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Message-ID: <20230213081701.GA10665@breakpoint.cc>
Date: Mon, 13 Feb 2023 09:17:01 +0100
From: Florian Westphal <fw@...len.de>
To: Hangyu Hua <hbh25y@...il.com>
Cc: Florian Westphal <fw@...len.de>,
Pablo Neira Ayuso <pablo@...filter.org>,
kadlec@...filter.org, davem@...emloft.net, edumazet@...gle.com,
kuba@...nel.org, pabeni@...hat.com,
netfilter-devel@...r.kernel.org, coreteam@...filter.org,
netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: netfilter: fix possible refcount leak in
ctnetlink_create_conntrack()
Hangyu Hua <hbh25y@...il.com> wrote:
> On 12/2/2023 20:53, Florian Westphal wrote:
> > Pablo Neira Ayuso <pablo@...filter.org> wrote:
> > > > One way would be to return 0 in that case (in
> > > > nf_conntrack_hash_check_insert()). What do you think?
> > >
> > > This is misleading to the user that adds an entry via ctnetlink?
> > >
> > > ETIMEDOUT also looks a bit confusing to report to userspace.
> > > Rewinding: if the intention is to deal with stale conntrack extension,
> > > for example, helper module has been removed while this entry was
> > > added. Then, probably call EAGAIN so nfnetlink has a chance to retry
> > > transparently?
> >
> > Seems we first need to add a "bool *inserted" so we know when the ct
> > entry went public.
> >
> I don't think so.
>
> nf_conntrack_hash_check_insert(struct nf_conn *ct)
> {
> ...
> /* The caller holds a reference to this object */
> refcount_set(&ct->ct_general.use, 2); // [1]
> __nf_conntrack_hash_insert(ct, hash, reply_hash);
> nf_conntrack_double_unlock(hash, reply_hash);
> NF_CT_STAT_INC(net, insert);
> local_bh_enable();
>
> if (!nf_ct_ext_valid_post(ct->ext)) {
> nf_ct_kill(ct); // [2]
> NF_CT_STAT_INC_ATOMIC(net, drop);
> return -ETIMEDOUT;
> }
> ...
> }
>
> We set ct->ct_general.use to 2 in nf_conntrack_hash_check_insert()([1]).
> nf_ct_kill willn't put the last refcount. So ct->master will not be freed in
> this way. But this means the situation not only causes ct->master's refcount
> leak but also releases ct whose refcount is still 1 in nf_conntrack_free()
> (in ctnetlink_create_conntrack() err1).
at [2] The refcount could be > 1, as entry became public. Other CPU
might have obtained a reference.
> I think it may be a good idea to set ct->ct_general.use to 0 after
> nf_ct_kill() ([2]) to put the caller's reference. What do you think?
We can't, see above. We need something similar to this (not even compile
tested):
diff --git a/net/netfilter/nf_conntrack_bpf.c b/net/netfilter/nf_conntrack_bpf.c
index 24002bc61e07..b9e0e01dae43 100644
--- a/net/netfilter/nf_conntrack_bpf.c
+++ b/net/netfilter/nf_conntrack_bpf.c
@@ -379,12 +379,16 @@ bpf_skb_ct_lookup(struct __sk_buff *skb_ctx, struct bpf_sock_tuple *bpf_tuple,
struct nf_conn *bpf_ct_insert_entry(struct nf_conn___init *nfct_i)
{
struct nf_conn *nfct = (struct nf_conn *)nfct_i;
+ bool inserted;
int err;
nfct->status |= IPS_CONFIRMED;
- err = nf_conntrack_hash_check_insert(nfct);
+ err = nf_conntrack_hash_check_insert(nfctm, &inserted);
if (err < 0) {
- nf_conntrack_free(nfct);
+ if (inserted)
+ nf_ct_put(nfct);
+ else
+ nf_conntrack_free(nfct);
return NULL;
}
return nfct;
diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index 496c4920505b..5f7b1fd744ef 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -872,7 +872,7 @@ static bool nf_ct_ext_valid_post(struct nf_ct_ext *ext)
}
int
-nf_conntrack_hash_check_insert(struct nf_conn *ct)
+nf_conntrack_hash_check_insert(struct nf_conn *ct, bool *inserted)
{
const struct nf_conntrack_zone *zone;
struct net *net = nf_ct_net(ct);
@@ -884,12 +884,11 @@ nf_conntrack_hash_check_insert(struct nf_conn *ct)
unsigned int sequence;
int err = -EEXIST;
+ *inserted = false;
zone = nf_ct_zone(ct);
- if (!nf_ct_ext_valid_pre(ct->ext)) {
- NF_CT_STAT_INC_ATOMIC(net, insert_failed);
- return -ETIMEDOUT;
- }
+ if (!nf_ct_ext_valid_pre(ct->ext))
+ return -EAGAIN;
local_bh_disable();
do {
@@ -924,6 +923,7 @@ nf_conntrack_hash_check_insert(struct nf_conn *ct)
goto chaintoolong;
}
+ *inserted = true;
smp_wmb();
/* The caller holds a reference to this object */
refcount_set(&ct->ct_general.use, 2);
@@ -934,8 +934,7 @@ nf_conntrack_hash_check_insert(struct nf_conn *ct)
if (!nf_ct_ext_valid_post(ct->ext)) {
nf_ct_kill(ct);
- NF_CT_STAT_INC_ATOMIC(net, drop);
- return -ETIMEDOUT;
+ return -EAGAIN;
}
return 0;
diff --git a/net/netfilter/nf_conntrack_netlink.c b/net/netfilter/nf_conntrack_netlink.c
index 1286ae7d4609..7ada6350c34d 100644
--- a/net/netfilter/nf_conntrack_netlink.c
+++ b/net/netfilter/nf_conntrack_netlink.c
@@ -2244,8 +2244,10 @@ ctnetlink_create_conntrack(struct net *net,
int err = -EINVAL;
struct nf_conntrack_helper *helper;
struct nf_conn_tstamp *tstamp;
+ bool inserted;
u64 timeout;
+restart:
ct = nf_conntrack_alloc(net, zone, otuple, rtuple, GFP_ATOMIC);
if (IS_ERR(ct))
return ERR_PTR(-ENOMEM);
@@ -2373,10 +2375,26 @@ ctnetlink_create_conntrack(struct net *net,
if (tstamp)
tstamp->start = ktime_get_real_ns();
- err = nf_conntrack_hash_check_insert(ct);
- if (err < 0)
- goto err2;
+ err = nf_conntrack_hash_check_insert(ct, &inserted);
+ if (err < 0) {
+ if (inserted) {
+ nf_ct_put(ct);
+ rcu_read_unlock();
+ if (err == -EAGAIN)
+ goto restart;
+ return err;
+ }
+ if (ct->master)
+ nf_ct_put(ct->master);
+
+ if (err == -EAGAIN) {
+ rcu_read_unlock();
+ nf_conntrack_free(ct);
+ goto restart;
+ }
+ goto err2;
+ }
rcu_read_unlock();
return ct;
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