[PATCH v2] netdevsim: fix use-after-free in nsim_create and __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.

The same window exists on nsim_create()'s error path:
nsim_ethtool_init() creates debugfs files under ddir with pointers into
ns before nsim_init_netdevsim()/nsim_init_netdevsim_vf() which can fail,
and the err_free_netdev label calls free_netdev() while those debugfs
entries are still live.

Fix both paths by calling debugfs_remove_recursive() on the port's
ddir before every free_netdev() call. The subsequent
nsim_dev_port_debugfs_exit() calls become harmless no-ops since ddir is
set to NULL.

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>
---
v2:
- Also fix the same use-after-free window on the error path of nsim_create() as suggested by Simon Horman.
- Shorten the code comment in nsim_destroy() to be more concise.

 drivers/net/netdevsim/netdev.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
index 27e5f109f933..f2824e75cddd 100644
--- a/drivers/net/netdevsim/netdev.c
+++ b/drivers/net/netdevsim/netdev.c
@@ -1165,6 +1165,8 @@ struct netdevsim *nsim_create(struct nsim_dev *nsim_dev,
 	return ns;
 
 err_free_netdev:
+	debugfs_remove_recursive(nsim_dev_port->ddir);
+	nsim_dev_port->ddir = NULL;
 	free_netdev(dev);
 	return ERR_PTR(err);
 }
@@ -1214,6 +1216,13 @@ 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