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inet: fix lwtunnel_valid_encap_type() lock imbalance
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After blamed commit rtm_to_fib_config() now calls
lwtunnel_valid_encap_type{_attr}() without RTNL held,
triggering an unlock balance in __rtnl_unlock,
as reported by syzbot [1]

IPv6 and rtm_to_nh_config() are not yet converted.

Add a temporary @rtnl_is_held parameter to lwtunnel_valid_encap_type()
and lwtunnel_valid_encap_type_attr().

While we are at it replace the two rcu_dereference()
in lwtunnel_valid_encap_type() with more appropriate
rcu_access_pointer().

[1]
syz-executor245/5836 is trying to release lock (rtnl_mutex) at:
 [<ffffffff89d0e38c>] __rtnl_unlock+0x6c/0xf0 net/core/rtnetlink.c:142
but there are no more locks to release!

other info that might help us debug this:
no locks held by syz-executor245/5836.

stack backtrace:
CPU: 0 UID: 0 PID: 5836 Comm: syz-executor245 Not tainted 6.14.0-rc4-syzkaller-00873-g3424291dd242 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025
Call Trace:
 <TASK>
  __dump_stack lib/dump_stack.c:94 [inline]
  dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
  print_unlock_imbalance_bug+0x25b/0x2d0 kernel/locking/lockdep.c:5289
  __lock_release kernel/locking/lockdep.c:5518 [inline]
  lock_release+0x47e/0xa30 kernel/locking/lockdep.c:5872
  __mutex_unlock_slowpath+0xec/0x800 kernel/locking/mutex.c:891
  __rtnl_unlock+0x6c/0xf0 net/core/rtnetlink.c:142
  lwtunnel_valid_encap_type+0x38a/0x5f0 net/core/lwtunnel.c:169
  lwtunnel_valid_encap_type_attr+0x113/0x270 net/core/lwtunnel.c:209
  rtm_to_fib_config+0x949/0x14e0 net/ipv4/fib_frontend.c:808
  inet_rtm_newroute+0xf6/0x2a0 net/ipv4/fib_frontend.c:917
  rtnetlink_rcv_msg+0x791/0xcf0 net/core/rtnetlink.c:6919
  netlink_rcv_skb+0x206/0x480 net/netlink/af_netlink.c:2534
  netlink_unicast_kernel net/netlink/af_netlink.c:1313 [inline]
  netlink_unicast+0x7f6/0x990 net/netlink/af_netlink.c:1339
  netlink_sendmsg+0x8de/0xcb0 net/netlink/af_netlink.c:1883
  sock_sendmsg_nosec net/socket.c:709 [inline]

Fixes: 1dd2af7 ("ipv4: fib: Convert RTM_NEWROUTE and RTM_DELROUTE to per-netns RTNL.")
Reported-by: syzbot+3f18ef0f7df107a3f6a0@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/67c6f87a.050a0220.38b91b.0147.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250304125918.2763514-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
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Eric Dumazet authored and Jakub Kicinski committed Mar 6, 2025
1 parent e0c8368 commit f130a0c
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Showing 5 changed files with 28 additions and 20 deletions.
12 changes: 8 additions & 4 deletions include/net/lwtunnel.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -116,9 +116,11 @@ int lwtunnel_encap_add_ops(const struct lwtunnel_encap_ops *op,
int lwtunnel_encap_del_ops(const struct lwtunnel_encap_ops *op,
unsigned int num);
int lwtunnel_valid_encap_type(u16 encap_type,
struct netlink_ext_ack *extack);
struct netlink_ext_ack *extack,
bool rtnl_is_held);
int lwtunnel_valid_encap_type_attr(struct nlattr *attr, int len,
struct netlink_ext_ack *extack);
struct netlink_ext_ack *extack,
bool rtnl_is_held);
int lwtunnel_build_state(struct net *net, u16 encap_type,
struct nlattr *encap,
unsigned int family, const void *cfg,
Expand Down Expand Up @@ -201,13 +203,15 @@ static inline int lwtunnel_encap_del_ops(const struct lwtunnel_encap_ops *op,
}

static inline int lwtunnel_valid_encap_type(u16 encap_type,
struct netlink_ext_ack *extack)
struct netlink_ext_ack *extack,
bool rtnl_is_held)
{
NL_SET_ERR_MSG(extack, "CONFIG_LWTUNNEL is not enabled in this kernel");
return -EOPNOTSUPP;
}
static inline int lwtunnel_valid_encap_type_attr(struct nlattr *attr, int len,
struct netlink_ext_ack *extack)
struct netlink_ext_ack *extack,
bool rtnl_is_held)
{
/* return 0 since we are not walking attr looking for
* RTA_ENCAP_TYPE attribute on nexthops.
Expand Down
23 changes: 12 additions & 11 deletions net/core/lwtunnel.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -147,7 +147,8 @@ int lwtunnel_build_state(struct net *net, u16 encap_type,
}
EXPORT_SYMBOL_GPL(lwtunnel_build_state);

int lwtunnel_valid_encap_type(u16 encap_type, struct netlink_ext_ack *extack)
int lwtunnel_valid_encap_type(u16 encap_type, struct netlink_ext_ack *extack,
bool rtnl_is_held)
{
const struct lwtunnel_encap_ops *ops;
int ret = -EINVAL;
Expand All @@ -158,21 +159,19 @@ int lwtunnel_valid_encap_type(u16 encap_type, struct netlink_ext_ack *extack)
return ret;
}

rcu_read_lock();
ops = rcu_dereference(lwtun_encaps[encap_type]);
rcu_read_unlock();
ops = rcu_access_pointer(lwtun_encaps[encap_type]);
#ifdef CONFIG_MODULES
if (!ops) {
const char *encap_type_str = lwtunnel_encap_str(encap_type);

if (encap_type_str) {
__rtnl_unlock();
if (rtnl_is_held)
__rtnl_unlock();
request_module("rtnl-lwt-%s", encap_type_str);
rtnl_lock();
if (rtnl_is_held)
rtnl_lock();

rcu_read_lock();
ops = rcu_dereference(lwtun_encaps[encap_type]);
rcu_read_unlock();
ops = rcu_access_pointer(lwtun_encaps[encap_type]);
}
}
#endif
Expand All @@ -185,7 +184,8 @@ int lwtunnel_valid_encap_type(u16 encap_type, struct netlink_ext_ack *extack)
EXPORT_SYMBOL_GPL(lwtunnel_valid_encap_type);

int lwtunnel_valid_encap_type_attr(struct nlattr *attr, int remaining,
struct netlink_ext_ack *extack)
struct netlink_ext_ack *extack,
bool rtnl_is_held)
{
struct rtnexthop *rtnh = (struct rtnexthop *)attr;
struct nlattr *nla_entype;
Expand All @@ -207,7 +207,8 @@ int lwtunnel_valid_encap_type_attr(struct nlattr *attr, int remaining,
encap_type = nla_get_u16(nla_entype);

if (lwtunnel_valid_encap_type(encap_type,
extack) != 0)
extack,
rtnl_is_held) != 0)
return -EOPNOTSUPP;
}
}
Expand Down
4 changes: 2 additions & 2 deletions net/ipv4/fib_frontend.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -807,7 +807,7 @@ static int rtm_to_fib_config(struct net *net, struct sk_buff *skb,
case RTA_MULTIPATH:
err = lwtunnel_valid_encap_type_attr(nla_data(attr),
nla_len(attr),
extack);
extack, false);
if (err < 0)
goto errout;
cfg->fc_mp = nla_data(attr);
Expand All @@ -825,7 +825,7 @@ static int rtm_to_fib_config(struct net *net, struct sk_buff *skb,
case RTA_ENCAP_TYPE:
cfg->fc_encap_type = nla_get_u16(attr);
err = lwtunnel_valid_encap_type(cfg->fc_encap_type,
extack);
extack, false);
if (err < 0)
goto errout;
break;
Expand Down
3 changes: 2 additions & 1 deletion net/ipv4/nexthop.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -3187,7 +3187,8 @@ static int rtm_to_nh_config(struct net *net, struct sk_buff *skb,
}

cfg->nh_encap_type = nla_get_u16(tb[NHA_ENCAP_TYPE]);
err = lwtunnel_valid_encap_type(cfg->nh_encap_type, extack);
err = lwtunnel_valid_encap_type(cfg->nh_encap_type,
extack, true);
if (err < 0)
goto out;

Expand Down
6 changes: 4 additions & 2 deletions net/ipv6/route.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -5128,7 +5128,8 @@ static int rtm_to_fib6_config(struct sk_buff *skb, struct nlmsghdr *nlh,
cfg->fc_mp_len = nla_len(tb[RTA_MULTIPATH]);

err = lwtunnel_valid_encap_type_attr(cfg->fc_mp,
cfg->fc_mp_len, extack);
cfg->fc_mp_len,
extack, true);
if (err < 0)
goto errout;
}
Expand All @@ -5147,7 +5148,8 @@ static int rtm_to_fib6_config(struct sk_buff *skb, struct nlmsghdr *nlh,
if (tb[RTA_ENCAP_TYPE]) {
cfg->fc_encap_type = nla_get_u16(tb[RTA_ENCAP_TYPE]);

err = lwtunnel_valid_encap_type(cfg->fc_encap_type, extack);
err = lwtunnel_valid_encap_type(cfg->fc_encap_type,
extack, true);
if (err < 0)
goto errout;
}
Expand Down

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