Re: [PATCH net] net: qualcomm: rmnet: set skb->dev for deaggregated MAP commands

[Eric Dumazet]

On Thu, Aug 20, 2026 at 10:11 AM Xiang Mei <xmei5@xxxxxxx> wrote:
>
> rmnet_map_deaggregate() allocates each sub-frame with alloc_skb() and
> leaves skb->dev NULL. __rmnet_map_ingress_handler() assigns
> skb->dev = ep->egress_dev only on the data path, but a MAP command frame
> is dispatched to rmnet_map_command() before that, so rmnet_map_send_ack()
> runs netif_tx_lock(skb->dev) on a NULL device. An unprivileged user
> reaches this by unsharing a user+net namespace, creating an rmnet link
> over a tap device with INGRESS_DEAGGREGATION and INGRESS_MAP_COMMANDS,
> and writing an aggregated frame carrying a flow-control command to the
> tap fd.
>
> Assign port->dev before dispatch: the command was received on the real
> device tracked by port->dev and the ACK must egress on that same device.
> The non-deaggregated path already has skb->dev == real_dev.
>
>   BUG: KASAN: null-ptr-deref in _raw_spin_lock (kernel/locking/spinlock.c:158)
>   Write of size 4 at addr 00000000000004b4 by task exploit/144
>   Call Trace:
>    _raw_spin_lock (kernel/locking/spinlock.c:158)
>    netif_tx_lock (net/sched/sch_generic.c:497)
>    rmnet_map_command (drivers/net/ethernet/qualcomm/rmnet/rmnet_map_command.c:67)
>    rmnet_rx_handler (drivers/net/ethernet/qualcomm/rmnet/rmnet_handlers.c:125)
>    __netif_receive_skb_core.constprop.0 (net/core/dev.c:6103)
>    ...
>    __netif_receive_skb_one_core (net/core/dev.c:6214)
>    netif_receive_skb (net/core/dev.c:6474)
>    tun_get_user (drivers/net/tun.c:1966)
>    tun_chr_write_iter (drivers/net/tun.c:2012)
>    vfs_write (fs/read_write.c:687)
>    ksys_write (fs/read_write.c:739)
>    do_syscall_64 (arch/x86/entry/syscall_64.c:94)
>    entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
>   Kernel panic - not syncing: Fatal exception in interrupt
>
> Fixes: 378e25357ac7 ("net: qualcomm: rmnet: Remove unnecessary device assignment")
> Reported-by: co+4638111fe2a12980@xxxxxxx
> Closes: https://lore.kernel.org/netdev/ijg79FFMfIvKJbivdJEKvTO90Q9dTvyBkJck@xxxxxxx/T/#u
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Xiang Mei <xmei5@xxxxxxx>
> ---
>  drivers/net/ethernet/qualcomm/rmnet/rmnet_map_command.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_command.c b/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_command.c
> index add0f5ade2e6..3944925566fc 100644
> --- a/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_command.c
> +++ b/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_command.c
> @@ -79,6 +79,8 @@ void rmnet_map_command(struct sk_buff *skb, struct rmnet_port *port)
>         unsigned char command_name;
>         unsigned char rc = 0;
>
> +       skb->dev = port->dev;
> +

This works, but the suggested patch is cleaner IMO

https://bugs.sh/b/4638111fe2a12980/patch.diff