[PATCH net] macvlan: observe an RCU grace period in macvlan_common_newlink() error path

[PATCH net] macvlan: observe an RCU grace period in macvlan_common_newlink() error path

[Eric Dumazet]

valis reported that a race condition still happens after my prior patch.

macvlan_common_newlink() might have made @dev visible before
detecting an error, and its caller will directly call free_netdev(dev).

We must respect an RCU period, either in macvlan or the core networking
stack.

After adding a temporary mdelay(1000) in macvlan_forward_source_one()
to open the race window, valis repro was:

ip link add p1 type veth peer p2
ip link set address 00:00:00:00:00:20 dev p1
ip link set up dev p1
ip link set up dev p2
ip link add mv0 link p2 type macvlan mode source

(ip link add invalid% link p2 type macvlan mode source macaddr add
00:00:00:00:00:20 &) ; sleep 0.5 ; ping -c1 -I p1 1.2.3.4
PING 1.2.3.4 (1.2.3.4): 56 data bytes
RTNETLINK answers: Invalid argument

BUG: KASAN: slab-use-after-free in macvlan_forward_source
(drivers/net/macvlan.c:408 drivers/net/macvlan.c:444)
Read of size 8 at addr ffff888016bb89c0 by task e/175

CPU: 1 UID: 1000 PID: 175 Comm: e Not tainted 6.19.0-rc8+ #33 NONE
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
Call Trace:
<IRQ>
dump_stack_lvl (lib/dump_stack.c:123)
print_report (mm/kasan/report.c:379 mm/kasan/report.c:482)
? macvlan_forward_source (drivers/net/macvlan.c:408 drivers/net/macvlan.c:444)
kasan_report (mm/kasan/report.c:597)
? macvlan_forward_source (drivers/net/macvlan.c:408 drivers/net/macvlan.c:444)
macvlan_forward_source (drivers/net/macvlan.c:408 drivers/net/macvlan.c:444)
? tasklet_init (kernel/softirq.c:983)
macvlan_handle_frame (drivers/net/macvlan.c:501)

Allocated by task 169:
kasan_save_stack (mm/kasan/common.c:58)
kasan_save_track (./arch/x86/include/asm/current.h:25
mm/kasan/common.c:70 mm/kasan/common.c:79)
__kasan_kmalloc (mm/kasan/common.c:419)
__kvmalloc_node_noprof (./include/linux/kasan.h:263 mm/slub.c:5657
mm/slub.c:7140)
alloc_netdev_mqs (net/core/dev.c:12012)
rtnl_create_link (net/core/rtnetlink.c:3648)
rtnl_newlink (net/core/rtnetlink.c:3830 net/core/rtnetlink.c:3957
net/core/rtnetlink.c:4072)
rtnetlink_rcv_msg (net/core/rtnetlink.c:6958)
netlink_rcv_skb (net/netlink/af_netlink.c:2550)
netlink_unicast (net/netlink/af_netlink.c:1319 net/netlink/af_netlink.c:1344)
netlink_sendmsg (net/netlink/af_netlink.c:1894)
__sys_sendto (net/socket.c:727 net/socket.c:742 net/socket.c:2206)
__x64_sys_sendto (net/socket.c:2209)
do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94)
entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:131)

Freed by task 169:
kasan_save_stack (mm/kasan/common.c:58)
kasan_save_track (./arch/x86/include/asm/current.h:25
mm/kasan/common.c:70 mm/kasan/common.c:79)
kasan_save_free_info (mm/kasan/generic.c:587)
__kasan_slab_free (mm/kasan/common.c:287)
kfree (mm/slub.c:6674 mm/slub.c:6882)
rtnl_newlink (net/core/rtnetlink.c:3845 net/core/rtnetlink.c:3957
net/core/rtnetlink.c:4072)
rtnetlink_rcv_msg (net/core/rtnetlink.c:6958)
netlink_rcv_skb (net/netlink/af_netlink.c:2550)
netlink_unicast (net/netlink/af_netlink.c:1319 net/netlink/af_netlink.c:1344)
netlink_sendmsg (net/netlink/af_netlink.c:1894)
__sys_sendto (net/socket.c:727 net/socket.c:742 net/socket.c:2206)
__x64_sys_sendto (net/socket.c:2209)
do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94)
entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:131)

Fixes: f8db6475a836 ("macvlan: fix error recovery in macvlan_common_newlink()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: valis <sec@valis.email>
---
 drivers/net/macvlan.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/net/macvlan.c b/drivers/net/macvlan.c
index c509228be84d1bb836cc221c1022d4037cd5c883..4433b8e95b6acc89b074dde16262bd38e6ee6840 100644
--- a/drivers/net/macvlan.c
+++ b/drivers/net/macvlan.c
@@ -1572,6 +1572,11 @@ int macvlan_common_newlink(struct net_device *dev,
 		if (create)
 			macvlan_port_destroy(port->dev);
 	}
+	/* @dev might have been made visible before an error was detected.
+	 * Make sure to observe an RCU grace period before our caller
+	 * (rtnl_newlink()) frees it.
+	 */
+	synchronize_net();
 	return err;
 }
 EXPORT_SYMBOL_GPL(macvlan_common_newlink);
-- 
2.53.0.310.g728cabbaf7-goog

Re: [PATCH net] macvlan: observe an RCU grace period in macvlan_common_newlink() error path

[patchwork-bot+netdevbpf]

Hello:

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

On Fri, 13 Feb 2026 14:25:57 +0000 you wrote:
> valis reported that a race condition still happens after my prior patch.
> 
> macvlan_common_newlink() might have made @dev visible before
> detecting an error, and its caller will directly call free_netdev(dev).
> 
> We must respect an RCU period, either in macvlan or the core networking
> stack.
> 
> [...]

Here is the summary with links:
  - [net] macvlan: observe an RCU grace period in macvlan_common_newlink() error path
    https://git.kernel.org/netdev/net/c/e3f000f0dee1

You are awesome, thank you!
-- 
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