[SRU][J][PATCH 1/1] ax25: rcu protect dev->ax25_ptr
Cengiz Can
cengiz.can at canonical.com
Wed Sep 9 11:53:19 UTC 2026
From: Eric Dumazet <edumazet at google.com>
syzbot found a lockdep issue [1].
We should remove ax25 RTNL dependency in ax25_setsockopt()
This should also fix a variety of possible UAF in ax25.
[1]
WARNING: possible circular locking dependency detected
6.13.0-rc3-syzkaller-00762-g9268abe611b0 #0 Not tainted
------------------------------------------------------
syz.5.1818/12806 is trying to acquire lock:
ffffffff8fcb3988 (rtnl_mutex){+.+.}-{4:4}, at: ax25_setsockopt+0xa55/0xe90 net/ax25/af_ax25.c:680
but task is already holding lock:
ffff8880617ac258 (sk_lock-AF_AX25){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1618 [inline]
ffff8880617ac258 (sk_lock-AF_AX25){+.+.}-{0:0}, at: ax25_setsockopt+0x209/0xe90 net/ax25/af_ax25.c:574
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #1 (sk_lock-AF_AX25){+.+.}-{0:0}:
lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5849
lock_sock_nested+0x48/0x100 net/core/sock.c:3642
lock_sock include/net/sock.h:1618 [inline]
ax25_kill_by_device net/ax25/af_ax25.c:101 [inline]
ax25_device_event+0x24d/0x580 net/ax25/af_ax25.c:146
notifier_call_chain+0x1a5/0x3f0 kernel/notifier.c:85
__dev_notify_flags+0x207/0x400
dev_change_flags+0xf0/0x1a0 net/core/dev.c:9026
dev_ifsioc+0x7c8/0xe70 net/core/dev_ioctl.c:563
dev_ioctl+0x719/0x1340 net/core/dev_ioctl.c:820
sock_do_ioctl+0x240/0x460 net/socket.c:1234
sock_ioctl+0x626/0x8e0 net/socket.c:1339
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:906 [inline]
__se_sys_ioctl+0xf5/0x170 fs/ioctl.c:892
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f
-> #0 (rtnl_mutex){+.+.}-{4:4}:
check_prev_add kernel/locking/lockdep.c:3161 [inline]
check_prevs_add kernel/locking/lockdep.c:3280 [inline]
validate_chain+0x18ef/0x5920 kernel/locking/lockdep.c:3904
__lock_acquire+0x1397/0x2100 kernel/locking/lockdep.c:5226
lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5849
__mutex_lock_common kernel/locking/mutex.c:585 [inline]
__mutex_lock+0x1ac/0xee0 kernel/locking/mutex.c:735
ax25_setsockopt+0xa55/0xe90 net/ax25/af_ax25.c:680
do_sock_setsockopt+0x3af/0x720 net/socket.c:2324
__sys_setsockopt net/socket.c:2349 [inline]
__do_sys_setsockopt net/socket.c:2355 [inline]
__se_sys_setsockopt net/socket.c:2352 [inline]
__x64_sys_setsockopt+0x1ee/0x280 net/socket.c:2352
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f
other info that might help us debug this:
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(sk_lock-AF_AX25);
lock(rtnl_mutex);
lock(sk_lock-AF_AX25);
lock(rtnl_mutex);
*** DEADLOCK ***
1 lock held by syz.5.1818/12806:
#0: ffff8880617ac258 (sk_lock-AF_AX25){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1618 [inline]
#0: ffff8880617ac258 (sk_lock-AF_AX25){+.+.}-{0:0}, at: ax25_setsockopt+0x209/0xe90 net/ax25/af_ax25.c:574
stack backtrace:
CPU: 1 UID: 0 PID: 12806 Comm: syz.5.1818 Not tainted 6.13.0-rc3-syzkaller-00762-g9268abe611b0 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
print_circular_bug+0x13a/0x1b0 kernel/locking/lockdep.c:2074
check_noncircular+0x36a/0x4a0 kernel/locking/lockdep.c:2206
check_prev_add kernel/locking/lockdep.c:3161 [inline]
check_prevs_add kernel/locking/lockdep.c:3280 [inline]
validate_chain+0x18ef/0x5920 kernel/locking/lockdep.c:3904
__lock_acquire+0x1397/0x2100 kernel/locking/lockdep.c:5226
lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5849
__mutex_lock_common kernel/locking/mutex.c:585 [inline]
__mutex_lock+0x1ac/0xee0 kernel/locking/mutex.c:735
ax25_setsockopt+0xa55/0xe90 net/ax25/af_ax25.c:680
do_sock_setsockopt+0x3af/0x720 net/socket.c:2324
__sys_setsockopt net/socket.c:2349 [inline]
__do_sys_setsockopt net/socket.c:2355 [inline]
__se_sys_setsockopt net/socket.c:2352 [inline]
__x64_sys_setsockopt+0x1ee/0x280 net/socket.c:2352
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f7b62385d29
Fixes: c433570458e4 ("ax25: fix a use-after-free in ax25_fillin_cb()")
Reported-by: syzbot <syzkaller at googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet at google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu at amazon.com>
Link: https://patch.msgid.link/20250103210514.87290-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba at kernel.org>
(backported from commit 95fc45d1dea8e1253f8ec58abc5befb71553d666)
[bot_kybele: This tree's ax25_dev uses a legacy singly-linked list (->next) not
list_head, has no device_up field, and sets/clears dev->ax25_ptr on multiple
exit paths; adapted rcu_assign_pointer publish inside the lock and converted
all three plain "dev->ax25_ptr = NULL" clears to RCU_INIT_POINTER instead of
upstream's single list_del-based clear.]
CVE-2025-21812
Assisted-by: kybele:claude-opus-4.8
Signed-off-by: Cengiz Can <cengiz.can at canonical.com>
---
include/linux/netdevice.h | 2 +-
include/net/ax25.h | 10 +++++-----
net/ax25/af_ax25.c | 12 ++++++------
net/ax25/ax25_dev.c | 8 ++++----
net/ax25/ax25_ip.c | 3 ++-
net/ax25/ax25_out.c | 22 +++++++++++++++++-----
net/ax25/ax25_route.c | 2 ++
7 files changed, 37 insertions(+), 22 deletions(-)
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index ae79df28174e..58c571e4e559 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -2133,7 +2133,7 @@ struct net_device {
struct in_device __rcu *ip_ptr;
struct inet6_dev __rcu *ip6_ptr;
#if IS_ENABLED(CONFIG_AX25)
- void *ax25_ptr;
+ struct ax25_dev __rcu *ax25_ptr;
#endif
struct wireless_dev *ieee80211_ptr;
struct wpan_dev *ieee802154_ptr;
diff --git a/include/net/ax25.h b/include/net/ax25.h
index aadff553e4b7..8019389878c8 100644
--- a/include/net/ax25.h
+++ b/include/net/ax25.h
@@ -237,6 +237,7 @@ typedef struct ax25_dev {
ax25_dama_info dama;
#endif
refcount_t refcount;
+ struct rcu_head rcu;
} ax25_dev;
typedef struct ax25_cb {
@@ -298,9 +299,8 @@ static inline void ax25_dev_hold(ax25_dev *ax25_dev)
static inline void ax25_dev_put(ax25_dev *ax25_dev)
{
- if (refcount_dec_and_test(&ax25_dev->refcount)) {
- kfree(ax25_dev);
- }
+ if (refcount_dec_and_test(&ax25_dev->refcount))
+ kfree_rcu(ax25_dev, rcu);
}
static inline __be16 ax25_type_trans(struct sk_buff *skb, struct net_device *dev)
{
@@ -344,9 +344,9 @@ extern ax25_dev *ax25_dev_list;
extern spinlock_t ax25_dev_lock;
#if IS_ENABLED(CONFIG_AX25)
-static inline ax25_dev *ax25_dev_ax25dev(struct net_device *dev)
+static inline ax25_dev *ax25_dev_ax25dev(const struct net_device *dev)
{
- return dev->ax25_ptr;
+ return rcu_dereference_rtnl(dev->ax25_ptr);
}
#endif
diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
index b9df05f40513..5f70739a85e2 100644
--- a/net/ax25/af_ax25.c
+++ b/net/ax25/af_ax25.c
@@ -463,7 +463,7 @@ static int ax25_ctl_ioctl(const unsigned int cmd, void __user *arg)
goto out_put;
}
-static void ax25_fillin_cb_from_dev(ax25_cb *ax25, ax25_dev *ax25_dev)
+static void ax25_fillin_cb_from_dev(ax25_cb *ax25, const ax25_dev *ax25_dev)
{
ax25->rtt = msecs_to_jiffies(ax25_dev->values[AX25_VALUES_T1]) / 2;
ax25->t1 = msecs_to_jiffies(ax25_dev->values[AX25_VALUES_T1]);
@@ -673,22 +673,22 @@ static int ax25_setsockopt(struct socket *sock, int level, int optname,
break;
}
- rtnl_lock();
- dev = __dev_get_by_name(&init_net, devname);
+ rcu_read_lock();
+ dev = dev_get_by_name_rcu(&init_net, devname);
if (!dev) {
- rtnl_unlock();
+ rcu_read_unlock();
res = -ENODEV;
break;
}
ax25->ax25_dev = ax25_dev_ax25dev(dev);
if (!ax25->ax25_dev) {
- rtnl_unlock();
+ rcu_read_unlock();
res = -ENODEV;
break;
}
ax25_fillin_cb(ax25, ax25->ax25_dev);
- rtnl_unlock();
+ rcu_read_unlock();
break;
default:
diff --git a/net/ax25/ax25_dev.c b/net/ax25/ax25_dev.c
index fe730fad1c38..65d30da1bd4f 100644
--- a/net/ax25/ax25_dev.c
+++ b/net/ax25/ax25_dev.c
@@ -59,7 +59,6 @@ void ax25_dev_device_up(struct net_device *dev)
}
refcount_set(&ax25_dev->refcount, 1);
- dev->ax25_ptr = ax25_dev;
ax25_dev->dev = dev;
dev_hold(dev);
ax25_dev->forward = NULL;
@@ -86,6 +85,7 @@ void ax25_dev_device_up(struct net_device *dev)
spin_lock_bh(&ax25_dev_lock);
ax25_dev->next = ax25_dev_list;
ax25_dev_list = ax25_dev;
+ rcu_assign_pointer(dev->ax25_ptr, ax25_dev);
spin_unlock_bh(&ax25_dev_lock);
ax25_register_dev_sysctl(ax25_dev);
@@ -116,7 +116,7 @@ void ax25_dev_device_down(struct net_device *dev)
if ((s = ax25_dev_list) == ax25_dev) {
ax25_dev_list = s->next;
spin_unlock_bh(&ax25_dev_lock);
- dev->ax25_ptr = NULL;
+ RCU_INIT_POINTER(dev->ax25_ptr, NULL);
dev_put(dev);
ax25_dev_put(ax25_dev);
return;
@@ -126,7 +126,7 @@ void ax25_dev_device_down(struct net_device *dev)
if (s->next == ax25_dev) {
s->next = ax25_dev->next;
spin_unlock_bh(&ax25_dev_lock);
- dev->ax25_ptr = NULL;
+ RCU_INIT_POINTER(dev->ax25_ptr, NULL);
dev_put(dev);
ax25_dev_put(ax25_dev);
return;
@@ -135,7 +135,7 @@ void ax25_dev_device_down(struct net_device *dev)
s = s->next;
}
spin_unlock_bh(&ax25_dev_lock);
- dev->ax25_ptr = NULL;
+ RCU_INIT_POINTER(dev->ax25_ptr, NULL);
ax25_dev_put(ax25_dev);
}
diff --git a/net/ax25/ax25_ip.c b/net/ax25/ax25_ip.c
index 36249776c021..215d4ccf12b9 100644
--- a/net/ax25/ax25_ip.c
+++ b/net/ax25/ax25_ip.c
@@ -122,6 +122,7 @@ netdev_tx_t ax25_ip_xmit(struct sk_buff *skb)
if (dev == NULL)
dev = skb->dev;
+ rcu_read_lock();
if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL) {
kfree_skb(skb);
goto put;
@@ -202,7 +203,7 @@ netdev_tx_t ax25_ip_xmit(struct sk_buff *skb)
ax25_queue_xmit(skb, dev);
put:
-
+ rcu_read_unlock();
ax25_route_lock_unuse();
return NETDEV_TX_OK;
}
diff --git a/net/ax25/ax25_out.c b/net/ax25/ax25_out.c
index 22f2f66c6e0a..0e8614e0dbb2 100644
--- a/net/ax25/ax25_out.c
+++ b/net/ax25/ax25_out.c
@@ -39,10 +39,14 @@ ax25_cb *ax25_send_frame(struct sk_buff *skb, int paclen, ax25_address *src, ax2
* specified.
*/
if (paclen == 0) {
- if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL)
+ rcu_read_lock();
+ ax25_dev = ax25_dev_ax25dev(dev);
+ if (!ax25_dev) {
+ rcu_read_unlock();
return NULL;
-
+ }
paclen = ax25_dev->values[AX25_VALUES_PACLEN];
+ rcu_read_unlock();
}
/*
@@ -53,13 +57,19 @@ ax25_cb *ax25_send_frame(struct sk_buff *skb, int paclen, ax25_address *src, ax2
return ax25; /* It already existed */
}
- if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL)
+ rcu_read_lock();
+ ax25_dev = ax25_dev_ax25dev(dev);
+ if (!ax25_dev) {
+ rcu_read_unlock();
return NULL;
+ }
- if ((ax25 = ax25_create_cb()) == NULL)
+ if ((ax25 = ax25_create_cb()) == NULL) {
+ rcu_read_unlock();
return NULL;
-
+ }
ax25_fillin_cb(ax25, ax25_dev);
+ rcu_read_unlock();
ax25->source_addr = *src;
ax25->dest_addr = *dest;
@@ -358,7 +368,9 @@ void ax25_queue_xmit(struct sk_buff *skb, struct net_device *dev)
{
unsigned char *ptr;
+ rcu_read_lock();
skb->protocol = ax25_type_trans(skb, ax25_fwd_dev(dev));
+ rcu_read_unlock();
ptr = skb_push(skb, 1);
*ptr = 0x00; /* KISS */
diff --git a/net/ax25/ax25_route.c b/net/ax25/ax25_route.c
index 9751207f7757..5307c907f575 100644
--- a/net/ax25/ax25_route.c
+++ b/net/ax25/ax25_route.c
@@ -407,6 +407,7 @@ int ax25_rt_autobind(ax25_cb *ax25, ax25_address *addr)
ax25_route_lock_unuse();
return -EHOSTUNREACH;
}
+ rcu_read_lock();
if ((ax25->ax25_dev = ax25_dev_ax25dev(ax25_rt->dev)) == NULL) {
err = -EHOSTUNREACH;
goto put;
@@ -443,6 +444,7 @@ int ax25_rt_autobind(ax25_cb *ax25, ax25_address *addr)
}
put:
+ rcu_read_unlock();
ax25_route_lock_unuse();
return err;
}
--
2.53.0
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