[PATCH net] net: qede: initialize extack in qede_flow_spec_to_rule()

[Anirudh Virdi <avirdi@redhat.com>]

qede_flow_spec_to_rule() declares a netlink_ext_ack on the stack
without initializing it. qede_parse_flow_attr() and
qede_flow_spec_validate() only set extack._msg on error paths via
NL_SET_ERR_MSG_MOD(), so on a successful parse the err_out block
reads an uninitialized pointer. If that garbage happens to be
non-NULL, DP_NOTICE("%s", extack._msg) dereferences it inside
vsnprintf, causing a page fault and kernel panic.

This manifests when configuring ethtool ntuple flow rules (ethtool -U
flow-type tcp4/udp4) on kernels built with CONFIG_INIT_STACK_NONE=y.
With CONFIG_INIT_STACK_ALL_ZERO=y the bug is masked because the
compiler zero-initializes extack, making _msg NULL.

Reproduced on RHEL 9.8 (kernel-5.14.0-687.47.1.el9_8) and mainline
(v7.3-rc6) with the following reproducer:

  IFACE=ens2f0
  ip link set $IFACE up
  ethtool -K $IFACE ntuple on
  for i in $(seq 1 100); do
    ethtool -U $IFACE flow-type udp4 src-ip 192.168.2.$i action -1
  done

The loop panics reliably after 10-15 iterations with:

  BUG: unable to handle page fault
  RIP: string+0x49/0xd0
  Call Trace:
   vsnprintf
   qede_flow_spec_to_rule+0x2b6/0x4b0 [qede]
   qede_add_cls_rule
   ethtool_set_rxnfc

Zero-initialize extack so _msg is NULL unless explicitly set by an
error path.

Fixes: eb705d734525 ("net: qede: use faked extack in qede_flow_spec_to_rule()")
Signed-off-by: Anirudh Virdi <avirdi@redhat.com>
---
 drivers/net/ethernet/qlogic/qede/qede_filter.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/qlogic/qede/qede_filter.c b/drivers/net/ethernet/qlogic/qede/qede_filter.c
index ab8b818588e2..e63422c945f9 100644
--- a/drivers/net/ethernet/qlogic/qede/qede_filter.c
+++ b/drivers/net/ethernet/qlogic/qede/qede_filter.c
@@ -1992,7 +1992,7 @@ static int qede_flow_spec_to_rule(struct qede_dev *edev,
 {
 	struct ethtool_rx_flow_spec_input input = {};
 	struct ethtool_rx_flow_rule *flow;
-	struct netlink_ext_ack extack;
+	struct netlink_ext_ack extack = {};
 	__be16 proto;
 	int err;
 
-- 
2.55.0