[PATCH net v2] 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: <redacted>
Closes: https://lore.kernel.org/all/68135815.050a0220.3a872c.000e.GAE@google.com/
Reported-by: <redacted>
Closes: https://lore.kernel.org/all/681357d6.050a0220.14dd7d.000d.GAE@google.com/
Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
---
Changes in v2:
- Don't test for ifa_family=AF_MCTP
- Fail if strict_check is set and header is short
- Link to v1: https://lore.kernel.org/r/20250505-mctp-addr-dump-v1-1-a997013f99b8@codeconstruct.com.au
---
 net/mctp/device.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/net/mctp/device.c b/net/mctp/device.c
index 8e0724c56723de328592bfe5c6fc8085cd3102fe..7c0dcf3df3196207af6e1a1c002f388265c49fa1 100644
--- 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;
+	int ifindex = 0, rc;
 
-	hdr = nlmsg_data(cb->nlh);
-	// filter by ifindex if requested
-	ifindex = hdr->ifa_index;
+	/* 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) {

---
base-commit: ebd297a2affadb6f6f4d2e5d975c1eda18ac762d
change-id: 20250505-mctp-addr-dump-673e0fdc7894

Best regards,
-- 
Matt Johnston [off-list ref]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski [off-list ref]:

On Thu, 08 May 2025 13:18:32 +0800 you wrote:

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).

[...]

Here is the summary with links:
  - [net,v2] net: mctp: Don't access ifa_index when missing
    https://git.kernel.org/netdev/net/c/f11cf946c0a9

You are awesome, thank you!
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