[PATCH net] gve: fix out-of-bounds read in gve_get_flow_rule_entry()

gve_get_flow_rule_entry() checks the requested location against the
first and last cached rules, but rules_cache_num can be 0 while
rules_cache_synced is true: GVE_FLOW_RULE_QUERY_RULES returns the rules
from the requested location onwards, so a request past the last
installed rule - or any request when no rules are installed - returns
zero rules, which the driver accepts and marks as synced.

The next query evaluates rules_cache[*cache_num - 1] with
*cache_num == 0. The zero-initialised cache does not trip the earlier
comparison, *cache_num - 1 wraps to 0xFFFFFFFF in u32, and the read
faults about 352 GiB past the cache. ETHTOOL_GRXCLSRULE needs no
capability, so an unprivileged user can trigger it:

    BUG: unable to handle page fault for address: ff240996c4c64fa8
    #PF: supervisor read access in kernel mode
    RIP: gve_get_flow_rule_entry+0xd4/0x110 [gve]

Treat an empty cache as a cache miss and re-query.

Fixes: 6f3bc487565d ("gve: Add flow steering ethtool support")
Cc: <redacted>
Assisted-by: LLM
Signed-off-by: Aamir Ahmed <redacted>
---
Reproducer available on request.

 drivers/net/ethernet/google/gve/gve_flow_rule.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/net/ethernet/google/gve/gve_flow_rule.c b/drivers/net/ethernet/google/gve/gve_flow_rule.c
index 2c80cda..db8b9a3 100644
--- a/drivers/net/ethernet/google/gve/gve_flow_rule.c
+++ b/drivers/net/ethernet/google/gve/gve_flow_rule.c

@@ -206,6 +206,7 @@ int gve_get_flow_rule_entry(struct gve_priv *priv, struct ethtool_rxnfc *cmd)
 		return -EOPNOTSUPP;
 
 	if (!priv->flow_rules_cache.rules_cache_synced ||
+	    !*cache_num ||
 	    fsp->location < be32_to_cpu(rules_cache[0].location) ||
 	    fsp->location > be32_to_cpu(rules_cache[*cache_num - 1].location)) {
 		err = gve_adminq_query_flow_rules(priv, GVE_FLOW_RULE_QUERY_RULES, fsp->location);

-- 
2.55.0