net: mctp: Don't access ifa_index when missing

In mctp_dump_addrinfo, ifa_index can be used to filter interfaces, but
only when the struct ifaddrmsg is provided. Otherwise it will be
comparing to uninitialised memory - reproducible in the syzkaller case from
dhcpd, or busybox "ip addr show".

The kernel MCTP implementation has always filtered by ifa_index, so
existing userspace programs expecting to dump MCTP addresses must
already be passing a valid ifa_index value (either 0 or a real index).

BUG: KMSAN: uninit-value in mctp_dump_addrinfo+0x208/0xac0 net/mctp/device.c:128
 mctp_dump_addrinfo+0x208/0xac0 net/mctp/device.c:128
 rtnl_dump_all+0x3ec/0x5b0 net/core/rtnetlink.c:4380
 rtnl_dumpit+0xd5/0x2f0 net/core/rtnetlink.c:6824
 netlink_dump+0x97b/0x1690 net/netlink/af_netlink.c:2309

Fixes: 583be982d934 ("mctp: Add device handling and netlink interface")
Reported-by: syzbot+e76d52dadc089b9d197f@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/68135815.050a0220.3a872c.000e.GAE@google.com/
Reported-by: syzbot+1065a199625a388fce60@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/681357d6.050a0220.14dd7d.000d.GAE@google.com/
Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Link: https://patch.msgid.link/20250508-mctp-addr-dump-v2-1-c8a53fd2dd66@codeconstruct.com.au
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/mctp/device.c b/net/mctp/device.c
index 8e0724c56723de328592bfe5c6fc8085cd3102fe..7c0dcf3df3196207af6e1a1c002f388265c49fa1 100644 (file)
--- a/net/mctp/device.c
+++ b/net/mctp/device.c
@@ -117,11 +117,18 @@ static int mctp_dump_addrinfo(struct sk_buff *skb, struct netlink_callback *cb)
        struct net_device *dev;
        struct ifaddrmsg *hdr;
        struct mctp_dev *mdev;
-       int ifindex, rc;
-
-       hdr = nlmsg_data(cb->nlh);
-       // filter by ifindex if requested
-       ifindex = hdr->ifa_index;
+       int ifindex = 0, rc;
+
+       /* Filter by ifindex if a header is provided */
+       if (cb->nlh->nlmsg_len >= nlmsg_msg_size(sizeof(*hdr))) {
+               hdr = nlmsg_data(cb->nlh);
+               ifindex = hdr->ifa_index;
+       } else {
+               if (cb->strict_check) {
+                       NL_SET_ERR_MSG(cb->extack, "mctp: Invalid header for addr dump request");
+                       return -EINVAL;
+               }
+       }
 
        rcu_read_lock();
        for_each_netdev_dump(net, dev, mcb->ifindex) {