Re: [PATCH net] net: devmem: fix TX binding UAF on netdevice unregister

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

On 09/10, Felix Hoffmann wrote:
> RX dma-buf bindings are invalidated by their memory provider when a
> netdevice is unregistered. TX bindings have no bound RX queues and no
> equivalent uninstall callback, so their physical and virtual netdevice
> pointers remain live after the devices are freed.
> 
> Closing the owning netlink socket after device removal then makes
> netdev_nl_sock_priv_destroy() dereference the freed physical netdevice to
> hold and lock it. KASAN reports a slab-use-after-free and the kernel can
> panic.
> 
> The binding can also outlive the device used for its dma-buf attachment.
> Since dma_buf_attach() does not hold a reference to that device, deferred
> binding cleanup can pass a freed device to dma_buf_unmap_attachment().
> 
> Invalidate TX bindings that refer to either the physical or virtual
> netdevice during unregister. Keep a reference on the exact DMA device
> until the attachment is unmapped. The netlink socket destructor then uses
> the existing device-gone path, while delayed dma-buf cleanup retains a
> valid DMA device.
> 
> NETDEV_CMD_BIND_TX does not require GENL_ADMIN_PERM. The failure was
> reproduced with the binding owned by UID 65534 across module removal.
> 
> Fixes: bd61848900bf ("net: devmem: Implement TX path")
> Cc: stable@vger.kernel.org
> Assisted-by: Codex:gpt-5
> Signed-off-by: Felix Hoffmann <f3lix.dev@gmx.de>
> ---
> The reproducer is available privately on request.

As I mentioned on https://lore.kernel.org/netdev/aqC4XSt7JrrTv4sG@devvm7509.cco0.facebook.com/
I'd like us to have an in-tree selftest for that. 
And Dragos also had a suggestion to use a new helper for checking if
there are any outstanding tx dmabufs attached...