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mptcp: disable active MPTCP in case of blackhole
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An MPTCP firewall blackhole can be detected if the following SYN
retransmission after a fallback to "plain" TCP is accepted.

In case of blackhole, a similar technique to the one used by TFO is
used: MPTCP can be disabled for a certain period of time, 1h by default.
This time period will grow exponentially when more blackhole issues get
detected right after MPTCP is re-enabled and will reset to the initial
value when the blackhole issue goes away.

The blackhole period can be modified thanks to a new sysctl knob:
blackhole_timeout. Two new MIB counters help understanding what's
happening:

- 'Blackhole', incremented when a blackhole has been detected.
- 'MPCapableSYNTXDisabled', incremented when an MPTCP connection
  directly falls back to TCP during the blackhole period.

Because the technique is similar to the one used by TFO, an important
part of the new code is similar to what was can find in tcp_fastopen.c,
with some adaptations to the MPTCP case.

Closes: multipath-tcp/mptcp_net-next#57
Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>
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matttbe authored and intel-lab-lkp committed Sep 2, 2024
1 parent 78c14d2 commit ee14c6e
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Showing 7 changed files with 151 additions and 8 deletions.
11 changes: 11 additions & 0 deletions Documentation/networking/mptcp-sysctl.rst
Original file line number Diff line number Diff line change
Expand Up @@ -34,6 +34,17 @@ available_schedulers - STRING
Shows the available schedulers choices that are registered. More packet
schedulers may be available, but not loaded.

blackhole_timeout - INTEGER (seconds)
Initial time period in second to disable MPTCP on active MPTCP sockets
when a MPTCP firewall blackhole issue happens. This time period will
grow exponentially when more blackhole issues get detected right after
MPTCP is re-enabled and will reset to the initial value when the
blackhole issue goes away.

0 to disable the blackhole detection.

Default: 3600

checksum_enabled - BOOLEAN
Control whether DSS checksum can be enabled.

Expand Down
121 changes: 117 additions & 4 deletions net/mptcp/ctrl.c
Original file line number Diff line number Diff line change
Expand Up @@ -28,8 +28,11 @@ struct mptcp_pernet {
#endif

unsigned int add_addr_timeout;
unsigned int blackhole_timeout;
unsigned int close_timeout;
unsigned int stale_loss_cnt;
atomic_t active_disable_times;
unsigned long active_disable_stamp;
u8 mptcp_enabled;
u8 checksum_enabled;
u8 allow_join_initial_addr_port;
Expand Down Expand Up @@ -88,6 +91,8 @@ static void mptcp_pernet_set_defaults(struct mptcp_pernet *pernet)
{
pernet->mptcp_enabled = 1;
pernet->add_addr_timeout = TCP_RTO_MAX;
pernet->blackhole_timeout = 3600;
atomic_set(&pernet->active_disable_times, 0);
pernet->close_timeout = TCP_TIMEWAIT_LEN;
pernet->checksum_enabled = 0;
pernet->allow_join_initial_addr_port = 1;
Expand Down Expand Up @@ -152,6 +157,20 @@ static int proc_available_schedulers(const struct ctl_table *ctl,
return ret;
}

static int proc_blackhole_detect_timeout(const struct ctl_table *table,
int write, void *buffer, size_t *lenp,
loff_t *ppos)
{
struct mptcp_pernet *pernet = mptcp_get_pernet(current->nsproxy->net_ns);
int ret;

ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
if (write && ret == 0)
atomic_set(&pernet->active_disable_times, 0);

return ret;
}

static struct ctl_table mptcp_sysctl_table[] = {
{
.procname = "enabled",
Expand Down Expand Up @@ -218,6 +237,13 @@ static struct ctl_table mptcp_sysctl_table[] = {
.mode = 0644,
.proc_handler = proc_dointvec_jiffies,
},
{
.procname = "blackhole_timeout",
.maxlen = sizeof(unsigned int),
.mode = 0644,
.proc_handler = proc_blackhole_detect_timeout,
.extra1 = SYSCTL_ZERO,
},
};

static int mptcp_pernet_new_table(struct net *net, struct mptcp_pernet *pernet)
Expand All @@ -241,6 +267,7 @@ static int mptcp_pernet_new_table(struct net *net, struct mptcp_pernet *pernet)
table[6].data = &pernet->scheduler;
/* table[7] is for available_schedulers which is read-only info */
table[8].data = &pernet->close_timeout;
table[9].data = &pernet->blackhole_timeout;

hdr = register_net_sysctl_sz(net, MPTCP_SYSCTL_PATH, table,
ARRAY_SIZE(mptcp_sysctl_table));
Expand Down Expand Up @@ -278,6 +305,88 @@ static void mptcp_pernet_del_table(struct mptcp_pernet *pernet) {}

#endif /* CONFIG_SYSCTL */

/* The following code block is to deal with middle box issues with MPTCP,
* similar to what is done with TFO.
* The proposed solution is to disable active MPTCP globally when SYN+MPC are
* dropped, while SYN without MPC aren't. In this case, active side MPTCP is
* disabled globally for 1hr at first. Then if it happens again, it is disabled
* for 2h, then 4h, 8h, ...
* The timeout is reset back to 1hr when a successful active MPTCP connection is
* fully established.
*/

/* Disable active MPTCP and record current jiffies and active_disable_times */
void mptcp_active_disable(struct sock *sk)
{
struct net *net = sock_net(sk);
struct mptcp_pernet *pernet;

pernet = mptcp_get_pernet(net);

if (!READ_ONCE(pernet->blackhole_timeout))
return;

/* Paired with READ_ONCE() in mptcp_active_should_disable() */
WRITE_ONCE(pernet->active_disable_stamp, jiffies);

/* Paired with smp_rmb() in mptcp_active_should_disable().
* We want pernet->active_disable_stamp to be updated first.
*/
smp_mb__before_atomic();
atomic_inc(&pernet->active_disable_times);

MPTCP_INC_STATS(net, MPTCP_MIB_BLACKHOLE);
}

/* Calculate timeout for MPTCP active disable
* Return true if we are still in the active MPTCP disable period
* Return false if timeout already expired and we should use active MPTCP
*/
bool mptcp_active_should_disable(struct sock *ssk)
{
struct net *net = sock_net(ssk);
unsigned int blackhole_timeout;
struct mptcp_pernet *pernet;
unsigned long timeout;
int disable_times;
int multiplier;

pernet = mptcp_get_pernet(net);
blackhole_timeout = READ_ONCE(pernet->blackhole_timeout);

if (!blackhole_timeout)
return false;

disable_times = atomic_read(&pernet->active_disable_times);
if (!disable_times)
return false;

/* Paired with smp_mb__before_atomic() in mptcp_active_disable() */
smp_rmb();

/* Limit timeout to max: 2^6 * initial timeout */
multiplier = 1 << min(disable_times - 1, 6);

/* Paired with the WRITE_ONCE() in mptcp_active_disable(). */
timeout = READ_ONCE(pernet->active_disable_stamp) +
multiplier * blackhole_timeout * HZ;

return time_before(jiffies, timeout);
}

/* Enable active MPTCP and reset active_disable_times if needed */
void mptcp_active_enable(struct sock *sk)
{
struct mptcp_pernet *pernet = mptcp_get_pernet(sock_net(sk));

if (atomic_read(&pernet->active_disable_times)) {
struct dst_entry *dst = sk_dst_get(sk);

if (dst && dst->dev && (dst->dev->flags & IFF_LOOPBACK))
atomic_set(&pernet->active_disable_times, 0);
}
}

/* Check the number of retransmissions, and fallback to TCP if needed */
void mptcp_active_detect_blackhole(struct sock *ssk, bool expired)
{
Expand All @@ -290,10 +399,14 @@ void mptcp_active_detect_blackhole(struct sock *ssk, bool expired)
timeouts = inet_csk(ssk)->icsk_retransmits;
subflow = mptcp_subflow_ctx(ssk);

if (subflow->request_mptcp && ssk->sk_state == TCP_SYN_SENT &&
(timeouts == 2 || (timeouts < 2 && expired))) {
MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_MPCAPABLEACTIVEDROP);
mptcp_subflow_early_fallback(mptcp_sk(subflow->conn), subflow);
if (subflow->request_mptcp && ssk->sk_state == TCP_SYN_SENT) {
if (timeouts == 2 || (timeouts < 2 && expired)) {
MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_MPCAPABLEACTIVEDROP);
subflow->mpc_drop = 1;
mptcp_subflow_early_fallback(mptcp_sk(subflow->conn), subflow);
} else {
subflow->mpc_drop = 0;
}
}
}

Expand Down
2 changes: 2 additions & 0 deletions net/mptcp/mib.c
Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,7 @@ static const struct snmp_mib mptcp_snmp_list[] = {
SNMP_MIB_ITEM("MPCapableFallbackACK", MPTCP_MIB_MPCAPABLEPASSIVEFALLBACK),
SNMP_MIB_ITEM("MPCapableFallbackSYNACK", MPTCP_MIB_MPCAPABLEACTIVEFALLBACK),
SNMP_MIB_ITEM("MPCapableSYNTXDrop", MPTCP_MIB_MPCAPABLEACTIVEDROP),
SNMP_MIB_ITEM("MPCapableSYNTXDisabled", MPTCP_MIB_MPCAPABLEACTIVEDISABLED),
SNMP_MIB_ITEM("MPFallbackTokenInit", MPTCP_MIB_TOKENFALLBACKINIT),
SNMP_MIB_ITEM("MPTCPRetrans", MPTCP_MIB_RETRANSSEGS),
SNMP_MIB_ITEM("MPJoinNoTokenFound", MPTCP_MIB_JOINNOTOKEN),
Expand Down Expand Up @@ -74,6 +75,7 @@ static const struct snmp_mib mptcp_snmp_list[] = {
SNMP_MIB_ITEM("RcvWndConflictUpdate", MPTCP_MIB_RCVWNDCONFLICTUPDATE),
SNMP_MIB_ITEM("RcvWndConflict", MPTCP_MIB_RCVWNDCONFLICT),
SNMP_MIB_ITEM("MPCurrEstab", MPTCP_MIB_CURRESTAB),
SNMP_MIB_ITEM("Blackhole", MPTCP_MIB_BLACKHOLE),
SNMP_MIB_SENTINEL
};

Expand Down
2 changes: 2 additions & 0 deletions net/mptcp/mib.h
Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,7 @@ enum linux_mptcp_mib_field {
MPTCP_MIB_MPCAPABLEPASSIVEFALLBACK,/* Server-side fallback during 3-way handshake */
MPTCP_MIB_MPCAPABLEACTIVEFALLBACK, /* Client-side fallback during 3-way handshake */
MPTCP_MIB_MPCAPABLEACTIVEDROP, /* Client-side fallback due to a MPC drop */
MPTCP_MIB_MPCAPABLEACTIVEDISABLED, /* Client-side disabled due to past issues */
MPTCP_MIB_TOKENFALLBACKINIT, /* Could not init/allocate token */
MPTCP_MIB_RETRANSSEGS, /* Segments retransmitted at the MPTCP-level */
MPTCP_MIB_JOINNOTOKEN, /* Received MP_JOIN but the token was not found */
Expand Down Expand Up @@ -75,6 +76,7 @@ enum linux_mptcp_mib_field {
*/
MPTCP_MIB_RCVWNDCONFLICT, /* Conflict with while updating msk rcv wnd */
MPTCP_MIB_CURRESTAB, /* Current established MPTCP connections */
MPTCP_MIB_BLACKHOLE, /* A blackhole has been detected */
__MPTCP_MIB_MAX
};

Expand Down
11 changes: 8 additions & 3 deletions net/mptcp/protocol.c
Original file line number Diff line number Diff line change
Expand Up @@ -3737,9 +3737,14 @@ static int mptcp_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)
if (rcu_access_pointer(tcp_sk(ssk)->md5sig_info))
mptcp_subflow_early_fallback(msk, subflow);
#endif
if (subflow->request_mptcp && mptcp_token_new_connect(ssk)) {
MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_TOKENFALLBACKINIT);
mptcp_subflow_early_fallback(msk, subflow);
if (subflow->request_mptcp) {
if (mptcp_active_should_disable(sk)) {
MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_MPCAPABLEACTIVEDISABLED);
mptcp_subflow_early_fallback(msk, subflow);
} else if (mptcp_token_new_connect(ssk) < 0) {
MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_TOKENFALLBACKINIT);
mptcp_subflow_early_fallback(msk, subflow);
}
}

WRITE_ONCE(msk->write_seq, subflow->idsn);
Expand Down
8 changes: 7 additions & 1 deletion net/mptcp/protocol.h
Original file line number Diff line number Diff line change
Expand Up @@ -531,7 +531,8 @@ struct mptcp_subflow_context {
valid_csum_seen : 1, /* at least one csum validated */
is_mptfo : 1, /* subflow is doing TFO */
close_event_done : 1, /* has done the post-closed part */
__unused : 9;
mpc_drop : 1, /* the MPC option has been dropped in a rtx */
__unused : 8;
bool data_avail;
bool scheduled;
u32 remote_nonce;
Expand Down Expand Up @@ -697,6 +698,11 @@ unsigned int mptcp_stale_loss_cnt(const struct net *net);
unsigned int mptcp_close_timeout(const struct sock *sk);
int mptcp_get_pm_type(const struct net *net);
const char *mptcp_get_scheduler(const struct net *net);

void mptcp_active_disable(struct sock *sk);
bool mptcp_active_should_disable(struct sock *ssk);
void mptcp_active_enable(struct sock *sk);

void mptcp_get_available_schedulers(char *buf, size_t maxlen);
void __mptcp_subflow_fully_established(struct mptcp_sock *msk,
struct mptcp_subflow_context *subflow,
Expand Down
4 changes: 4 additions & 0 deletions net/mptcp/subflow.c
Original file line number Diff line number Diff line change
Expand Up @@ -546,6 +546,7 @@ static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)
subflow->mp_capable = 1;
MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_MPCAPABLEACTIVEACK);
mptcp_finish_connect(sk);
mptcp_active_enable(parent);
mptcp_propagate_state(parent, sk, subflow, &mp_opt);
} else if (subflow->request_join) {
u8 hmac[SHA256_DIGEST_SIZE];
Expand Down Expand Up @@ -591,6 +592,9 @@ static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)
MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_JOINPORTSYNACKRX);
}
} else if (mptcp_check_fallback(sk)) {
/* It looks like MPTCP is blocked, while TCP is not */
if (subflow->mpc_drop)
mptcp_active_disable(parent);
fallback:
mptcp_propagate_state(parent, sk, subflow, NULL);
}
Expand Down

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