net: core: prevent NULL deref in generic_hwtstamp_ioctl_lower()

net: core: prevent NULL deref in generic_hwtstamp_ioctl_lower()

The ethtool tsconfig Netlink path can trigger a null pointer
dereference. A call chain such as:

  tsconfig_prepare_data() ->
  dev_get_hwtstamp_phylib() ->
  vlan_hwtstamp_get() ->
  generic_hwtstamp_get_lower() ->
  generic_hwtstamp_ioctl_lower()

results in generic_hwtstamp_ioctl_lower() being called with
kernel_cfg->ifr as NULL.

The generic_hwtstamp_ioctl_lower() function does not expect
a NULL ifr and dereferences it, leading to a system crash.

Fix this by adding a NULL check for kernel_cfg->ifr in
generic_hwtstamp_ioctl_lower(). If ifr is NULL, return -EINVAL.

Fixes: 6e9e2eed4f39 ("net: ethtool: Add support for tsconfig command to get/set hwtstamp config")
Closes: https://lore.kernel.org/cd6a7056-fa6d-43f8-b78a-f5e811247ba8@linux.dev
Signed-off-by: Jiaming Zhang <r772577952@gmail.com>
Reviewed-by: Kory Maincent <kory.maincent@bootlin.com>
Link: https://patch.msgid.link/20251111173652.749159-2-r772577952@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>