[PATCH net 4/5] net: dsa: mv88e6xxx: bound the policy rule dump by the caller's buffer size

mv88e6xxx_get_rxnfc() uses rxnfc->rule_cnt as the write index while
dumping the policy IDR, clobbering the input value before it has been
looked at.  That input is the number of entries the caller had room for.
ETHTOOL_GRXCLSRLALL requires no CAP_NET_ADMIN and the ioctl sizes the
buffer from the rule_cnt userspace passes in, so once an admin has
installed policy rules any user can ask for fewer slots than there are
rules and run off the end of the allocation.  A rule_cnt of 0 leaves the
buffer pointer NULL and the walk dereferences it.

Count into a local so the caller's limit survives the walk, and stop with
-EMSGSIZE once it is reached.

Fixes: da7dc8755304 ("net: dsa: mv88e6xxx: add RXNFC support")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
CC: andrew@lunn.ch
CC: olteanv@gmail.com
CC: vivien.didelot@gmail.com
CC: f.fainelli@gmail.com
---
 drivers/net/dsa/mv88e6xxx/chip.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/drivers/net/dsa/mv88e6xxx/chip.c b/drivers/net/dsa/mv88e6xxx/chip.c
index 80b877c74513..7f68a0c55802 100644
--- a/drivers/net/dsa/mv88e6xxx/chip.c
+++ b/drivers/net/dsa/mv88e6xxx/chip.c

@@ -2438,6 +2438,7 @@ static int mv88e6xxx_get_rxnfc(struct dsa_switch *ds, int port,
 	struct ethtool_rx_flow_spec *fs = &rxnfc->fs;
 	struct mv88e6xxx_chip *chip = ds->priv;
 	struct mv88e6xxx_policy *policy;
+	u32 cnt = 0;
 	int err;
 	int id;
 

@@ -2463,11 +2464,18 @@ static int mv88e6xxx_get_rxnfc(struct dsa_switch *ds, int port,
 		break;
 	case ETHTOOL_GRXCLSRLALL:
 		rxnfc->data = 0;
-		rxnfc->rule_cnt = 0;
-		idr_for_each_entry(&chip->policies, policy, id)
-			if (policy->port == port)
-				rule_locs[rxnfc->rule_cnt++] = id;
 		err = 0;
+		idr_for_each_entry(&chip->policies, policy, id) {
+			if (policy->port != port)
+				continue;
+			if (cnt == rxnfc->rule_cnt) {
+				err = -EMSGSIZE;
+				break;
+			}
+			rule_locs[cnt++] = id;
+		}
+		if (!err)
+			rxnfc->rule_cnt = cnt;
 		break;
 	default:
 		err = -EOPNOTSUPP;

-- 
2.55.0