Re: [PATCH net] net: mctp: Don't access ifa_index when missing

[Simon Horman <horms@kernel.org>]

On Mon, May 05, 2025 at 05:05:12PM +0800, Matt Johnston 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).
> 
> 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>

Thanks Matt,

FWIIW, this looks good to me.

Reviewed-by: Simon Horman <horms@kernel.org>

> ---
>  net/mctp/device.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/net/mctp/device.c b/net/mctp/device.c
> index 8e0724c56723de328592bfe5c6fc8085cd3102fe..7780acdb99dedca1cd6a17e4d6bf917c7f7f370f 100644
> --- a/net/mctp/device.c
> +++ b/net/mctp/device.c
> @@ -117,11 +117,17 @@ 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);

FWIIW, I think the scope of the declaration of hdr can be reduced to this block.
(Less positive ease, so to speak.)

> +		/* Userspace programs providing AF_MCTP must be expecting ifa_index filter
> +		 * behaviour, as will those setting strict_check.
> +		 */
> +		if (hdr->ifa_family == AF_MCTP || cb->strict_check)
> +			ifindex = hdr->ifa_index;
> +	}
>  
>  	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 <matt@codeconstruct.com.au>
>