Subject: Re: [PATCH net 1/2] net: openvswitch: limit the number of recursions from action sets

From: Aaron Conole <[email-masked]>
Date: Tue, 06 Feb 2024 09:55:03 -0500

Eric Dumazet <[email-masked]> writes:

> On Tue, Feb 6, 2024 at 2:11 PM Aaron Conole <[email-masked]> wrote:
>>
>> The ovs module allows for some actions to recursively contain an action
>> list for complex scenarios, such as sampling, checking lengths, etc.
>> When these actions are copied into the internal flow table, they are
>> evaluated to validate that such actions make sense, and these calls
>> happen recursively.
>>
>> The ovs-vswitchd userspace won't emit more than 16 recursion levels
>> deep.  However, the module has no such limit and will happily accept
>> limits larger than 16 levels nested.  Prevent this by tracking the
>> number of recursions happening and manually limiting it to 16 levels
>> nested.
>>
>> The initial implementation of the sample action would track this depth
>> and prevent more than 3 levels of recursion, but this was removed to
>> support the clone use case, rather than limited at the current userspace
>> limit.
>>
>> Fixes: 798c166173ff ("openvswitch: Optimize sample action for the clone use cases")
>> Signed-off-by: Aaron Conole <[email-masked]>
>> ---
>>  net/openvswitch/flow_netlink.c | 33 ++++++++++++++++++++++++++++-----
>>  1 file changed, 28 insertions(+), 5 deletions(-)
>>
>> diff --git a/net/openvswitch/flow_netlink.c b/net/openvswitch/flow_netlink.c
>> index 88965e2068ac..ba5cfa67a720 100644
>> --- a/net/openvswitch/flow_netlink.c
>> +++ b/net/openvswitch/flow_netlink.c
>> @@ -48,6 +48,9 @@ struct ovs_len_tbl {
>>
>>  #define OVS_ATTR_NESTED -1
>>  #define OVS_ATTR_VARIABLE -2
>> +#define OVS_COPY_ACTIONS_MAX_DEPTH 16
>> +
>> +static DEFINE_PER_CPU(int, copy_actions_depth);
>>
>>  static bool actions_may_change_flow(const struct nlattr *actions)
>>  {
>> @@ -3148,11 +3151,11 @@ static int copy_action(const struct nlattr *from,
>>         return 0;
>>  }
>>
>> -static int __ovs_nla_copy_actions(struct net *net, const struct nlattr *attr,
>> -                                 const struct sw_flow_key *key,
>> -                                 struct sw_flow_actions **sfa,
>> -                                 __be16 eth_type, __be16 vlan_tci,
>> -                                 u32 mpls_label_count, bool log)
>> +static int ___ovs_nla_copy_actions(struct net *net, const struct nlattr *attr,
>> +                                  const struct sw_flow_key *key,
>> +                                  struct sw_flow_actions **sfa,
>> +                                  __be16 eth_type, __be16 vlan_tci,
>> +                                  u32 mpls_label_count, bool log)
>>  {
>>         u8 mac_proto = ovs_key_mac_proto(key);
>>         const struct nlattr *a;
>> @@ -3478,6 +3481,26 @@ static int __ovs_nla_copy_actions(struct net *net, const struct nlattr *attr,
>>         return 0;
>>  }
>>
>> +static int __ovs_nla_copy_actions(struct net *net, const struct nlattr *attr,
>> +                                 const struct sw_flow_key *key,
>> +                                 struct sw_flow_actions **sfa,
>> +                                 __be16 eth_type, __be16 vlan_tci,
>> +                                 u32 mpls_label_count, bool log)
>> +{
>> +       int level = this_cpu_read(copy_actions_depth);
>> +       int ret;
>> +
>> +       if (level > OVS_COPY_ACTIONS_MAX_DEPTH)
>> +               return -EOVERFLOW;
>> +
>
> This code seems to run in process context.
>
> Using per cpu limit would not work, unless you disabled migration ?

Oops - I didn't consider it.

Given that, maybe the best approach would not to rely on per-cpu
counter. I'll respin in the next series with a depth counter that I pass
to the function instead and compare that.  I guess that should address
migration and eliminate the need for per-cpu counter.

Does it make sense?

>> +       __this_cpu_inc(copy_actions_depth);
>> +       ret = ___ovs_nla_copy_actions(net, attr, key, sfa, eth_type,
>> +                                     vlan_tci, mpls_label_count, log);
>> +       __this_cpu_dec(copy_actions_depth);
>> +
>> +       return ret;
>> +}
>> +
>>  /* 'key' must be the masked key. */
>>  int ovs_nla_copy_actions(struct net *net, const struct nlattr *attr,
>>                          const struct sw_flow_key *key,
>> --
>> 2.41.0
>>