@xxxxxxxxxxxxxxxxxxxxxxxxx
    Closes: https://syzkaller.appspot.com/bug?extid=332fe1e67018625f63c9
    Fixes: 73d33bd063c4 ("l2tp: avoid using drain_workqueue in l2tp_pre_exit_net")
    Signed-off-by: James Chapman <jchapman@xxxxxxxxxxx>
    Link: https://patch.msgid.link/20241118140411.1582555-1-jchapman@xxxxxxxxxxx
    Signed-off-by: Paolo Abeni <pabeni@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 3eec23ac5ab10..369a2f2e459cd 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1870,15 +1870,31 @@ static __net_exit void l2tp_pre_exit_net(struct net *net)
 	}
 }
 
+static int l2tp_idr_item_unexpected(int id, void *p, void *data)
+{
+	const char *idr_name = data;
+
+	pr_err("l2tp: %s IDR not empty at net %d exit\n", idr_name, id);
+	WARN_ON_ONCE(1);
+	return 1;
+}
+
 static __net_exit void l2tp_exit_net(struct net *net)
 {
 	struct l2tp_net *pn = l2tp_pernet(net);
 
-	WARN_ON_ONCE(!idr_is_empty(&pn->l2tp_v2_session_idr));
+	/* Our per-net IDRs should be empty. Check that is so, to
+	 * help catch cleanup races or refcnt leaks.
+	 */
+	idr_for_each(&pn->l2tp_v2_session_idr, l2tp_idr_item_unexpected,
+		     "v2_session");
+	idr_for_each(&pn->l2tp_v3_session_idr, l2tp_idr_item_unexpected,
+		     "v3_session");
+	idr_for_each(&pn->l2tp_tunnel_idr, l2tp_idr_item_unexpected,
+		     "tunnel");
+
 	idr_destroy(&pn->l2tp_v2_session_idr);
-	WARN_ON_ONCE(!idr_is_empty(&pn->l2tp_v3_session_idr));
 	idr_destroy(&pn->l2tp_v3_session_idr);
-	WARN_ON_ONCE(!idr_is_empty(&pn->l2tp_tunnel_idr));
 	idr_destroy(&pn->l2tp_tunnel_idr);
 }