[PATCH net] batman-adv: Hold rtnl lock during MTU update via netlink

[PATCH net] batman-adv: Hold rtnl lock during MTU update via netlink

[Sven Eckelmann]

The automatic recalculation of the maximum allowed MTU is usually triggered
by code sections which are already rtnl lock protected by callers outside
of batman-adv. But when the fragmentation setting is changed via
batman-adv's own batadv genl family, then the rtnl lock is not yet taken.

But dev_set_mtu requires that the caller holds the rtnl lock because it
uses netdevice notifiers. And this code will then fail the check for this
lock:

  RTNL: assertion failed at net/core/dev.c (1953)

Cc: stable@vger.kernel.org
Reported-by: syzbot+f8812454d9b3ac00d282@syzkaller.appspotmail.com
Fixes: c6a953cce8d0 ("batman-adv: Trigger events for auto adjusted MTU")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
---
This problem was just identified by syzbot [1]. I hope it is ok to directly
send this patch to netdev instead of creating a single-patch PR from
the batadv/net branch. If you still prefer a PR then we can also prepare
it.

[1] https://lore.kernel.org/r/0000000000009bbb4b0603717cde@google.com
---
 net/batman-adv/netlink.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/batman-adv/netlink.c b/net/batman-adv/netlink.c
index ad5714f737be..6efbc9275aec 100644
--- a/net/batman-adv/netlink.c
+++ b/net/batman-adv/netlink.c
@@ -495,7 +495,10 @@ static int batadv_netlink_set_mesh(struct sk_buff *skb, struct genl_info *info)
 		attr = info->attrs[BATADV_ATTR_FRAGMENTATION_ENABLED];
 
 		atomic_set(&bat_priv->fragmentation, !!nla_get_u8(attr));
+
+		rtnl_lock();
 		batadv_update_min_mtu(bat_priv->soft_iface);
+		rtnl_unlock();
 	}
 
 	if (info->attrs[BATADV_ATTR_GW_BANDWIDTH_DOWN]) {

---
base-commit: 421d467dc2d483175bad4fb76a31b9e5a3d744cf
change-id: 20230821-batadv-missing-mtu-rtnl-lock-bc4cee67731d

Best regards,
-- 
Sven Eckelmann <sven@narfation.org>

[PATCH net] batman-adv: Hold rtnl lock during MTU update via netlink

[Sven Eckelmann]

The automatic recalculation of the maximum allowed MTU is usually triggered
by code sections which are already rtnl lock protected by callers outside
of batman-adv. But when the fragmentation setting is changed via
batman-adv's own batadv genl family, then the rtnl lock is not yet taken.

But dev_set_mtu requires that the caller holds the rtnl lock because it
uses netdevice notifiers. And this code will then fail the check for this
lock:

  RTNL: assertion failed at net/core/dev.c (1953)

Cc: stable@vger.kernel.org
Reported-by: syzbot+f8812454d9b3ac00d282@syzkaller.appspotmail.com
Fixes: c6a953cce8d0 ("batman-adv: Trigger events for auto adjusted MTU")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
---
This problem was just identified by syzbot [1]. I hope it is ok to directly
send this patch to netdev instead of creating a single-patch PR from
the batadv/net branch. If you still prefer a PR then we can also prepare
it.

[1] https://lore.kernel.org/r/0000000000009bbb4b0603717cde@google.com
---
 net/batman-adv/netlink.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/batman-adv/netlink.c b/net/batman-adv/netlink.c
index ad5714f737be..6efbc9275aec 100644
--- a/net/batman-adv/netlink.c
+++ b/net/batman-adv/netlink.c
@@ -495,7 +495,10 @@ static int batadv_netlink_set_mesh(struct sk_buff *skb, struct genl_info *info)
 		attr = info->attrs[BATADV_ATTR_FRAGMENTATION_ENABLED];
 
 		atomic_set(&bat_priv->fragmentation, !!nla_get_u8(attr));
+
+		rtnl_lock();
 		batadv_update_min_mtu(bat_priv->soft_iface);
+		rtnl_unlock();
 	}
 
 	if (info->attrs[BATADV_ATTR_GW_BANDWIDTH_DOWN]) {

---
base-commit: 421d467dc2d483175bad4fb76a31b9e5a3d744cf
change-id: 20230821-batadv-missing-mtu-rtnl-lock-bc4cee67731d

Best regards,
-- 
Sven Eckelmann <sven@narfation.org>

Re: [PATCH net] batman-adv: Hold rtnl lock during MTU update via netlink

[Simon Horman]

On Mon, Aug 21, 2023 at 09:48:48PM +0200, Sven Eckelmann wrote:
> The automatic recalculation of the maximum allowed MTU is usually triggered
> by code sections which are already rtnl lock protected by callers outside
> of batman-adv. But when the fragmentation setting is changed via
> batman-adv's own batadv genl family, then the rtnl lock is not yet taken.
> 
> But dev_set_mtu requires that the caller holds the rtnl lock because it
> uses netdevice notifiers. And this code will then fail the check for this
> lock:
> 
>   RTNL: assertion failed at net/core/dev.c (1953)
> 
> Cc: stable@vger.kernel.org
> Reported-by: syzbot+f8812454d9b3ac00d282@syzkaller.appspotmail.com
> Fixes: c6a953cce8d0 ("batman-adv: Trigger events for auto adjusted MTU")
> Signed-off-by: Sven Eckelmann <sven@narfation.org>

Reviewed-by: Simon Horman <horms@kernel.org>

> ---
> This problem was just identified by syzbot [1]. I hope it is ok to directly
> send this patch to netdev instead of creating a single-patch PR from
> the batadv/net branch. If you still prefer a PR then we can also prepare
> it.
> 
> [1] https://lore.kernel.org/r/0000000000009bbb4b0603717cde@google.com

...

Re: [PATCH net] batman-adv: Hold rtnl lock during MTU update via netlink

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Mon, 21 Aug 2023 21:48:48 +0200 you wrote:
> The automatic recalculation of the maximum allowed MTU is usually triggered
> by code sections which are already rtnl lock protected by callers outside
> of batman-adv. But when the fragmentation setting is changed via
> batman-adv's own batadv genl family, then the rtnl lock is not yet taken.
> 
> But dev_set_mtu requires that the caller holds the rtnl lock because it
> uses netdevice notifiers. And this code will then fail the check for this
> lock:
> 
> [...]

Here is the summary with links:
  - [net] batman-adv: Hold rtnl lock during MTU update via netlink
    https://git.kernel.org/netdev/net/c/987aae75fc10

You are awesome, thank you!
-- 
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