ethtool: Fix uninitialized number of lanes

In the Linux kernel, the following vulnerability has been resolved:

ethtool: Fix uninitialized number of lanes

It is not possible to set the number of lanes when setting link modes

using the legacy IOCTL ethtool interface. Since 'struct

ethtool_link_ksettings' is not initialized in this path, drivers receive

an uninitialized number of lanes in 'struct

ethtool_link_ksettings::lanes'.

When this information is later queried from drivers, it results in the

ethtool code making decisions based on uninitialized memory, leading to

the following KMSAN splat [1]. In practice, this most likely only

happens with the tun driver that simply returns whatever it got in the

set operation.

As far as I can tell, this uninitialized memory is not leaked to user

space thanks to the 'ethtool_ops->cap_link_lanes_supported' check in

linkmodes_prepare_data().

Fix by initializing the structure in the IOCTL path. Did not find any

more call sites that pass an uninitialized structure when calling

'ethtool_ops::set_link_ksettings()'.

[1]

BUG: KMSAN: uninit-value in ethnl_update_linkmodes net/ethtool/linkmodes.c:273 [inline]

BUG: KMSAN: uninit-value in ethnl_set_linkmodes+0x190b/0x19d0 net/ethtool/linkmodes.c:333

ethnl_update_linkmodes net/ethtool/linkmodes.c:273 [inline]

ethnl_set_linkmodes+0x190b/0x19d0 net/ethtool/linkmodes.c:333

ethnl_default_set_doit+0x88d/0xde0 net/ethtool/netlink.c:640

genl_family_rcv_msg_doit net/netlink/genetlink.c:968 [inline]

genl_family_rcv_msg net/netlink/genetlink.c:1048 [inline]

genl_rcv_msg+0x141a/0x14c0 net/netlink/genetlink.c:1065

netlink_rcv_skb+0x3f8/0x750 net/netlink/af_netlink.c:2577

genl_rcv+0x40/0x60 net/netlink/genetlink.c:1076

netlink_unicast_kernel net/netlink/af_netlink.c:1339 [inline]

netlink_unicast+0xf41/0x1270 net/netlink/af_netlink.c:1365

netlink_sendmsg+0x127d/0x1430 net/netlink/af_netlink.c:1942

sock_sendmsg_nosec net/socket.c:724 [inline]

sock_sendmsg net/socket.c:747 [inline]

____sys_sendmsg+0xa24/0xe40 net/socket.c:2501

___sys_sendmsg+0x2a1/0x3f0 net/socket.c:2555

__sys_sendmsg net/socket.c:2584 [inline]

__do_sys_sendmsg net/socket.c:2593 [inline]

__se_sys_sendmsg net/socket.c:2591 [inline]

__x64_sys_sendmsg+0x36b/0x540 net/socket.c:2591

do_syscall_x64 arch/x86/entry/common.c:50 [inline]

do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80

entry_SYSCALL_64_after_hwframe+0x63/0xcd

Uninit was stored to memory at:

tun_get_link_ksettings+0x37/0x60 drivers/net/tun.c:3544

__ethtool_get_link_ksettings+0x17b/0x260 net/ethtool/ioctl.c:441

ethnl_set_linkmodes+0xee/0x19d0 net/ethtool/linkmodes.c:327

ethnl_default_set_doit+0x88d/0xde0 net/ethtool/netlink.c:640

genl_family_rcv_msg_doit net/netlink/genetlink.c:968 [inline]

genl_family_rcv_msg net/netlink/genetlink.c:1048 [inline]

genl_rcv_msg+0x141a/0x14c0 net/netlink/genetlink.c:1065

netlink_rcv_skb+0x3f8/0x750 net/netlink/af_netlink.c:2577

genl_rcv+0x40/0x60 net/netlink/genetlink.c:1076

netlink_unicast_kernel net/netlink/af_netlink.c:1339 [inline]

netlink_unicast+0xf41/0x1270 net/netlink/af_netlink.c:1365

netlink_sendmsg+0x127d/0x1430 net/netlink/af_netlink.c:1942

sock_sendmsg_nosec net/socket.c:724 [inline]

sock_sendmsg net/socket.c:747 [inline]

____sys_sendmsg+0xa24/0xe40 net/socket.c:2501

___sys_sendmsg+0x2a1/0x3f0 net/socket.c:2555

__sys_sendmsg net/socket.c:2584 [inline]

__do_sys_sendmsg net/socket.c:2593 [inline]

__se_sys_sendmsg net/socket.c:2591 [inline]

__x64_sys_sendmsg+0x36b/0x540 net/socket.c:2591

do_syscall_x64 arch/x86/entry/common.c:50 [inline]

do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80

entry_SYSCALL_64_after_hwframe+0x63/0xcd

Uninit was stored to memory at:

tun_set_link_ksettings+0x37/0x60 drivers/net/tun.c:3553

ethtool_set_link_ksettings+0x600/0x690 net/ethtool/ioctl.c:609

__dev_ethtool net/ethtool/ioctl.c:3024 [inline]

dev_ethtool+0x1db9/0x2a70 net/ethtool/ioctl.c:3078

dev_ioctl+0xb07/0x1270 net/core/dev_ioctl.c:524

sock_do_ioctl+0x295/0x540 net/socket.c:1213

sock_i

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