[PATCH net-next v1] ipv4: hold a consistent view of rt->dst.dev under RCU

[PATCH net-next v1] ipv4: hold a consistent view of rt->dst.dev under RCU

[xuanqiang.luo]

From: Xuanqiang Luo <luoxuanqiang@kylinos.cn>

rt_flush_dev() walks the per-CPU uncached route list and rewrites
rt->dst.dev in-place to blackhole_netdev under spin_lock_bh().
This lock does not exclude RCU readers, which may load rt->dst.dev
multiple times within a single rcu_read_lock() region.

ip_rt_send_redirect() is a typical example: it reads rt->dst.dev
three times to obtain in_dev, the L3 master ifindex, and net.
A concurrent device unregistration can repoint rt->dst.dev to
blackhole_netdev between those reads, making the reader combine
state from two different net_devices — for instance, an in_dev
from the real device but a netns and peer lookup from the blackhole
device, causing ICMP redirects to be issued against the wrong
namespace.  ip_rt_get_source() has the same problem: it reads
rt->dst.dev four times to obtain the output ifindex, the netns,
and the source address, so a concurrent flush can cause the source
selection to mix state from different devices.

Take a single READ_ONCE() snapshot of rt->dst.dev at the start of
each affected RCU reader and use that snapshot throughout, so
concurrent flushes cannot cause mid-function inconsistency.
Publish the in-place write in rt_flush_dev() with WRITE_ONCE() to
pair with the readers.

Fixes: caacf05e5ad1a ("ipv4: Properly purge netdev references on uncached routes.")
Signed-off-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
---
 net/ipv4/route.c | 29 +++++++++++++++++------------
 1 file changed, 17 insertions(+), 12 deletions(-)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 3f3de5164d6e5..e14325c4929ab 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -873,6 +873,7 @@ static void ipv4_negative_advice(struct sock *sk,
 void ip_rt_send_redirect(struct sk_buff *skb)
 {
 	struct rtable *rt = skb_rtable(skb);
+	struct net_device *dev;
 	struct in_device *in_dev;
 	struct inet_peer *peer;
 	struct net *net;
@@ -880,15 +881,16 @@ void ip_rt_send_redirect(struct sk_buff *skb)
 	int vif;
 
 	rcu_read_lock();
-	in_dev = __in_dev_get_rcu(rt->dst.dev);
+	dev = READ_ONCE(rt->dst.dev);
+	in_dev = __in_dev_get_rcu(dev);
 	if (!in_dev || !IN_DEV_TX_REDIRECTS(in_dev)) {
 		rcu_read_unlock();
 		return;
 	}
 	log_martians = IN_DEV_LOG_MARTIANS(in_dev);
-	vif = l3mdev_master_ifindex_rcu(rt->dst.dev);
+	vif = l3mdev_master_ifindex_rcu(dev);
 
-	net = dev_net(rt->dst.dev);
+	net = dev_net(dev);
 	peer = inet_getpeer_v4(net->ipv4.peers, ip_hdr(skb)->saddr, vif);
 	if (!peer) {
 		rcu_read_unlock();
@@ -1287,29 +1289,32 @@ void ip_rt_get_source(u8 *addr, struct sk_buff *skb, struct rtable *rt)
 {
 	__be32 src;
 
-	if (rt_is_output_route(rt))
+	rcu_read_lock();
+	if (rt_is_output_route(rt)) {
 		src = ip_hdr(skb)->saddr;
-	else {
+	} else {
 		struct fib_result res;
 		struct iphdr *iph = ip_hdr(skb);
+		struct net_device *dev = READ_ONCE(rt->dst.dev);
+		struct net *net = dev_net(dev);
 		struct flowi4 fl4 = {
 			.daddr = iph->daddr,
 			.saddr = iph->saddr,
 			.flowi4_dscp = ip4h_dscp(iph),
-			.flowi4_oif = rt->dst.dev->ifindex,
+			.flowi4_oif = dev->ifindex,
 			.flowi4_iif = skb->dev->ifindex,
 			.flowi4_mark = skb->mark,
 		};
 
-		rcu_read_lock();
-		if (fib_lookup(dev_net(rt->dst.dev), &fl4, &res, 0) == 0)
-			src = fib_result_prefsrc(dev_net(rt->dst.dev), &res);
+		if (fib_lookup(net, &fl4, &res, 0) == 0)
+			src = fib_result_prefsrc(net, &res);
 		else
-			src = inet_select_addr(rt->dst.dev,
+			src = inet_select_addr(dev,
 					       rt_nexthop(rt, iph->daddr),
 					       RT_SCOPE_UNIVERSE);
-		rcu_read_unlock();
 	}
+	rcu_read_unlock();
+
 	memcpy(addr, &src, 4);
 }
 
@@ -1565,7 +1570,7 @@ void rt_flush_dev(struct net_device *dev)
 		list_for_each_entry_safe(rt, safe, &ul->head, dst.rt_uncached) {
 			if (rt->dst.dev != dev)
 				continue;
-			rt->dst.dev = blackhole_netdev;
+			WRITE_ONCE(rt->dst.dev, blackhole_netdev);
 			netdev_ref_replace(dev, blackhole_netdev,
 					   &rt->dst.dev_tracker, GFP_ATOMIC);
 			list_del_init(&rt->dst.rt_uncached);
-- 
2.43.0

[PATCH net-next v2] ipv4: hold a consistent view of rt->dst.dev under RCU

[xuanqiang.luo]

From: Xuanqiang Luo <luoxuanqiang@kylinos.cn>

rt_flush_dev() walks the per-CPU uncached route list and rewrites
rt->dst.dev in-place to blackhole_netdev under spin_lock_bh().
This lock does not exclude RCU readers, which may load rt->dst.dev
multiple times within a single rcu_read_lock() region.

ip_rt_send_redirect() is a typical example: it reads rt->dst.dev
three times to obtain in_dev, the L3 master ifindex, and net.
A concurrent device unregistration can repoint rt->dst.dev to
blackhole_netdev between those reads, making the reader combine
state from two different net_devices — for instance, an in_dev
from the real device but a netns and peer lookup from the blackhole
device.  ip_rt_get_source() has the same problem: it reads
rt->dst.dev four times to obtain the output ifindex, the netns,
and the source address, so a concurrent flush can cause the source
selection to mix state from different devices.

Take a single dst_dev_rcu() snapshot of rt->dst.dev at the start
of each affected RCU reader and use that snapshot throughout, so
concurrent flushes cannot cause mid-function inconsistency.
Publish the in-place write in rt_flush_dev() with rcu_assign_pointer()
to match the readers.

Fixes: caacf05e5ad1a ("ipv4: Properly purge netdev references on uncached routes.")
Signed-off-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
---
v2:
- Use dst_dev_rcu() and dev_net_rcu() for the RCU readers.
- Use rcu_assign_pointer() when publishing the uncached route device
  replacement.
- Slightly adjust the commit message wording because this issue was found
  by inspection, not from an observed user-visible failure.

v1: https://lore.kernel.org/all/20260630094250.29386-1-xuanqiang.luo@linux.dev/

 net/ipv4/route.c | 29 +++++++++++++++++------------
 1 file changed, 17 insertions(+), 12 deletions(-)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 3f3de5164d6e5..57f38467e6d0c 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -873,6 +873,7 @@ static void ipv4_negative_advice(struct sock *sk,
 void ip_rt_send_redirect(struct sk_buff *skb)
 {
 	struct rtable *rt = skb_rtable(skb);
+	struct net_device *dev;
 	struct in_device *in_dev;
 	struct inet_peer *peer;
 	struct net *net;
@@ -880,15 +881,16 @@ void ip_rt_send_redirect(struct sk_buff *skb)
 	int vif;
 
 	rcu_read_lock();
-	in_dev = __in_dev_get_rcu(rt->dst.dev);
+	dev = dst_dev_rcu(&rt->dst);
+	in_dev = __in_dev_get_rcu(dev);
 	if (!in_dev || !IN_DEV_TX_REDIRECTS(in_dev)) {
 		rcu_read_unlock();
 		return;
 	}
 	log_martians = IN_DEV_LOG_MARTIANS(in_dev);
-	vif = l3mdev_master_ifindex_rcu(rt->dst.dev);
+	vif = l3mdev_master_ifindex_rcu(dev);
 
-	net = dev_net(rt->dst.dev);
+	net = dev_net_rcu(dev);
 	peer = inet_getpeer_v4(net->ipv4.peers, ip_hdr(skb)->saddr, vif);
 	if (!peer) {
 		rcu_read_unlock();
@@ -1287,29 +1289,32 @@ void ip_rt_get_source(u8 *addr, struct sk_buff *skb, struct rtable *rt)
 {
 	__be32 src;
 
-	if (rt_is_output_route(rt))
+	rcu_read_lock();
+	if (rt_is_output_route(rt)) {
 		src = ip_hdr(skb)->saddr;
-	else {
+	} else {
 		struct fib_result res;
 		struct iphdr *iph = ip_hdr(skb);
+		struct net_device *dev = dst_dev_rcu(&rt->dst);
+		struct net *net = dev_net_rcu(dev);
 		struct flowi4 fl4 = {
 			.daddr = iph->daddr,
 			.saddr = iph->saddr,
 			.flowi4_dscp = ip4h_dscp(iph),
-			.flowi4_oif = rt->dst.dev->ifindex,
+			.flowi4_oif = dev->ifindex,
 			.flowi4_iif = skb->dev->ifindex,
 			.flowi4_mark = skb->mark,
 		};
 
-		rcu_read_lock();
-		if (fib_lookup(dev_net(rt->dst.dev), &fl4, &res, 0) == 0)
-			src = fib_result_prefsrc(dev_net(rt->dst.dev), &res);
+		if (fib_lookup(net, &fl4, &res, 0) == 0)
+			src = fib_result_prefsrc(net, &res);
 		else
-			src = inet_select_addr(rt->dst.dev,
+			src = inet_select_addr(dev,
 					       rt_nexthop(rt, iph->daddr),
 					       RT_SCOPE_UNIVERSE);
-		rcu_read_unlock();
 	}
+	rcu_read_unlock();
+
 	memcpy(addr, &src, 4);
 }
 
@@ -1565,7 +1570,7 @@ void rt_flush_dev(struct net_device *dev)
 		list_for_each_entry_safe(rt, safe, &ul->head, dst.rt_uncached) {
 			if (rt->dst.dev != dev)
 				continue;
-			rt->dst.dev = blackhole_netdev;
+			rcu_assign_pointer(rt->dst.dev_rcu, blackhole_netdev);
 			netdev_ref_replace(dev, blackhole_netdev,
 					   &rt->dst.dev_tracker, GFP_ATOMIC);
 			list_del_init(&rt->dst.rt_uncached);
-- 
2.43.0

[PATCH net-next v2] ipv4: hold a consistent view of rt->dst.dev under RCU

[xuanqiang.luo]

From: Xuanqiang Luo <luoxuanqiang@kylinos.cn>

rt_flush_dev() walks the per-CPU uncached route list and rewrites
rt->dst.dev in-place to blackhole_netdev under spin_lock_bh().
This lock does not exclude RCU readers, which may load rt->dst.dev
multiple times within a single rcu_read_lock() region.

ip_rt_send_redirect() is a typical example: it reads rt->dst.dev
three times to obtain in_dev, the L3 master ifindex, and net.
A concurrent device unregistration can repoint rt->dst.dev to
blackhole_netdev between those reads, making the reader combine
state from two different net_devices — for instance, an in_dev
from the real device but a netns and peer lookup from the blackhole
device.  ip_rt_get_source() has the same problem: it reads
rt->dst.dev four times to obtain the output ifindex, the netns,
and the source address, so a concurrent flush can cause the source
selection to mix state from different devices.

Take a single dst_dev_rcu() snapshot of rt->dst.dev at the start
of each affected RCU reader and use that snapshot throughout, so
concurrent flushes cannot cause mid-function inconsistency.
Publish the in-place write in rt_flush_dev() with rcu_assign_pointer()
to match the readers.

Fixes: caacf05e5ad1a ("ipv4: Properly purge netdev references on uncached routes.")
Signed-off-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
---
v2:
- Use dst_dev_rcu() and dev_net_rcu() for the RCU readers.
- Use rcu_assign_pointer() when publishing the uncached route device
  replacement.
- Slightly adjust the commit message wording because this issue was found
  by inspection, not from an observed user-visible failure.

v1: https://lore.kernel.org/all/20260630094250.29386-1-xuanqiang.luo@linux.dev/

 net/ipv4/route.c | 29 +++++++++++++++++------------
 1 file changed, 17 insertions(+), 12 deletions(-)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 3f3de5164d6e5..57f38467e6d0c 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -873,6 +873,7 @@ static void ipv4_negative_advice(struct sock *sk,
 void ip_rt_send_redirect(struct sk_buff *skb)
 {
 	struct rtable *rt = skb_rtable(skb);
+	struct net_device *dev;
 	struct in_device *in_dev;
 	struct inet_peer *peer;
 	struct net *net;
@@ -880,15 +881,16 @@ void ip_rt_send_redirect(struct sk_buff *skb)
 	int vif;
 
 	rcu_read_lock();
-	in_dev = __in_dev_get_rcu(rt->dst.dev);
+	dev = dst_dev_rcu(&rt->dst);
+	in_dev = __in_dev_get_rcu(dev);
 	if (!in_dev || !IN_DEV_TX_REDIRECTS(in_dev)) {
 		rcu_read_unlock();
 		return;
 	}
 	log_martians = IN_DEV_LOG_MARTIANS(in_dev);
-	vif = l3mdev_master_ifindex_rcu(rt->dst.dev);
+	vif = l3mdev_master_ifindex_rcu(dev);
 
-	net = dev_net(rt->dst.dev);
+	net = dev_net_rcu(dev);
 	peer = inet_getpeer_v4(net->ipv4.peers, ip_hdr(skb)->saddr, vif);
 	if (!peer) {
 		rcu_read_unlock();
@@ -1287,29 +1289,32 @@ void ip_rt_get_source(u8 *addr, struct sk_buff *skb, struct rtable *rt)
 {
 	__be32 src;
 
-	if (rt_is_output_route(rt))
+	rcu_read_lock();
+	if (rt_is_output_route(rt)) {
 		src = ip_hdr(skb)->saddr;
-	else {
+	} else {
 		struct fib_result res;
 		struct iphdr *iph = ip_hdr(skb);
+		struct net_device *dev = dst_dev_rcu(&rt->dst);
+		struct net *net = dev_net_rcu(dev);
 		struct flowi4 fl4 = {
 			.daddr = iph->daddr,
 			.saddr = iph->saddr,
 			.flowi4_dscp = ip4h_dscp(iph),
-			.flowi4_oif = rt->dst.dev->ifindex,
+			.flowi4_oif = dev->ifindex,
 			.flowi4_iif = skb->dev->ifindex,
 			.flowi4_mark = skb->mark,
 		};
 
-		rcu_read_lock();
-		if (fib_lookup(dev_net(rt->dst.dev), &fl4, &res, 0) == 0)
-			src = fib_result_prefsrc(dev_net(rt->dst.dev), &res);
+		if (fib_lookup(net, &fl4, &res, 0) == 0)
+			src = fib_result_prefsrc(net, &res);
 		else
-			src = inet_select_addr(rt->dst.dev,
+			src = inet_select_addr(dev,
 					       rt_nexthop(rt, iph->daddr),
 					       RT_SCOPE_UNIVERSE);
-		rcu_read_unlock();
 	}
+	rcu_read_unlock();
+
 	memcpy(addr, &src, 4);
 }
 
@@ -1565,7 +1570,7 @@ void rt_flush_dev(struct net_device *dev)
 		list_for_each_entry_safe(rt, safe, &ul->head, dst.rt_uncached) {
 			if (rt->dst.dev != dev)
 				continue;
-			rt->dst.dev = blackhole_netdev;
+			rcu_assign_pointer(rt->dst.dev_rcu, blackhole_netdev);
 			netdev_ref_replace(dev, blackhole_netdev,
 					   &rt->dst.dev_tracker, GFP_ATOMIC);
 			list_del_init(&rt->dst.rt_uncached);
-- 
2.43.0

Re: [PATCH net-next v2] ipv4: hold a consistent view of rt->dst.dev under RCU

[Ido Schimmel]

On Wed, Jul 01, 2026 at 11:24:34AM +0800, xuanqiang.luo@linux.dev wrote:
> From: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
> 
> rt_flush_dev() walks the per-CPU uncached route list and rewrites
> rt->dst.dev in-place to blackhole_netdev under spin_lock_bh().
> This lock does not exclude RCU readers, which may load rt->dst.dev
> multiple times within a single rcu_read_lock() region.
> 
> ip_rt_send_redirect() is a typical example: it reads rt->dst.dev
> three times to obtain in_dev, the L3 master ifindex, and net.
> A concurrent device unregistration can repoint rt->dst.dev to
> blackhole_netdev between those reads, making the reader combine
> state from two different net_devices — for instance, an in_dev
> from the real device but a netns and peer lookup from the blackhole
> device.  ip_rt_get_source() has the same problem: it reads
> rt->dst.dev four times to obtain the output ifindex, the netns,
> and the source address, so a concurrent flush can cause the source
> selection to mix state from different devices.

Why only change ip_rt_send_redirect() and ip_rt_get_source() when the
patch is titled "ipv4: hold a consistent view of rt->dst.dev under RCU"?
What is the criterion?

> 
> Take a single dst_dev_rcu() snapshot of rt->dst.dev at the start
> of each affected RCU reader and use that snapshot throughout, so
> concurrent flushes cannot cause mid-function inconsistency.
> Publish the in-place write in rt_flush_dev() with rcu_assign_pointer()
> to match the readers.

The rt_flush_dev() change should be a separate change. Note that
dst_dev_put() was already converted to use rcu_assign_pointer().

> 
> Fixes: caacf05e5ad1a ("ipv4: Properly purge netdev references on uncached routes.")

Please remove the Fixes tag given you are targeting net-next.

> Signed-off-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
> ---
> v2:
> - Use dst_dev_rcu() and dev_net_rcu() for the RCU readers.
> - Use rcu_assign_pointer() when publishing the uncached route device
>   replacement.
> - Slightly adjust the commit message wording because this issue was found
>   by inspection, not from an observed user-visible failure.
> 
> v1: https://lore.kernel.org/all/20260630094250.29386-1-xuanqiang.luo@linux.dev/
> 
>  net/ipv4/route.c | 29 +++++++++++++++++------------
>  1 file changed, 17 insertions(+), 12 deletions(-)
> 
> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> index 3f3de5164d6e5..57f38467e6d0c 100644
> --- a/net/ipv4/route.c
> +++ b/net/ipv4/route.c
> @@ -873,6 +873,7 @@ static void ipv4_negative_advice(struct sock *sk,
>  void ip_rt_send_redirect(struct sk_buff *skb)
>  {
>  	struct rtable *rt = skb_rtable(skb);
> +	struct net_device *dev;

https://docs.kernel.org/process/maintainer-netdev.html#local-variable-ordering-reverse-xmas-tree-rcs

Same in other places.

Re: [PATCH net-next v2] ipv4: hold a consistent view of rt->dst.dev under RCU

[luoxuanqiang]

在 2026/7/6 00:46, Ido Schimmel 写道:
> On Wed, Jul 01, 2026 at 11:24:34AM +0800,xuanqiang.luo@linux.dev wrote:
>> From: Xuanqiang Luo<luoxuanqiang@kylinos.cn>
>>
>> rt_flush_dev() walks the per-CPU uncached route list and rewrites
>> rt->dst.dev in-place to blackhole_netdev under spin_lock_bh().
>> This lock does not exclude RCU readers, which may load rt->dst.dev
>> multiple times within a single rcu_read_lock() region.
>>
>> ip_rt_send_redirect() is a typical example: it reads rt->dst.dev
>> three times to obtain in_dev, the L3 master ifindex, and net.
>> A concurrent device unregistration can repoint rt->dst.dev to
>> blackhole_netdev between those reads, making the reader combine
>> state from two different net_devices — for instance, an in_dev
>> from the real device but a netns and peer lookup from the blackhole
>> device.  ip_rt_get_source() has the same problem: it reads
>> rt->dst.dev four times to obtain the output ifindex, the netns,
>> and the source address, so a concurrent flush can cause the source
>> selection to mix state from different devices.
> Why only change ip_rt_send_redirect() and ip_rt_get_source() when the
> patch is titled "ipv4: hold a consistent view of rt->dst.dev under RCU"?
> What is the criterion?

Thanks! You are right, the subject is too broad. I will make them
more accurate in the next version.

>> Take a single dst_dev_rcu() snapshot of rt->dst.dev at the start
>> of each affected RCU reader and use that snapshot throughout, so
>> concurrent flushes cannot cause mid-function inconsistency.
>> Publish the in-place write in rt_flush_dev() with rcu_assign_pointer()
>> to match the readers.
> The rt_flush_dev() change should be a separate change. Note that
> dst_dev_put() was already converted to use rcu_assign_pointer().
>
I will split the rt_flush_dev() change into a separate patch.

>> Fixes: caacf05e5ad1a ("ipv4: Properly purge netdev references on uncached routes.")
> Please remove the Fixes tag given you are targeting net-next.

Just to clarify: is the suggestion to drop the Fixes tag here solely
because this patch is targeted at net-next? Or are there any other
reasons?

>> Signed-off-by: Xuanqiang Luo<luoxuanqiang@kylinos.cn>
>> ---
>> v2:
>> - Use dst_dev_rcu() and dev_net_rcu() for the RCU readers.
>> - Use rcu_assign_pointer() when publishing the uncached route device
>>    replacement.
>> - Slightly adjust the commit message wording because this issue was found
>>    by inspection, not from an observed user-visible failure.
>>
>> v1:https://lore.kernel.org/all/20260630094250.29386-1-xuanqiang.luo@linux.dev/
>>
>>   net/ipv4/route.c | 29 +++++++++++++++++------------
>>   1 file changed, 17 insertions(+), 12 deletions(-)
>>
>> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
>> index 3f3de5164d6e5..57f38467e6d0c 100644
>> --- a/net/ipv4/route.c
>> +++ b/net/ipv4/route.c
>> @@ -873,6 +873,7 @@ static void ipv4_negative_advice(struct sock *sk,
>>   void ip_rt_send_redirect(struct sk_buff *skb)
>>   {
>>   	struct rtable *rt = skb_rtable(skb);
>> +	struct net_device *dev;
> https://docs.kernel.org/process/maintainer-netdev.html#local-variable-ordering-reverse-xmas-tree-rcs
>
> Same in other places.

Thanks for pointing this out!
I will fix the local variable ordering in the next version.

Re: [PATCH net-next v2] ipv4: hold a consistent view of rt->dst.dev under RCU

[Paolo Abeni]

On 7/7/26 3:26 AM, luoxuanqiang wrote:
> 在 2026/7/6 00:46, Ido Schimmel 写道:
>> On Wed, Jul 01, 2026 at 11:24:34AM +0800,xuanqiang.luo@linux.dev wrote:
>>> From: Xuanqiang Luo<luoxuanqiang@kylinos.cn>
>>>
>>> rt_flush_dev() walks the per-CPU uncached route list and rewrites
>>> rt->dst.dev in-place to blackhole_netdev under spin_lock_bh().
>>> This lock does not exclude RCU readers, which may load rt->dst.dev
>>> multiple times within a single rcu_read_lock() region.
>>>
>>> ip_rt_send_redirect() is a typical example: it reads rt->dst.dev
>>> three times to obtain in_dev, the L3 master ifindex, and net.
>>> A concurrent device unregistration can repoint rt->dst.dev to
>>> blackhole_netdev between those reads, making the reader combine
>>> state from two different net_devices — for instance, an in_dev
>>> from the real device but a netns and peer lookup from the blackhole
>>> device.  ip_rt_get_source() has the same problem: it reads
>>> rt->dst.dev four times to obtain the output ifindex, the netns,
>>> and the source address, so a concurrent flush can cause the source
>>> selection to mix state from different devices.
>> Why only change ip_rt_send_redirect() and ip_rt_get_source() when the
>> patch is titled "ipv4: hold a consistent view of rt->dst.dev under RCU"?
>> What is the criterion?
> 
> Thanks! You are right, the subject is too broad. I will make them
> more accurate in the next version.
> 
>>> Take a single dst_dev_rcu() snapshot of rt->dst.dev at the start
>>> of each affected RCU reader and use that snapshot throughout, so
>>> concurrent flushes cannot cause mid-function inconsistency.
>>> Publish the in-place write in rt_flush_dev() with rcu_assign_pointer()
>>> to match the readers.
>> The rt_flush_dev() change should be a separate change. Note that
>> dst_dev_put() was already converted to use rcu_assign_pointer().
>>
> I will split the rt_flush_dev() change into a separate patch.
> 
>>> Fixes: caacf05e5ad1a ("ipv4: Properly purge netdev references on uncached routes.")
>> Please remove the Fixes tag given you are targeting net-next.
> 
> Just to clarify: is the suggestion to drop the Fixes tag here solely
> because this patch is targeted at net-next? Or are there any other
> reasons?

Generally speaking, yes: net-next patches should not include a fixes tag
unless the blamed commit is on net-next only.

More specifically, this patch is really a behavior improvement and not a
vertical fix, as such we want to avoid it propagating on stable trees,
as the fixes tag sometimes does.

/P

Re: [PATCH net-next v2] ipv4: hold a consistent view of rt->dst.dev under RCU

[luoxuanqiang]

在 2026/7/7 17:02, Paolo Abeni 写道:
> On 7/7/26 3:26 AM, luoxuanqiang wrote:
>> 在 2026/7/6 00:46, Ido Schimmel 写道:
>>> On Wed, Jul 01, 2026 at 11:24:34AM +0800,xuanqiang.luo@linux.dev wrote:
>>>> From: Xuanqiang Luo<luoxuanqiang@kylinos.cn>
>>>>
>>>> rt_flush_dev() walks the per-CPU uncached route list and rewrites
>>>> rt->dst.dev in-place to blackhole_netdev under spin_lock_bh().
>>>> This lock does not exclude RCU readers, which may load rt->dst.dev
>>>> multiple times within a single rcu_read_lock() region.
>>>>
>>>> ip_rt_send_redirect() is a typical example: it reads rt->dst.dev
>>>> three times to obtain in_dev, the L3 master ifindex, and net.
>>>> A concurrent device unregistration can repoint rt->dst.dev to
>>>> blackhole_netdev between those reads, making the reader combine
>>>> state from two different net_devices — for instance, an in_dev
>>>> from the real device but a netns and peer lookup from the blackhole
>>>> device.  ip_rt_get_source() has the same problem: it reads
>>>> rt->dst.dev four times to obtain the output ifindex, the netns,
>>>> and the source address, so a concurrent flush can cause the source
>>>> selection to mix state from different devices.
>>> Why only change ip_rt_send_redirect() and ip_rt_get_source() when the
>>> patch is titled "ipv4: hold a consistent view of rt->dst.dev under RCU"?
>>> What is the criterion?
>> Thanks! You are right, the subject is too broad. I will make them
>> more accurate in the next version.
>>
>>>> Take a single dst_dev_rcu() snapshot of rt->dst.dev at the start
>>>> of each affected RCU reader and use that snapshot throughout, so
>>>> concurrent flushes cannot cause mid-function inconsistency.
>>>> Publish the in-place write in rt_flush_dev() with rcu_assign_pointer()
>>>> to match the readers.
>>> The rt_flush_dev() change should be a separate change. Note that
>>> dst_dev_put() was already converted to use rcu_assign_pointer().
>>>
>> I will split the rt_flush_dev() change into a separate patch.
>>
>>>> Fixes: caacf05e5ad1a ("ipv4: Properly purge netdev references on uncached routes.")
>>> Please remove the Fixes tag given you are targeting net-next.
>> Just to clarify: is the suggestion to drop the Fixes tag here solely
>> because this patch is targeted at net-next? Or are there any other
>> reasons?
> Generally speaking, yes: net-next patches should not include a fixes tag
> unless the blamed commit is on net-next only.
>
> More specifically, this patch is really a behavior improvement and not a
> vertical fix, as such we want to avoid it propagating on stable trees,
> as the fixes tag sometimes does.
>
> /P
>
Many thanks for the detailed explanation!