[PATCH net v3] net: rtnetlink: zero ifla_vf_broadcast to avoid stack infoleak in rtnl_fill_vfinfo

[Kai Zen <kai.aizen.dev@gmail.com>]

rtnl_fill_vfinfo() declares struct ifla_vf_broadcast on the stack
without initialisation:

	struct ifla_vf_broadcast vf_broadcast;

The struct contains a single fixed 32-byte field:

	/* include/uapi/linux/if_link.h */
	struct ifla_vf_broadcast {
		__u8 broadcast[32];
	};

The function then copies dev->broadcast into it using dev->addr_len
as the length:

	memcpy(vf_broadcast.broadcast, dev->broadcast, dev->addr_len);

On Ethernet devices (the overwhelming majority of SR-IOV NICs)
dev->addr_len is 6, so only the first 6 bytes of broadcast[] are
written. The remaining 26 bytes retain whatever was previously on
the kernel stack. The full struct is then handed to userspace via:

	nla_put(skb, IFLA_VF_BROADCAST,
		sizeof(vf_broadcast), &vf_broadcast)

leaking up to 26 bytes of uninitialised kernel stack per VF per
RTM_GETLINK request, repeatable.

The other vf_* structs in the same function are explicitly zeroed
for exactly this reason - see the memset() calls for ivi,
vf_vlan_info, node_guid and port_guid a few lines above.
vf_broadcast was simply missed when it was added.

Reachability: any unprivileged local process can open AF_NETLINK /
NETLINK_ROUTE without capabilities and send RTM_GETLINK with an
IFLA_EXT_MASK attribute carrying RTEXT_FILTER_VF. The kernel walks
each VF and emits IFLA_VF_BROADCAST, leaking 26 bytes of stack per
VF per request. Stack residue at this call site can include return
addresses and transient sensitive data; KASAN with stack
instrumentation, or KMSAN, will flag the nla_put() when reproduced.

Zero the on-stack struct before the partial memcpy, matching the
existing pattern used for the other vf_* structs in the same
function.

Fixes: 75345f888f70 ("ipoib: show VF broadcast address")
Cc: stable@vger.kernel.org
Signed-off-by: Kai Zen <kai.aizen.dev@gmail.com>
---
 net/core/rtnetlink.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index b613bb6e0..df042da42 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -1572,6 +1572,7 @@ static noinline_for_stack int rtnl_fill_vfinfo(struct sk_buff *skb,
 		port_guid.vf = ivi.vf;
 
 	memcpy(vf_mac.mac, ivi.mac, sizeof(ivi.mac));
+	memset(&vf_broadcast, 0, sizeof(vf_broadcast));
 	memcpy(vf_broadcast.broadcast, dev->broadcast, dev->addr_len);
 	vf_vlan.vlan = ivi.vlan;
 	vf_vlan.qos = ivi.qos;
-- 
2.43.0