Re: [PATCH v2] netdevsim: fix use-after-free in nsim_create and __nsim_dev_port_del

[Simon Horman <horms@kernel.org>]

On Tue, Jun 23, 2026 at 08:14:47PM +0530, Hrushiraj Gandhi wrote:
> 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.

Thanks for the updates.

Reviewed-by: Simon Horman <horms@kernel.org>