CVE-2024-26951: wireguard: netlink: check for dangling peer via is_dead instead of empty list

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

wireguard: netlink: check for dangling peer via isdead instead of empty list

If all peers are removed via wgpeerremoveall(), rather than setting peerlist to empty, the peer is added to a temporary list with a head on the stack of wgpeerremoveall(). If a netlink dump is resumed and the cursored peer is one that has been removed via wgpeerremoveall(), it will iterate from that peer and then attempt to dump freed peers.

Fix this by instead checking peer->isdead, which was explictly created for this purpose. Also move up the deviceupdatelock lockdep assertion, since reading isdead relies on that.

It can be reproduced by a small script like:

echo "Setting config..." ip link add dev wg0 type wireguard wg setconf wg0 /big-config ( while true; do echo "Showing config..." wg showconf wg0 > /dev/null done ) & sleep 4 wg setconf wg0 <(printf "[Peer]\nPublicKey=$(wg genkey)\n")

Resulting in:

BUG: KASAN: slab-use-after-free in lockacquire+0x182a/0x1b20 Read of size 8 at addr ffff88811956ec70 by task wg/59 CPU: 2 PID: 59 Comm: wg Not tainted 6.8.0-rc2-debug+ #5 Call Trace: <TASK> dumpstacklvl+0x47/0x70 printaddressdescription.constprop.0+0x2c/0x380 printreport+0xab/0x250 kasanreport+0xba/0xf0 lockacquire+0x182a/0x1b20 lockacquire+0x191/0x4b0 downread+0x80/0x440 getpeer+0x140/0xcb0 wggetdevicedump+0x471/0x1130