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genetlink: hold RCU in genlmsg_mcast()
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While running net selftests with CONFIG_PROVE_RCU_LIST=y I saw
one lockdep splat [1].

genlmsg_mcast() uses for_each_net_rcu(), and must therefore hold RCU.

Instead of letting all callers guard genlmsg_multicast_allns()
with a rcu_read_lock()/rcu_read_unlock() pair, do it in genlmsg_mcast().

This also means the @flags parameter is useless, we need to always use
GFP_ATOMIC.

[1]
[10882.424136] =============================
[10882.424166] WARNING: suspicious RCU usage
[10882.424309] 6.12.0-rc2-virtme #1156 Not tainted
[10882.424400] -----------------------------
[10882.424423] net/netlink/genetlink.c:1940 RCU-list traversed in non-reader section!!
[10882.424469]
other info that might help us debug this:

[10882.424500]
rcu_scheduler_active = 2, debug_locks = 1
[10882.424744] 2 locks held by ip/15677:
[10882.424791] #0: ffffffffb6b491b0 (cb_lock){++++}-{3:3}, at: genl_rcv (net/netlink/genetlink.c:1219)
[10882.426334] #1: ffffffffb6b49248 (genl_mutex){+.+.}-{3:3}, at: genl_rcv_msg (net/netlink/genetlink.c:61 net/netlink/genetlink.c:57 net/netlink/genetlink.c:1209)
[10882.426465]
stack backtrace:
[10882.426805] CPU: 14 UID: 0 PID: 15677 Comm: ip Not tainted 6.12.0-rc2-virtme #1156
[10882.426919] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[10882.427046] Call Trace:
[10882.427131]  <TASK>
[10882.427244] dump_stack_lvl (lib/dump_stack.c:123)
[10882.427335] lockdep_rcu_suspicious (kernel/locking/lockdep.c:6822)
[10882.427387] genlmsg_multicast_allns (net/netlink/genetlink.c:1940 (discriminator 7) net/netlink/genetlink.c:1977 (discriminator 7))
[10882.427436] l2tp_tunnel_notify.constprop.0 (net/l2tp/l2tp_netlink.c:119) l2tp_netlink
[10882.427683] l2tp_nl_cmd_tunnel_create (net/l2tp/l2tp_netlink.c:253) l2tp_netlink
[10882.427748] genl_family_rcv_msg_doit (net/netlink/genetlink.c:1115)
[10882.427834] genl_rcv_msg (net/netlink/genetlink.c:1195 net/netlink/genetlink.c:1210)
[10882.427877] ? __pfx_l2tp_nl_cmd_tunnel_create (net/l2tp/l2tp_netlink.c:186) l2tp_netlink
[10882.427927] ? __pfx_genl_rcv_msg (net/netlink/genetlink.c:1201)
[10882.427959] netlink_rcv_skb (net/netlink/af_netlink.c:2551)
[10882.428069] genl_rcv (net/netlink/genetlink.c:1220)
[10882.428095] netlink_unicast (net/netlink/af_netlink.c:1332 net/netlink/af_netlink.c:1357)
[10882.428140] netlink_sendmsg (net/netlink/af_netlink.c:1901)
[10882.428210] ____sys_sendmsg (net/socket.c:729 (discriminator 1) net/socket.c:744 (discriminator 1) net/socket.c:2607 (discriminator 1))

Fixes: 33f72e6 ("l2tp : multicast notification to the registered listeners")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: James Chapman <jchapman@katalix.com>
Cc: Tom Parkin <tparkin@katalix.com>
Cc: Johannes Berg <johannes.berg@intel.com>
Link: https://patch.msgid.link/20241011171217.3166614-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
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Eric Dumazet authored and kuba-moo committed Oct 16, 2024
1 parent 1833d8a commit 56440d7
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Showing 5 changed files with 20 additions and 25 deletions.
2 changes: 1 addition & 1 deletion drivers/target/target_core_user.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -2130,7 +2130,7 @@ static int tcmu_netlink_event_send(struct tcmu_dev *udev,
}

ret = genlmsg_multicast_allns(&tcmu_genl_family, skb, 0,
TCMU_MCGRP_CONFIG, GFP_KERNEL);
TCMU_MCGRP_CONFIG);

/* Wait during an add as the listener may not be up yet */
if (ret == 0 ||
Expand Down
3 changes: 1 addition & 2 deletions include/net/genetlink.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -531,13 +531,12 @@ static inline int genlmsg_multicast(const struct genl_family *family,
* @skb: netlink message as socket buffer
* @portid: own netlink portid to avoid sending to yourself
* @group: offset of multicast group in groups array
* @flags: allocation flags
*
* This function must hold the RTNL or rcu_read_lock().
*/
int genlmsg_multicast_allns(const struct genl_family *family,
struct sk_buff *skb, u32 portid,
unsigned int group, gfp_t flags);
unsigned int group);

/**
* genlmsg_unicast - unicast a netlink message
Expand Down
4 changes: 2 additions & 2 deletions net/l2tp/l2tp_netlink.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -116,7 +116,7 @@ static int l2tp_tunnel_notify(struct genl_family *family,
NLM_F_ACK, tunnel, cmd);

if (ret >= 0) {
ret = genlmsg_multicast_allns(family, msg, 0, 0, GFP_ATOMIC);
ret = genlmsg_multicast_allns(family, msg, 0, 0);
/* We don't care if no one is listening */
if (ret == -ESRCH)
ret = 0;
Expand Down Expand Up @@ -144,7 +144,7 @@ static int l2tp_session_notify(struct genl_family *family,
NLM_F_ACK, session, cmd);

if (ret >= 0) {
ret = genlmsg_multicast_allns(family, msg, 0, 0, GFP_ATOMIC);
ret = genlmsg_multicast_allns(family, msg, 0, 0);
/* We don't care if no one is listening */
if (ret == -ESRCH)
ret = 0;
Expand Down
28 changes: 14 additions & 14 deletions net/netlink/genetlink.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1501,15 +1501,11 @@ static int genl_ctrl_event(int event, const struct genl_family *family,
if (IS_ERR(msg))
return PTR_ERR(msg);

if (!family->netnsok) {
if (!family->netnsok)
genlmsg_multicast_netns(&genl_ctrl, &init_net, msg, 0,
0, GFP_KERNEL);
} else {
rcu_read_lock();
genlmsg_multicast_allns(&genl_ctrl, msg, 0,
0, GFP_ATOMIC);
rcu_read_unlock();
}
else
genlmsg_multicast_allns(&genl_ctrl, msg, 0, 0);

return 0;
}
Expand Down Expand Up @@ -1929,23 +1925,23 @@ static int __init genl_init(void)

core_initcall(genl_init);

static int genlmsg_mcast(struct sk_buff *skb, u32 portid, unsigned long group,
gfp_t flags)
static int genlmsg_mcast(struct sk_buff *skb, u32 portid, unsigned long group)
{
struct sk_buff *tmp;
struct net *net, *prev = NULL;
bool delivered = false;
int err;

rcu_read_lock();
for_each_net_rcu(net) {
if (prev) {
tmp = skb_clone(skb, flags);
tmp = skb_clone(skb, GFP_ATOMIC);
if (!tmp) {
err = -ENOMEM;
goto error;
}
err = nlmsg_multicast(prev->genl_sock, tmp,
portid, group, flags);
portid, group, GFP_ATOMIC);
if (!err)
delivered = true;
else if (err != -ESRCH)
Expand All @@ -1954,27 +1950,31 @@ static int genlmsg_mcast(struct sk_buff *skb, u32 portid, unsigned long group,

prev = net;
}
err = nlmsg_multicast(prev->genl_sock, skb, portid, group, GFP_ATOMIC);

rcu_read_unlock();

err = nlmsg_multicast(prev->genl_sock, skb, portid, group, flags);
if (!err)
delivered = true;
else if (err != -ESRCH)
return err;
return delivered ? 0 : -ESRCH;
error:
rcu_read_unlock();

kfree_skb(skb);
return err;
}

int genlmsg_multicast_allns(const struct genl_family *family,
struct sk_buff *skb, u32 portid,
unsigned int group, gfp_t flags)
unsigned int group)
{
if (WARN_ON_ONCE(group >= family->n_mcgrps))
return -EINVAL;

group = family->mcgrp_offset + group;
return genlmsg_mcast(skb, portid, group, flags);
return genlmsg_mcast(skb, portid, group);
}
EXPORT_SYMBOL(genlmsg_multicast_allns);

Expand Down
8 changes: 2 additions & 6 deletions net/wireless/nl80211.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -17986,10 +17986,8 @@ void nl80211_common_reg_change_event(enum nl80211_commands cmd_id,

genlmsg_end(msg, hdr);

rcu_read_lock();
genlmsg_multicast_allns(&nl80211_fam, msg, 0,
NL80211_MCGRP_REGULATORY, GFP_ATOMIC);
rcu_read_unlock();
NL80211_MCGRP_REGULATORY);

return;

Expand Down Expand Up @@ -18722,10 +18720,8 @@ void nl80211_send_beacon_hint_event(struct wiphy *wiphy,

genlmsg_end(msg, hdr);

rcu_read_lock();
genlmsg_multicast_allns(&nl80211_fam, msg, 0,
NL80211_MCGRP_REGULATORY, GFP_ATOMIC);
rcu_read_unlock();
NL80211_MCGRP_REGULATORY);

return;

Expand Down

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