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

[Xiang Mei <xmei5@asu.edu>]

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@bugs.sh
Closes: https://lore.kernel.org/netdev/ijg79FFMfIvKJbivdJEKvTO90Q9dTvyBkJck@bugs.sh/T/#u
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Xiang Mei <xmei5@asu.edu>
---
 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;
+
 	/* Command data immediately follows the MAP header */
 	cmd = (struct rmnet_map_control_command *)(map_header + 1);
 	command_name = cmd->command_name;
-- 
2.43.0