Re: [PATCH net v2 1/4] net: devmem: unmap dma_buf synchronously on queue uninstall

[Stanislav Fomichev <sdf.kernel@gmail.com>]

On 10/12, Mina Almasry wrote:
> When a bound net_device is unregistered while userspace still holds a
> netlink or TCP socket reference on a devmem binding,
> mp_dmabuf_devmem_uninstall() clears binding->dev to NULL so
> netdev_nl_sock_priv_destroy() will not touch the unregistered
> net_device, but leaves the dma_buf attachment mapped until
> __net_devmem_dmabuf_binding_free() runs later on socket close.
> 
> Because dma_buf_attach() does not take a reference on dma_dev, device
> removal frees dma_dev while the attachment is still mapped. Closing the
> netlink socket afterwards triggers a slab use-after-free in
> dma_unmap_sg_attrs():
> 
>   BUG: KASAN: slab-use-after-free in dma_unmap_sg_attrs+0x76/0x2b0
>   Read of size 8 at addr ffff88810fae64b0 by task kworker/3:1/53
>   Workqueue: events __net_devmem_dmabuf_binding_free
>   Call Trace:
>    dma_unmap_sg_attrs+0x76/0x2b0
>    unmap_udmabuf+0x8e/0x100
>    dma_buf_unmap_attachment+0x145/0x330
>    dma_buf_unmap_attachment_unlocked+0x96/0x130
>    __net_devmem_dmabuf_binding_free+0x156/0x2e0
>   Freed by task 9:
>    kfree+0x1b8/0x550
>    nsim_bus_dev_release+0xe/0x60
>    device_release+0xcf/0x250
> 
> Add net_devmem_dmabuf_binding_unmap() serialized under binding->lock
> and null out binding->sgt and binding->attachment after unmapping so
> unmapping is idempotent across queue uninstall and deferred binding
> destruction. Call it synchronously when the last bound RX queue is
> uninstalled in mp_dmabuf_devmem_uninstall(), except when a leased
> queue is unleased: the physical device is still registered and, if
> restarting its queue failed, may still be using the binding.
> 
> The same use-after-free happens when the netlink socket is closed
> before the device is removed but RX tokens or the pending release work
> keep the binding alive. Such bindings are no longer installed on any
> queue and were already erased from net_devmem_dmabuf_bindings. Keep
> bindings in net_devmem_dmabuf_bindings until they are freed, hide
> unbound ones from TX lookups with an xarray mark, and on
> NETDEV_UNREGISTER clear binding->dev of unbound RX bindings for the
> device and unmap them, unless the device is a virtual device leasing

[..]

> queues, for the same reason as above. net_devmem_lock serializes that
> walk with the final free.

Can you explain net_devmem_lock a bit more? In
net_devmem_dev_unregister, why can you not net_devmem_lookup_dmabuf and
then rely on the binding->lock? The rest looks good, but the
new lock seems a bit excessive?