net: fix crash when config small gso_max_size/gso_ipv4_max_size

Config a small gso_max_size/gso_ipv4_max_size will lead to an underflow
in sk_dst_gso_max_size(), which may trigger a BUG_ON crash,
because sk->sk_gso_max_size would be much bigger than device limits.
Call Trace:
tcp_write_xmit
    tso_segs = tcp_init_tso_segs(skb, mss_now);
        tcp_set_skb_tso_segs
            tcp_skb_pcount_set
                // skb->len = 524288, mss_now = 8
                // u16 tso_segs = 524288/8 = 65535 -> 0
                tso_segs = DIV_ROUND_UP(skb->len, mss_now)
    BUG_ON(!tso_segs)
Add check for the minimum value of gso_max_size and gso_ipv4_max_size.

Fixes: 46e6b99 ("rtnetlink: allow GSO maximums to be set on device creation")
Fixes: 9eefedd ("net: add gso_ipv4_max_size and gro_ipv4_max_size per device")
Signed-off-by: Wang Liang <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Wang Liang

net/core/rtnetlink.c

@@ -2032,7 +2032,7 @@ static const struct nla_policy ifla_policy[IFLA_MAX+1] = {
 	[IFLA_NUM_TX_QUEUES]	= { .type = NLA_U32 },
 	[IFLA_NUM_RX_QUEUES]	= { .type = NLA_U32 },
 	[IFLA_GSO_MAX_SEGS]	= { .type = NLA_U32 },
-	[IFLA_GSO_MAX_SIZE]	= { .type = NLA_U32 },
+	[IFLA_GSO_MAX_SIZE]	= NLA_POLICY_MIN(NLA_U32, MAX_TCP_HEADER + 1),
 	[IFLA_PHYS_PORT_ID]	= { .type = NLA_BINARY, .len = MAX_PHYS_ITEM_ID_LEN },
 	[IFLA_CARRIER_CHANGES]	= { .type = NLA_U32 },  /* ignored */
 	[IFLA_PHYS_SWITCH_ID]	= { .type = NLA_BINARY, .len = MAX_PHYS_ITEM_ID_LEN },
@@ -2057,7 +2057,7 @@ static const struct nla_policy ifla_policy[IFLA_MAX+1] = {
 	[IFLA_TSO_MAX_SIZE]	= { .type = NLA_REJECT },
 	[IFLA_TSO_MAX_SEGS]	= { .type = NLA_REJECT },
 	[IFLA_ALLMULTI]		= { .type = NLA_REJECT },
-	[IFLA_GSO_IPV4_MAX_SIZE]	= { .type = NLA_U32 },
+	[IFLA_GSO_IPV4_MAX_SIZE]	= NLA_POLICY_MIN(NLA_U32, MAX_TCP_HEADER + 1),
 	[IFLA_GRO_IPV4_MAX_SIZE]	= { .type = NLA_U32 },
 };