[PATCH 5.15 496/570] net: mctp: Dont access ifa_index when missing

Greg Kroah-Hartman
2026-04-13T16:39:24Z

From: Matt Johnston <matt@codeconstruct.com.au>

[ Upstream commit f11cf946c0a92c560a890d68e4775723353599e1 ]

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

[ The context change is due to the commit 2d45eeb7d5d7
("mctp: no longer rely on net->dev_index_head[]") in v6.14
which is irrelevant to the logic of this patch. ]

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>
Signed-off-by: Johnny Hao <johnny_haocn@sina.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/mctp/device.c | 19 +++++++++++++------
1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/net/mctp/device.c b/net/mctp/device.c
index c00a2550e2e0e..aec7ffad2666a 100644
--- a/net/mctp/device.c
+++ b/net/mctp/device.c
@@ -99,12 +99,19 @@ 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;
- int idx, rc;
-
- hdr = nlmsg_data(cb->nlh);
- // filter by ifindex if requested
- ifindex = hdr->ifa_index;
+ int idx;
+ 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 (; mcb->h < NETDEV_HASHENTRIES; mcb->h++, mcb->idx = 0) {
--
2.53.0