[PATCH] netdevsim: fix use-after-free in __nsim_dev_port_del

[Hrushiraj Gandhi <hrushirajg23@gmail.com>]

debugfs files created under a port's ddir (ethtool/get_err,
ethtool/set_err, ring params, bpf_offloaded_id, udp_ports/inject_error,
etc.) store raw pointers directly into the netdevsim struct, which lives
in the net_device private data kmalloc slab.

If these files outlive the netdevsim struct, a concurrent reader can
trigger a slab-use-after-free by passing debugfs_file_get() (which only
checks dentry lifetime) and then dereferencing the freed data pointer
in debugfs_u32_get().

In __nsim_dev_port_del(), nsim_destroy() is called before
nsim_dev_port_debugfs_exit(). However, nsim_destroy() calls free_netdev()
at its end, while nsim_dev_port_debugfs_exit() removes the port's
debugfs directory. This means the slab is freed before the debugfs
files are removed.

Fix by calling debugfs_remove_recursive(ns->nsim_dev_port->ddir) in
nsim_destroy() right before free_netdev(). This ensures all per-port
debugfs files are destroyed synchronously before the backing memory is
freed. The subsequent call to nsim_dev_port_debugfs_exit() in
__nsim_dev_port_del() becomes a harmless no-op.

Reported-by: syzbot+6c25f4750230faf70be9@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6c25f4750230faf70be9
Fixes: e05b2d141fef ("netdevsim: move netdev creation/destruction to dev probe")
Signed-off-by: Hrushiraj Gandhi <hrushirajg23@gmail.com>
---
 drivers/net/netdevsim/netdev.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
index 27e5f109f933..08136e7990cb 100644
--- a/drivers/net/netdevsim/netdev.c
+++ b/drivers/net/netdevsim/netdev.c
@@ -1214,6 +1214,13 @@ void nsim_destroy(struct netdevsim *ns)
 		ns->page = NULL;
 	}
 
+	/*
+	 * Remove per-port debugfs files before free_netdev() releases the
+	 * netdevsim struct to prevent use-after-free in concurrent readers.
+	 */
+	debugfs_remove_recursive(ns->nsim_dev_port->ddir);
+	ns->nsim_dev_port->ddir = NULL;
+
 	free_netdev(dev);
 }
 
-- 
2.47.3