[RT PATCH 1/2] net: add back the missing serialization in ip_send_unicast_reply()

Some time ago Sami Pietikäinen reported a crash on -RT in
ip_send_unicast_reply() which was later fixed by Nicholas Mc Guire
(v3.12.8-rt11). Later (v3.18.8) the code was reworked and I dropped the
patch. As it turns out it was mistake.
I have reports that the same crash is possible with a similar backtrace.
It seems that vanilla protects access to this_cpu_ptr() via
local_bh_disable(). This does not work on -RT since we can have
NET_RX and NET_TX running in parallel on the same CPU.
This is brings back the old locks.

|Unable to handle kernel NULL pointer dereference at virtual address 00000010
|PC is at __ip_make_skb+0x198/0x3e8
|[<c04e39d8>] (__ip_make_skb) from [<c04e3ca8>] (ip_push_pending_frames+0x20/0x40)
|[<c04e3ca8>] (ip_push_pending_frames) from [<c04e3ff0>] (ip_send_unicast_reply+0x210/0x22c)
|[<c04e3ff0>] (ip_send_unicast_reply) from [<c04fbb54>] (tcp_v4_send_reset+0x190/0x1c0)
|[<c04fbb54>] (tcp_v4_send_reset) from [<c04fcc1c>] (tcp_v4_do_rcv+0x22c/0x288)
|[<c04fcc1c>] (tcp_v4_do_rcv) from [<c0474364>] (release_sock+0xb4/0x150)
|[<c0474364>] (release_sock) from [<c04ed904>] (tcp_close+0x240/0x454)
|[<c04ed904>] (tcp_close) from [<c0511408>] (inet_release+0x74/0x7c)
|[<c0511408>] (inet_release) from [<c0470728>] (sock_release+0x30/0xb0)
|[<c0470728>] (sock_release) from [<c0470abc>] (sock_close+0x1c/0x24)
|[<c0470abc>] (sock_close) from [<c0115ec4>] (__fput+0xe8/0x20c)
|[<c0115ec4>] (__fput) from [<c0116050>] (____fput+0x18/0x1c)
|[<c0116050>] (____fput) from [<c0058138>] (task_work_run+0xa4/0xb8)
|[<c0058138>] (task_work_run) from [<c0011478>] (do_work_pending+0xd0/0xe4)
|[<c0011478>] (do_work_pending) from [<c000e740>] (work_pending+0xc/0x20)
|Code: e3530001 8a000001 e3a00040 ea000011 (e5973010)

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 net/ipv4/tcp_ipv4.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index ad450509029b..c5521d1f1263 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c

@@ -62,6 +62,7 @@
 #include <linux/init.h>
 #include <linux/times.h>
 #include <linux/slab.h>
+#include <linux/locallock.h>
 
 #include <net/net_namespace.h>
 #include <net/icmp.h>

@@ -565,6 +566,7 @@ void tcp_v4_send_check(struct sock *sk, struct sk_buff *skb)
 }
 EXPORT_SYMBOL(tcp_v4_send_check);
 
+static DEFINE_LOCAL_IRQ_LOCK(tcp_sk_lock);
 /*
  *	This routine will send an RST to the other tcp.
  *

@@ -689,10 +691,13 @@ static void tcp_v4_send_reset(const struct sock *sk, struct sk_buff *skb)
 		     offsetof(struct inet_timewait_sock, tw_bound_dev_if));
 
 	arg.tos = ip_hdr(skb)->tos;
+
+	local_lock(tcp_sk_lock);
 	ip_send_unicast_reply(*this_cpu_ptr(net->ipv4.tcp_sk),
 			      skb, &TCP_SKB_CB(skb)->header.h4.opt,
 			      ip_hdr(skb)->saddr, ip_hdr(skb)->daddr,
 			      &arg, arg.iov[0].iov_len);
+	local_unlock(tcp_sk_lock);
 
 	TCP_INC_STATS_BH(net, TCP_MIB_OUTSEGS);
 	TCP_INC_STATS_BH(net, TCP_MIB_OUTRSTS);

@@ -774,10 +779,12 @@ static void tcp_v4_send_ack(struct net *net,
 	if (oif)
 		arg.bound_dev_if = oif;
 	arg.tos = tos;
+	local_lock(tcp_sk_lock);
 	ip_send_unicast_reply(*this_cpu_ptr(net->ipv4.tcp_sk),
 			      skb, &TCP_SKB_CB(skb)->header.h4.opt,
 			      ip_hdr(skb)->saddr, ip_hdr(skb)->daddr,
 			      &arg, arg.iov[0].iov_len);
+	local_unlock(tcp_sk_lock);
 
 	TCP_INC_STATS_BH(net, TCP_MIB_OUTSEGS);
 }

-- 
2.9.3

It looks like the this_cpu_ptr() access in icmp_sk() is protected with
local_bh_disable(). To avoid missing serialization in -RT I am adding
here a local lock. No crash has been observed, this is just precaution.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 net/ipv4/icmp.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
index c1f1d5030d37..63731fd6af3e 100644
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c

@@ -78,6 +78,7 @@
 #include <linux/string.h>
 #include <linux/netfilter_ipv4.h>
 #include <linux/slab.h>
+#include <linux/locallock.h>
 #include <net/snmp.h>
 #include <net/ip.h>
 #include <net/route.h>

@@ -205,6 +206,8 @@ static const struct icmp_control icmp_pointers[NR_ICMP_TYPES+1];
  *
  *	On SMP we have one ICMP socket per-cpu.
  */
+static DEFINE_LOCAL_IRQ_LOCK(icmp_sk_lock);
+
 static struct sock *icmp_sk(struct net *net)
 {
 	return *this_cpu_ptr(net->ipv4.icmp_sk);

@@ -216,12 +219,14 @@ static inline struct sock *icmp_xmit_lock(struct net *net)
 
 	local_bh_disable();
 
+	local_lock(icmp_sk_lock);
 	sk = icmp_sk(net);
 
 	if (unlikely(!spin_trylock(&sk->sk_lock.slock))) {
 		/* This can happen if the output path signals a
 		 * dst_link_failure() for an outgoing ICMP packet.
 		 */
+		local_unlock(icmp_sk_lock);
 		local_bh_enable();
 		return NULL;
 	}

@@ -231,6 +236,7 @@ static inline struct sock *icmp_xmit_lock(struct net *net)
 static inline void icmp_xmit_unlock(struct sock *sk)
 {
 	spin_unlock_bh(&sk->sk_lock.slock);
+	local_unlock(icmp_sk_lock);
 }
 
 int sysctl_icmp_msgs_per_sec __read_mostly = 1000;

@@ -359,6 +365,7 @@ static void icmp_push_reply(struct icmp_bxm *icmp_param,
 	struct sock *sk;
 	struct sk_buff *skb;
 
+	local_lock(icmp_sk_lock);
 	sk = icmp_sk(dev_net((*rt)->dst.dev));
 	if (ip_append_data(sk, fl4, icmp_glue_bits, icmp_param,
 			   icmp_param->data_len+icmp_param->head_len,

@@ -381,6 +388,7 @@ static void icmp_push_reply(struct icmp_bxm *icmp_param,
 		skb->ip_summed = CHECKSUM_NONE;
 		ip_push_pending_frames(sk, fl4);
 	}
+	local_unlock(icmp_sk_lock);
 }
 
 /*

-- 
2.9.3

On Wed, 31 Aug 2016 18:00:48 +0200
Sebastian Andrzej Siewior [off-list ref] wrote:

Interesting that I noticed in mainline, they have:

	local_bh_disable();

here.

I'm surprised we don't have a local_lock_bh() or something to that
effect.

-- Steve

On Wed, 2016-08-31 at 18:00 +0200, Sebastian Andrzej Siewior wrote:

It looks like the this_cpu_ptr() access in icmp_sk() is protected with
local_bh_disable(). To avoid missing serialization in -RT I am adding
here a local lock. No crash has been observed, this is just precaution.

Hmm...

Deadlock alert ?
Please read the comment few lines after, explaining why we have to use
spin_trylock().
Or maybe I should double check what is local_lock() in RT

 	sk = icmp_sk(net);
 
 	if (unlikely(!spin_trylock(&sk->sk_lock.slock))) {

...

 		/* This can happen if the output path signals a
 		 * dst_link_failure() for an outgoing ICMP packet.
 		 */

+		local_unlock(icmp_sk_lock);
 		local_bh_enable();
 		return NULL;

On 2016-08-31 12:15:53 [-0400], Steven Rostedt wrote:

quoted

@@ -689,10 +691,13 @@ static void tcp_v4_send_reset(const struct sock *sk, struct sk_buff *skb)
 		     offsetof(struct inet_timewait_sock, tw_bound_dev_if));
 
 	arg.tos = ip_hdr(skb)->tos;
+
+	local_lock(tcp_sk_lock);

Interesting that I noticed in mainline, they have:

	local_bh_disable();

here.

I'm surprised we don't have a local_lock_bh() or something to that
effect.

Turning local_bh_disable() into local_lock_bh(). One side effect would
be that the network driver will be flushed out / waited for completion
during socket write (due to the spin_lock_bh()). Not sure how much fun
all this will bring.
We could try this…

-- Steve

Sebastian

On Wed, 31 Aug 2016 09:37:43 -0700
Eric Dumazet [off-list ref] wrote:

On Wed, 2016-08-31 at 18:00 +0200, Sebastian Andrzej Siewior wrote:

quoted

It looks like the this_cpu_ptr() access in icmp_sk() is protected with
local_bh_disable(). To avoid missing serialization in -RT I am adding
here a local lock. No crash has been observed, this is just precaution.
  

Hmm...

quoted

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 net/ipv4/icmp.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
index c1f1d5030d37..63731fd6af3e 100644
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c

@@ -78,6 +78,7 @@
 #include <linux/string.h>
 #include <linux/netfilter_ipv4.h>
 #include <linux/slab.h>
+#include <linux/locallock.h>
 #include <net/snmp.h>
 #include <net/ip.h>
 #include <net/route.h>

@@ -205,6 +206,8 @@ static const struct icmp_control icmp_pointers[NR_ICMP_TYPES+1];
  *
  *	On SMP we have one ICMP socket per-cpu.
  */
+static DEFINE_LOCAL_IRQ_LOCK(icmp_sk_lock);
+
 static struct sock *icmp_sk(struct net *net)
 {
 	return *this_cpu_ptr(net->ipv4.icmp_sk);

@@ -216,12 +219,14 @@ static inline struct sock *icmp_xmit_lock(struct net *net)
 
 	local_bh_disable();
 
+	local_lock(icmp_sk_lock);  

Deadlock alert ?
Please read the comment few lines after, explaining why we have to use
spin_trylock().
Or maybe I should double check what is local_lock() in RT

And I don't know exactly what the deadlock scenario of the comment
below is. Is it racing with a softirq somehow?

Note, in RT softirqs can schedule out, and are preemptable. But I don't
know enough about this code to know if that is enough to not have a
deadlock here.

-- Steve

quoted

 	sk = icmp_sk(net);
 
 	if (unlikely(!spin_trylock(&sk->sk_lock.slock))) {  

...

quoted

 		/* This can happen if the output path signals a
 		 * dst_link_failure() for an outgoing ICMP packet.
 		 */  

quoted

+		local_unlock(icmp_sk_lock);
 		local_bh_enable();
 		return NULL;  

On 2016-08-31 09:37:43 [-0700], Eric Dumazet wrote:

quoted

--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c

@@ -216,12 +219,14 @@ static inline struct sock *icmp_xmit_lock(struct net *net)
 
 	local_bh_disable();
 
+	local_lock(icmp_sk_lock);

Deadlock alert ?
Please read the comment few lines after, explaining why we have to use
spin_trylock().
Or maybe I should double check what is local_lock() in RT

On !RT local_lock() is preempt_disable().
On RT local_lock() is a per-CPU sleeping spinlock which can be taken
recursively by the owner.

On a "normal" ping reply we have
- icmp_reply()
  - icmp_xmit_lock()
    - local_lock()
  - icmp_push_reply()
 icmp_send()
    - local_lock()

so that works. I didn't manage to resolve a possible dst_link_failure()
path but I would assume it is like the above (where the owner takes the
same local_lock() multiple times).

Sebastian

On Thu, 2016-09-01 at 10:11 +0200, Sebastian Andrzej Siewior wrote:

On !RT local_lock() is preempt_disable().
On RT local_lock() is a per-CPU sleeping spinlock which can be taken
recursively by the owner.

Ok, thanks for the explanation, the 'recursively' part I was not aware
of ;)