[PATCH 5.6 07/38] net_sched: add a temporary refcnt for struct tcindex_data

Greg Kroah-Hartman
2020-04-11T12:21:33Z

From: Cong Wang <xiyou.wangcong@gmail.com>

[ Upstream commit 304e024216a802a7dc8ba75d36de82fa136bbf3e ]

Although we intentionally use an ordered workqueue for all tc
filter works, the ordering is not guaranteed by RCU work,
given that tcf_queue_work() is esstenially a call_rcu().

This problem is demostrated by Thomas:

CPU 0:
tcf_queue_work()
tcf_queue_work(&r->rwork, tcindex_destroy_rexts_work);

-> Migration to CPU 1

CPU 1:
tcf_queue_work(&p->rwork, tcindex_destroy_work);

so the 2nd work could be queued before the 1st one, which leads
to a free-after-free.

Enforcing this order in RCU work is hard as it requires to change
RCU code too. Fortunately we can workaround this problem in tcindex
filter by taking a temporary refcnt, we only refcnt it right before
we begin to destroy it. This simplifies the code a lot as a full
refcnt requires much more changes in tcindex_set_parms().

Reported-by: syzbot+46f513c3033d592409d2@syzkaller.appspotmail.com
Fixes: 3d210534cc93 ("net_sched: fix a race condition in tcindex_destroy()")
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/sched/cls_tcindex.c | 44 ++++++++++++++++++++++++++++++++++++++------
1 file changed, 38 insertions(+), 6 deletions(-)

--- a/net/sched/cls_tcindex.c
+++ b/net/sched/cls_tcindex.c
@@ -11,6 +11,7 @@
#include <linux/skbuff.h>
#include <linux/errno.h>
#include <linux/slab.h>
+#include <linux/refcount.h>
#include <net/act_api.h>
#include <net/netlink.h>
#include <net/pkt_cls.h>
@@ -26,9 +27,12 @@
#define DEFAULT_HASH_SIZE 64 /* optimized for diffserv */

+struct tcindex_data;
+
struct tcindex_filter_result {
struct tcf_exts exts;
struct tcf_result res;
+ struct tcindex_data *p;
struct rcu_work rwork;
};

@@ -49,6 +53,7 @@ struct tcindex_data {
u32 hash; /* hash table size; 0 if undefined */
u32 alloc_hash; /* allocated size */
u32 fall_through; /* 0: only classify if explicit match */
+ refcount_t refcnt; /* a temporary refcnt for perfect hash */
struct rcu_work rwork;
};

@@ -57,6 +62,20 @@ static inline int tcindex_filter_is_set(
return tcf_exts_has_actions(&r->exts) || r->res.classid;
}

+static void tcindex_data_get(struct tcindex_data *p)
+{
+ refcount_inc(&p->refcnt);
+}
+
+static void tcindex_data_put(struct tcindex_data *p)
+{
+ if (refcount_dec_and_test(&p->refcnt)) {
+ kfree(p->perfect);
+ kfree(p->h);
+ kfree(p);
+ }
+}
+
static struct tcindex_filter_result *tcindex_lookup(struct tcindex_data *p,
u16 key)
{
@@ -141,6 +160,7 @@ static void __tcindex_destroy_rexts(stru
{
tcf_exts_destroy(&r->exts);
tcf_exts_put_net(&r->exts);
+ tcindex_data_put(r->p);
}

static void tcindex_destroy_rexts_work(struct work_struct *work)
@@ -212,6 +232,8 @@ found:
else
__tcindex_destroy_fexts(f);
} else {
+ tcindex_data_get(p);
+
if (tcf_exts_get_net(&r->exts))
tcf_queue_work(&r->rwork, tcindex_destroy_rexts_work);
else
@@ -228,9 +250,7 @@ static void tcindex_destroy_work(struct
struct tcindex_data,
rwork);

- kfree(p->perfect);
- kfree(p->h);
- kfree(p);
+ tcindex_data_put(p);
}

static inline int
@@ -248,9 +268,11 @@ static const struct nla_policy tcindex_p
};

static int tcindex_filter_result_init(struct tcindex_filter_result *r,
+ struct tcindex_data *p,
struct net *net)
{
memset(r, 0, sizeof(*r));
+ r->p = p;
return tcf_exts_init(&r->exts, net, TCA_TCINDEX_ACT,
TCA_TCINDEX_POLICE);
}
@@ -290,6 +312,7 @@ static int tcindex_alloc_perfect_hash(st
TCA_TCINDEX_ACT, TCA_TCINDEX_POLICE);
if (err < 0)
goto errout;
+ cp->perfect[i].p = cp;
}

return 0;
@@ -334,6 +357,7 @@ tcindex_set_parms(struct net *net, struc
cp->alloc_hash = p->alloc_hash;
cp->fall_through = p->fall_through;
cp->tp = tp;
+ refcount_set(&cp->refcnt, 1); /* Paired with tcindex_destroy_work() */

if (tb[TCA_TCINDEX_HASH])
cp->hash = nla_get_u32(tb[TCA_TCINDEX_HASH]);
@@ -366,7 +390,7 @@ tcindex_set_parms(struct net *net, struc
}
cp->h = p->h;

- err = tcindex_filter_result_init(&new_filter_result, net);
+ err = tcindex_filter_result_init(&new_filter_result, cp, net);
if (err < 0)
goto errout_alloc;
if (old_r)
@@ -434,7 +458,7 @@ tcindex_set_parms(struct net *net, struc
goto errout_alloc;
f->key = handle;
f->next = NULL;
- err = tcindex_filter_result_init(&f->result, net);
+ err = tcindex_filter_result_init(&f->result, cp, net);
if (err < 0) {
kfree(f);
goto errout_alloc;
@@ -447,7 +471,7 @@ tcindex_set_parms(struct net *net, struc
}

if (old_r && old_r != r) {
- err = tcindex_filter_result_init(old_r, net);
+ err = tcindex_filter_result_init(old_r, cp, net);
if (err < 0) {
kfree(f);
goto errout_alloc;
@@ -571,6 +595,14 @@ static void tcindex_destroy(struct tcf_p
for (i = 0; i < p->hash; i++) {
struct tcindex_filter_result *r = p->perfect + i;

+ /* tcf_queue_work() does not guarantee the ordering we
+ * want, so we have to take this refcnt temporarily to
+ * ensure 'p' is freed after all tcindex_filter_result
+ * here. Imperfect hash does not need this, because it
+ * uses linked lists rather than an array.
+ */
+ tcindex_data_get(p);
+
tcf_unbind_filter(tp, &r->res);
if (tcf_exts_get_net(&r->exts))
tcf_queue_work(&r->rwork,