KASAN: use-after-free Read in nbp_vlan_rcu_free

KASAN: use-after-free Read in nbp_vlan_rcu_free

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    e12e00e388de Merge tag 'kbuild-fixes-v4.20' of git://git.k..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14cdb6f5400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=8f215f21f041a0d7
dashboard link: https://syzkaller.appspot.com/bug?extid=04681da557a0e49a52e5
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+04681da557a0e49a52e5@syzkaller.appspotmail.com

device bridge_slave_1 left promiscuous mode
bridge0: port 2(bridge_slave_1) entered disabled state
device bridge_slave_0 left promiscuous mode
bridge0: port 1(bridge_slave_0) entered disabled state
==================================================================
BUG: KASAN: use-after-free in nbp_vlan_rcu_free+0x152/0x160  
net/bridge/br_vlan.c:200
Read of size 8 at addr ffff8881bbc44a18 by task swapper/1/0

CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.20.0-rc1+ #332
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
  <IRQ>
  __dump_stack lib/dump_stack.c:77 [inline]
  dump_stack+0x244/0x39d lib/dump_stack.c:113
  print_address_description.cold.7+0x9/0x1ff mm/kasan/report.c:256
  kasan_report_error mm/kasan/report.c:354 [inline]
  kasan_report.cold.8+0x242/0x309 mm/kasan/report.c:412
  __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:433
  nbp_vlan_rcu_free+0x152/0x160 net/bridge/br_vlan.c:200
  __rcu_reclaim kernel/rcu/rcu.h:240 [inline]
  rcu_do_batch kernel/rcu/tree.c:2437 [inline]
  invoke_rcu_callbacks kernel/rcu/tree.c:2716 [inline]
  rcu_process_callbacks+0x100a/0x1ac0 kernel/rcu/tree.c:2697
  __do_softirq+0x308/0xb7e kernel/softirq.c:292
  invoke_softirq kernel/softirq.c:373 [inline]
  irq_exit+0x17f/0x1c0 kernel/softirq.c:413
  exiting_irq arch/x86/include/asm/apic.h:536 [inline]
  smp_apic_timer_interrupt+0x1cb/0x760 arch/x86/kernel/apic/apic.c:1061
  apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:804
  </IRQ>
RIP: 0010:native_safe_halt+0x6/0x10 arch/x86/include/asm/irqflags.h:57
Code: e9 2c ff ff ff 48 89 c7 48 89 45 d8 e8 d3 43 f3 f9 48 8b 45 d8 e9 ca  
fe ff ff 48 89 df e8 c2 43 f3 f9 eb 82 55 48 89 e5 fb f4 <5d> c3 0f 1f 84  
00 00 00 00 00 55 48 89 e5 f4 5d c3 90 90 90 90 90
RSP: 0018:ffff8881d9b27cb8 EFLAGS: 00000286 ORIG_RAX: ffffffffffffff13
RAX: dffffc0000000000 RBX: 1ffff1103b364f9b RCX: 0000000000000000
RDX: 1ffffffff12a3f71 RSI: 0000000000000001 RDI: ffffffff8951fb88
RBP: ffff8881d9b27cb8 R08: ffff8881d9b14340 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8881d9b27d78
R13: ffffffff8a14e1e0 R14: 0000000000000000 R15: 0000000000000001
  arch_safe_halt arch/x86/include/asm/paravirt.h:151 [inline]
  default_idle+0xbf/0x490 arch/x86/kernel/process.c:498
  arch_cpu_idle+0x10/0x20 arch/x86/kernel/process.c:489
  default_idle_call+0x6d/0x90 kernel/sched/idle.c:93
  cpuidle_idle_call kernel/sched/idle.c:153 [inline]
  do_idle+0x49b/0x5c0 kernel/sched/idle.c:262
  cpu_startup_entry+0x18/0x20 kernel/sched/idle.c:353
  start_secondary+0x487/0x5f0 arch/x86/kernel/smpboot.c:271
  secondary_startup_64+0xa4/0xb0 arch/x86/kernel/head_64.S:243

Allocated by task 13858:
  save_stack+0x43/0xd0 mm/kasan/kasan.c:448
  set_track mm/kasan/kasan.c:460 [inline]
  kasan_kmalloc+0xc7/0xe0 mm/kasan/kasan.c:553
  kmem_cache_alloc_trace+0x152/0x750 mm/slab.c:3620
  kmalloc include/linux/slab.h:546 [inline]
  kzalloc include/linux/slab.h:741 [inline]
  br_vlan_add+0x6e5/0x1340 net/bridge/br_vlan.c:650
  br_vlan_init+0x339/0x3e0 net/bridge/br_vlan.c:1047
  br_dev_init+0x10d/0x2a0 net/bridge/br_device.c:134
  register_netdevice+0x332/0x11d0 net/core/dev.c:8459
  br_dev_newlink+0x2a/0x130 net/bridge/br_netlink.c:1309
  rtnl_newlink+0xf08/0x1da0 net/core/rtnetlink.c:3175
  rtnetlink_rcv_msg+0x46a/0xc20 net/core/rtnetlink.c:4947
  netlink_rcv_skb+0x172/0x440 net/netlink/af_netlink.c:2477
  rtnetlink_rcv+0x1c/0x20 net/core/rtnetlink.c:4965
  netlink_unicast_kernel net/netlink/af_netlink.c:1310 [inline]
  netlink_unicast+0x5a5/0x760 net/netlink/af_netlink.c:1336
  netlink_sendmsg+0xa18/0xfc0 net/netlink/af_netlink.c:1917
  sock_sendmsg_nosec net/socket.c:621 [inline]
  sock_sendmsg+0xd5/0x120 net/socket.c:631
  ___sys_sendmsg+0x7fd/0x930 net/socket.c:2116
  __sys_sendmsg+0x11d/0x280 net/socket.c:2154
  __do_sys_sendmsg net/socket.c:2163 [inline]
  __se_sys_sendmsg net/socket.c:2161 [inline]
  __x64_sys_sendmsg+0x78/0xb0 net/socket.c:2161
  do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
  entry_SYSCALL_64_after_hwframe+0x49/0xbe

Freed by task 9:
  save_stack+0x43/0xd0 mm/kasan/kasan.c:448
  set_track mm/kasan/kasan.c:460 [inline]
  __kasan_slab_free+0x102/0x150 mm/kasan/kasan.c:521
  kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:528
  __cache_free mm/slab.c:3498 [inline]
  kfree+0xcf/0x230 mm/slab.c:3817
  br_master_vlan_rcu_free+0xa8/0xe0 net/bridge/br_vlan.c:174
  __rcu_reclaim kernel/rcu/rcu.h:240 [inline]
  rcu_do_batch kernel/rcu/tree.c:2437 [inline]
  invoke_rcu_callbacks kernel/rcu/tree.c:2716 [inline]
  rcu_process_callbacks+0x100a/0x1ac0 kernel/rcu/tree.c:2697
  __do_softirq+0x308/0xb7e kernel/softirq.c:292

The buggy address belongs to the object at ffff8881bbc44a00
  which belongs to the cache kmalloc-96 of size 96
The buggy address is located 24 bytes inside of
  96-byte region [ffff8881bbc44a00, ffff8881bbc44a60)
The buggy address belongs to the page:
page:ffffea0006ef1100 count:1 mapcount:0 mapping:ffff8881da8004c0 index:0x0
flags: 0x2fffc0000000200(slab)
raw: 02fffc0000000200 ffffea0007471a08 ffffea000757b908 ffff8881da8004c0
raw: 0000000000000000 ffff8881bbc44000 0000000100000020 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
  ffff8881bbc44900: fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
  ffff8881bbc44980: fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
> ffff8881bbc44a00: fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
                             ^
  ffff8881bbc44a80: 00 00 00 00 00 00 00 00 00 00 00 00 fc fc fc fc
  ffff8881bbc44b00: fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
==================================================================


---
This bug is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this bug report. See:
https://goo.gl/tpsmEJ#bug-status-tracking for how to communicate with  
syzbot.

Re: KASAN: use-after-free Read in nbp_vlan_rcu_free

[nikolay]

On 12 November 2018 06:51:02 CET, syzbot <syzbot+04681da557a0e49a52e5@syzkaller.appspotmail.com> wrote:
>Hello,
>
>syzbot found the following crash on:
>
>HEAD commit:    e12e00e388de Merge tag 'kbuild-fixes-v4.20' of
>git://git.k..
>git tree:       upstream
>console output:
>https://syzkaller.appspot.com/x/log.txt?x=14cdb6f5400000
>kernel config: 
>https://syzkaller.appspot.com/x/.config?x=8f215f21f041a0d7
>dashboard link:
>https://syzkaller.appspot.com/bug?extid=04681da557a0e49a52e5
>compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>
>Unfortunately, I don't have any reproducer for this crash yet.
>
>IMPORTANT: if you fix the bug, please add the following tag to the
>commit:
>Reported-by: syzbot+04681da557a0e49a52e5@syzkaller.appspotmail.com

Thanks, I'm about to fly out for LPC. Will take a look in a day.

[PATCH net] net: bridge: fix per-port vlan stats use-after-free on destruction

[Nikolay Aleksandrov]

Syzbot reported a use-after-free of the global vlan context on port vlan
destruction. When I added per-port vlan stats I missed the fact that the
global vlan context can be freed before the per-port vlan rcu callback.
There're a few different ways to deal with this, I've chosen to add a
new private flag that is set only when per-port stats are allocated so
we can directly check it on destruction without dereferencing the global
context at all. The flag is internally controlled by the kernel and
user-space isn't allowed to set it.

Fixes: 9163a0fc1f0c ("net: bridge: add support for per-port vlan stats")
Reported-by: syzbot+04681da557a0e49a52e5@syzkaller.appspotmail.com
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
---
 include/uapi/linux/if_bridge.h | 3 +++
 net/bridge/br_netlink.c        | 7 ++++++-
 net/bridge/br_vlan.c           | 3 ++-
 3 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/if_bridge.h b/include/uapi/linux/if_bridge.h
index e41eda3c71f1..fa1f72276712 100644
--- a/include/uapi/linux/if_bridge.h
+++ b/include/uapi/linux/if_bridge.h
@@ -130,6 +130,9 @@ enum {
 #define BRIDGE_VLAN_INFO_RANGE_BEGIN	(1<<3) /* VLAN is start of vlan range */
 #define BRIDGE_VLAN_INFO_RANGE_END	(1<<4) /* VLAN is end of vlan range */
 #define BRIDGE_VLAN_INFO_BRENTRY	(1<<5) /* Global bridge VLAN entry */
+#define BRIDGE_VLAN_INFO_PORT_STATS	(1<<6) /* Per-port VLAN stats */
+
+#define BRIDGE_VLAN_INFO_PRIVATE_FLAGS BRIDGE_VLAN_INFO_PORT_STATS
 
 struct bridge_vlan_info {
 	__u16 flags;
diff --git a/net/bridge/br_netlink.c b/net/bridge/br_netlink.c
index 3345f1984542..c600fedcfb76 100644
--- a/net/bridge/br_netlink.c
+++ b/net/bridge/br_netlink.c
@@ -527,8 +527,12 @@ int br_getlink(struct sk_buff *skb, u32 pid, u32 seq,
 static int br_vlan_info(struct net_bridge *br, struct net_bridge_port *p,
 			int cmd, struct bridge_vlan_info *vinfo, bool *changed)
 {
+	int err = -EINVAL;
 	bool curr_change;
-	int err = 0;
+
+	/* don't allow user-space control over private flags */
+	if (vinfo->flags & BRIDGE_VLAN_INFO_PRIVATE_FLAGS)
+		return err;
 
 	switch (cmd) {
 	case RTM_SETLINK:
@@ -548,6 +552,7 @@ static int br_vlan_info(struct net_bridge *br, struct net_bridge_port *p,
 		break;
 
 	case RTM_DELLINK:
+		err = 0;
 		if (p) {
 			if (!nbp_vlan_delete(p, vinfo->vid))
 				*changed = true;
diff --git a/net/bridge/br_vlan.c b/net/bridge/br_vlan.c
index 8c9297a01947..004e1f8c5040 100644
--- a/net/bridge/br_vlan.c
+++ b/net/bridge/br_vlan.c
@@ -197,7 +197,7 @@ static void nbp_vlan_rcu_free(struct rcu_head *rcu)
 	v = container_of(rcu, struct net_bridge_vlan, rcu);
 	WARN_ON(br_vlan_is_master(v));
 	/* if we had per-port stats configured then free them here */
-	if (v->brvlan->stats != v->stats)
+	if (v->flags & BRIDGE_VLAN_INFO_PORT_STATS)
 		free_percpu(v->stats);
 	v->stats = NULL;
 	kfree(v);
@@ -264,6 +264,7 @@ static int __vlan_add(struct net_bridge_vlan *v, u16 flags)
 				err = -ENOMEM;
 				goto out_filt;
 			}
+			v->flags |= BRIDGE_VLAN_INFO_PORT_STATS;
 		} else {
 			v->stats = masterv->stats;
 		}
-- 
2.17.2

Re: [PATCH net] net: bridge: fix per-port vlan stats use-after-free on destruction

[Nikolay Aleksandrov]

On 11/13/18 3:01 AM, Nikolay Aleksandrov wrote:
> Syzbot reported a use-after-free of the global vlan context on port vlan
> destruction. When I added per-port vlan stats I missed the fact that the
> global vlan context can be freed before the per-port vlan rcu callback.
> There're a few different ways to deal with this, I've chosen to add a
> new private flag that is set only when per-port stats are allocated so
> we can directly check it on destruction without dereferencing the global
> context at all. The flag is internally controlled by the kernel and
> user-space isn't allowed to set it.
> 
> Fixes: 9163a0fc1f0c ("net: bridge: add support for per-port vlan stats")
> Reported-by: syzbot+04681da557a0e49a52e5@syzkaller.appspotmail.com
> Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
> ---

I'll post v2 with a cosmetic change - move the check up one level where
it's more logical to be and the rest of the checks are done.

[PATCH net v2] net: bridge: fix vlan stats use-after-free on destruction

[Nikolay Aleksandrov]

Syzbot reported a use-after-free of the global vlan context on port vlan
destruction. When I added per-port vlan stats I missed the fact that the
global vlan context can be freed before the per-port vlan rcu callback.
There're a few different ways to deal with this, I've chosen to add a
new private flag that is set only when per-port stats are allocated so
we can directly check it on destruction without dereferencing the global
context at all. The flag is internally controlled by the kernel and
user-space isn't allowed to set it.

Fixes: 9163a0fc1f0c ("net: bridge: add support for per-port vlan stats")
Reported-by: syzbot+04681da557a0e49a52e5@syzkaller.appspotmail.com
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
---
v2: cosmetic change, move the check to br_process_vlan_info where the
    other checks are done

 include/uapi/linux/if_bridge.h | 3 +++
 net/bridge/br_netlink.c        | 4 ++++
 net/bridge/br_vlan.c           | 3 ++-
 3 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/if_bridge.h b/include/uapi/linux/if_bridge.h
index e41eda3c71f1..fa1f72276712 100644
--- a/include/uapi/linux/if_bridge.h
+++ b/include/uapi/linux/if_bridge.h
@@ -130,6 +130,9 @@ enum {
 #define BRIDGE_VLAN_INFO_RANGE_BEGIN	(1<<3) /* VLAN is start of vlan range */
 #define BRIDGE_VLAN_INFO_RANGE_END	(1<<4) /* VLAN is end of vlan range */
 #define BRIDGE_VLAN_INFO_BRENTRY	(1<<5) /* Global bridge VLAN entry */
+#define BRIDGE_VLAN_INFO_PORT_STATS	(1<<6) /* Per-port VLAN stats */
+
+#define BRIDGE_VLAN_INFO_PRIVATE_FLAGS BRIDGE_VLAN_INFO_PORT_STATS
 
 struct bridge_vlan_info {
 	__u16 flags;
diff --git a/net/bridge/br_netlink.c b/net/bridge/br_netlink.c
index 3345f1984542..a017ed566b67 100644
--- a/net/bridge/br_netlink.c
+++ b/net/bridge/br_netlink.c
@@ -573,6 +573,10 @@ static int br_process_vlan_info(struct net_bridge *br,
 	if (!vinfo_curr->vid || vinfo_curr->vid >= VLAN_VID_MASK)
 		return -EINVAL;
 
+	/* don't allow user-space control over private flags */
+	if (vinfo_curr->flags & BRIDGE_VLAN_INFO_PRIVATE_FLAGS)
+		return -EINVAL;
+
 	if (vinfo_curr->flags & BRIDGE_VLAN_INFO_RANGE_BEGIN) {
 		/* check if we are already processing a range */
 		if (*vinfo_last)
diff --git a/net/bridge/br_vlan.c b/net/bridge/br_vlan.c
index 8c9297a01947..004e1f8c5040 100644
--- a/net/bridge/br_vlan.c
+++ b/net/bridge/br_vlan.c
@@ -197,7 +197,7 @@ static void nbp_vlan_rcu_free(struct rcu_head *rcu)
 	v = container_of(rcu, struct net_bridge_vlan, rcu);
 	WARN_ON(br_vlan_is_master(v));
 	/* if we had per-port stats configured then free them here */
-	if (v->brvlan->stats != v->stats)
+	if (v->flags & BRIDGE_VLAN_INFO_PORT_STATS)
 		free_percpu(v->stats);
 	v->stats = NULL;
 	kfree(v);
@@ -264,6 +264,7 @@ static int __vlan_add(struct net_bridge_vlan *v, u16 flags)
 				err = -ENOMEM;
 				goto out_filt;
 			}
+			v->flags |= BRIDGE_VLAN_INFO_PORT_STATS;
 		} else {
 			v->stats = masterv->stats;
 		}
-- 
2.17.2

[PATCH net v3] net: bridge: fix vlan stats use-after-free on destruction

[Nikolay Aleksandrov]

Syzbot reported a use-after-free of the global vlan context on port vlan
destruction. When I added per-port vlan stats I missed the fact that the
global vlan context can be freed before the per-port vlan rcu callback.
There're a few different ways to deal with this, I've chosen to add a
new private flag that is set only when per-port stats are allocated so
we can directly check it on destruction without dereferencing the global
context at all. The new field in net_bridge_vlan uses a hole.

v2: cosmetic change, move the check to br_process_vlan_info where the
    other checks are done
v3: add change log in the patch, add private (in-kernel only) flags in a
    hole in net_bridge_vlan struct and use that instead of mixing
    user-space flags with private flags

Fixes: 9163a0fc1f0c ("net: bridge: add support for per-port vlan stats")
Reported-by: syzbot+04681da557a0e49a52e5@syzkaller.appspotmail.com
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
---
After the jet lag mostly passed it has all come together in a cleaner
and less future error-prone way. :)

 net/bridge/br_private.h | 7 +++++++
 net/bridge/br_vlan.c    | 3 ++-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/net/bridge/br_private.h b/net/bridge/br_private.h
index 2920e06a5403..04c19a37e500 100644
--- a/net/bridge/br_private.h
+++ b/net/bridge/br_private.h
@@ -102,12 +102,18 @@ struct br_tunnel_info {
 	struct metadata_dst	*tunnel_dst;
 };
 
+/* private vlan flags */
+enum {
+	BR_VLFLAG_PER_PORT_STATS = BIT(0),
+};
+
 /**
  * struct net_bridge_vlan - per-vlan entry
  *
  * @vnode: rhashtable member
  * @vid: VLAN id
  * @flags: bridge vlan flags
+ * @priv_flags: private (in-kernel) bridge vlan flags
  * @stats: per-cpu VLAN statistics
  * @br: if MASTER flag set, this points to a bridge struct
  * @port: if MASTER flag unset, this points to a port struct
@@ -127,6 +133,7 @@ struct net_bridge_vlan {
 	struct rhash_head		tnode;
 	u16				vid;
 	u16				flags;
+	u16				priv_flags;
 	struct br_vlan_stats __percpu	*stats;
 	union {
 		struct net_bridge	*br;
diff --git a/net/bridge/br_vlan.c b/net/bridge/br_vlan.c
index 8c9297a01947..e84be08b8285 100644
--- a/net/bridge/br_vlan.c
+++ b/net/bridge/br_vlan.c
@@ -197,7 +197,7 @@ static void nbp_vlan_rcu_free(struct rcu_head *rcu)
 	v = container_of(rcu, struct net_bridge_vlan, rcu);
 	WARN_ON(br_vlan_is_master(v));
 	/* if we had per-port stats configured then free them here */
-	if (v->brvlan->stats != v->stats)
+	if (v->priv_flags & BR_VLFLAG_PER_PORT_STATS)
 		free_percpu(v->stats);
 	v->stats = NULL;
 	kfree(v);
@@ -264,6 +264,7 @@ static int __vlan_add(struct net_bridge_vlan *v, u16 flags)
 				err = -ENOMEM;
 				goto out_filt;
 			}
+			v->priv_flags |= BR_VLFLAG_PER_PORT_STATS;
 		} else {
 			v->stats = masterv->stats;
 		}
-- 
2.17.2

Re: [PATCH net v3] net: bridge: fix vlan stats use-after-free on destruction

[David Miller]

From: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Date: Fri, 16 Nov 2018 18:50:01 +0200

> Syzbot reported a use-after-free of the global vlan context on port vlan
> destruction. When I added per-port vlan stats I missed the fact that the
> global vlan context can be freed before the per-port vlan rcu callback.
> There're a few different ways to deal with this, I've chosen to add a
> new private flag that is set only when per-port stats are allocated so
> we can directly check it on destruction without dereferencing the global
> context at all. The new field in net_bridge_vlan uses a hole.
> 
> v2: cosmetic change, move the check to br_process_vlan_info where the
>     other checks are done
> v3: add change log in the patch, add private (in-kernel only) flags in a
>     hole in net_bridge_vlan struct and use that instead of mixing
>     user-space flags with private flags
> 
> Fixes: 9163a0fc1f0c ("net: bridge: add support for per-port vlan stats")
> Reported-by: syzbot+04681da557a0e49a52e5@syzkaller.appspotmail.com
> Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>

Applied.

Re: [PATCH net v3] net: bridge: fix vlan stats use-after-free on destruction

[Stephen Hemminger]

On Fri, 16 Nov 2018 18:50:01 +0200
Nikolay Aleksandrov <nikolay@cumulusnetworks.com> wrote:

> Syzbot reported a use-after-free of the global vlan context on port vlan
> destruction. When I added per-port vlan stats I missed the fact that the
> global vlan context can be freed before the per-port vlan rcu callback.
> There're a few different ways to deal with this, I've chosen to add a
> new private flag that is set only when per-port stats are allocated so
> we can directly check it on destruction without dereferencing the global
> context at all. The new field in net_bridge_vlan uses a hole.
> 
> v2: cosmetic change, move the check to br_process_vlan_info where the
>     other checks are done
> v3: add change log in the patch, add private (in-kernel only) flags in a
>     hole in net_bridge_vlan struct and use that instead of mixing
>     user-space flags with private flags
> 
> Fixes: 9163a0fc1f0c ("net: bridge: add support for per-port vlan stats")
> Reported-by: syzbot+04681da557a0e49a52e5@syzkaller.appspotmail.com
> Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>

Why not just use v->stats itself as the flag.
Since free of NULL is a nop it would be cleaner?

Re: [PATCH net v3] net: bridge: fix vlan stats use-after-free on destruction

[Nikolay Aleksandrov]

On 24/04/2020 03:05, Stephen Hemminger wrote:
> On Fri, 16 Nov 2018 18:50:01 +0200
> Nikolay Aleksandrov <nikolay@cumulusnetworks.com> wrote:
> 
>> Syzbot reported a use-after-free of the global vlan context on port vlan
>> destruction. When I added per-port vlan stats I missed the fact that the
>> global vlan context can be freed before the per-port vlan rcu callback.
>> There're a few different ways to deal with this, I've chosen to add a
>> new private flag that is set only when per-port stats are allocated so
>> we can directly check it on destruction without dereferencing the global
>> context at all. The new field in net_bridge_vlan uses a hole.
>>
>> v2: cosmetic change, move the check to br_process_vlan_info where the
>>     other checks are done
>> v3: add change log in the patch, add private (in-kernel only) flags in a
>>     hole in net_bridge_vlan struct and use that instead of mixing
>>     user-space flags with private flags
>>
>> Fixes: 9163a0fc1f0c ("net: bridge: add support for per-port vlan stats")
>> Reported-by: syzbot+04681da557a0e49a52e5@syzkaller.appspotmail.com
>> Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
> 
> Why not just use v->stats itself as the flag.
> Since free of NULL is a nop it would be cleaner?
> 

v->stats is *always* set while the vlan is published/visible, that's a guarantee
I don't want to break because I'll have to add null checks in the fast-path.

By the way this is a thread from 2018. :-)

Cheers,
 Nik

Re: [PATCH net v3] net: bridge: fix vlan stats use-after-free on destruction

[Stephen Hemminger]

On Fri, 16 Nov 2018 18:50:01 +0200
Nikolay Aleksandrov <nikolay@cumulusnetworks.com> wrote:

> +	if (v->priv_flags & BR_VLFLAG_PER_PORT_STATS)
>  		free_percpu(v->stats);

Why not not v->stats == NULL as a flag instead?

Then the fact that free_percpu(NULL) is a Nop would mean less code
in the bridge driver.