[PATCH net] net: dcb: initialize PFC value before calling getpfccfg

[Joas Antonio dos Santos]

dcbnl_getpfccfg() and dcbnl_cee_fill() pass the address of an
uninitialized stack u8 to the driver's getpfccfg() op and then put it
into the netlink reply unconditionally.  The op returns void, so the
core cannot tell whether the driver wrote the value.

i40e_dcbnl_get_pfc_cfg() and ice_dcbnl_get_pfc_cfg() return without
writing *setting unless the DCBX agent is host managed and in CEE mode.
Both drivers normally run with firmware managed DCBX
(DCB_CAP_DCBX_LLD_MANAGED), and i40e with FW LLDP disabled runs host
managed in IEEE mode, so on a DCB capable i40e/ice port the PFC values
in DCB_CMD_PFC_GCFG and DCB_CMD_CEE_GET replies, and in the CEE part of
RTNLGRP_DCB notifications, come from uninitialized stack.  The same u8
is reused for all eight priorities, so this is one byte per reply.

dcb_doit() only requires CAP_NET_ADMIN for RTM_SETDCB, so the GET
commands are available to unprivileged users.

Set the value to 0 before each call, like the PG config path already
presets its outputs to DCB_ATTR_VALUE_UNDEFINED.  0 is also what the
reply contains today on kernels built with CONFIG_INIT_STACK_ALL_ZERO,
so nothing changes for those.

Fixes: b94b013eb626 ("ice: Implement DCBNL support")
Fixes: 5effa78e7c94 ("i40e: Add netlink callbacks support for software based DCB")
Signed-off-by: Joas Antonio dos Santos <joasantonio108@xxxxxxxxx>
Assisted-by: Yaga Agent HackerSec Claude:claude-opus-5-5
---
Found by code inspection, compile-tested only (W=1).  I don't have
i40e/ice hardware to check the reply.  On kernels built with
CONFIG_INIT_STACK_ALL_ZERO the byte is already zero, so in practice
it only leaks on builds without stack auto-initialization.

 net/dcb/dcbnl.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/dcb/dcbnl.c b/net/dcb/dcbnl.c
index ab71f53128..ad0aebe0ad 100644
--- a/net/dcb/dcbnl.c
+++ b/net/dcb/dcbnl.c
@@ -281,6 +281,7 @@ static int dcbnl_getpfccfg(struct net_device *netdev, struct nlmsghdr *nlh,
 		if (!getall && !data[i])
 			continue;
 
+		value = 0;
 		netdev->dcbnl_ops->getpfccfg(netdev, i - DCB_PFC_UP_ATTR_0,
 		                             &value);
 		ret = nla_put_u8(skb, i, value);
@@ -1395,6 +1396,7 @@ static int dcbnl_cee_fill(struct sk_buff *skb, struct net_device *netdev)
 			goto nla_put_failure;
 
 		for (i = DCB_PFC_UP_ATTR_0; i <= DCB_PFC_UP_ATTR_7; i++) {
+			value = 0;
 			ops->getpfccfg(netdev, i - DCB_PFC_UP_ATTR_0, &value);
 			if (nla_put_u8(skb, i, value))
 				goto nla_put_failure;