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ALSA: seq: Fix lockdep warnings due to double mutex locks
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The port subscription code uses double mutex locks for source and
destination ports, and this may become racy once when wrongly set up.
It leads to lockdep warning splat, typically triggered by fuzzer like
syzkaller, although the actual deadlock hasn't been seen, so far.

This patch simplifies the handling by reducing to two single locks, so
that no lockdep warning will be trigger any longer.

By splitting to two actions, a still-in-progress element shall be
added in one list while handling another.  For ignoring this element,
a new check is added in deliver_to_subscribers().

Along with it, the code to add/remove the subscribers list element was
cleaned up and refactored.

BugLink: http://lkml.kernel.org/r/CACT4Y+aKQXV7xkBW9hpQbzaDO7LrUvohxWh-UwMxXjDy-yBD=A@mail.gmail.com
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
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Takashi Iwai committed Feb 3, 2016
1 parent 81f5775 commit 7f0973e
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Showing 2 changed files with 133 additions and 103 deletions.
3 changes: 3 additions & 0 deletions sound/core/seq/seq_clientmgr.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -678,6 +678,9 @@ static int deliver_to_subscribers(struct snd_seq_client *client,
else
down_read(&grp->list_mutex);
list_for_each_entry(subs, &grp->list_head, src_list) {
/* both ports ready? */
if (atomic_read(&subs->ref_count) != 2)
continue;
event->dest = subs->info.dest;
if (subs->info.flags & SNDRV_SEQ_PORT_SUBS_TIMESTAMP)
/* convert time according to flag with subscription */
Expand Down
233 changes: 130 additions & 103 deletions sound/core/seq/seq_ports.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -173,10 +173,6 @@ struct snd_seq_client_port *snd_seq_create_port(struct snd_seq_client *client,
}

/* */
enum group_type {
SRC_LIST, DEST_LIST
};

static int subscribe_port(struct snd_seq_client *client,
struct snd_seq_client_port *port,
struct snd_seq_port_subs_info *grp,
Expand All @@ -203,14 +199,28 @@ static struct snd_seq_client_port *get_client_port(struct snd_seq_addr *addr,
return NULL;
}

static void delete_and_unsubscribe_port(struct snd_seq_client *client,
struct snd_seq_client_port *port,
struct snd_seq_subscribers *subs,
bool is_src, bool ack);

static inline struct snd_seq_subscribers *
get_subscriber(struct list_head *p, bool is_src)
{
if (is_src)
return list_entry(p, struct snd_seq_subscribers, src_list);
else
return list_entry(p, struct snd_seq_subscribers, dest_list);
}

/*
* remove all subscribers on the list
* this is called from port_delete, for each src and dest list.
*/
static void clear_subscriber_list(struct snd_seq_client *client,
struct snd_seq_client_port *port,
struct snd_seq_port_subs_info *grp,
int grptype)
int is_src)
{
struct list_head *p, *n;

Expand All @@ -219,37 +229,28 @@ static void clear_subscriber_list(struct snd_seq_client *client,
struct snd_seq_client *c;
struct snd_seq_client_port *aport;

if (grptype == SRC_LIST) {
subs = list_entry(p, struct snd_seq_subscribers, src_list);
subs = get_subscriber(p, is_src);
if (is_src)
aport = get_client_port(&subs->info.dest, &c);
} else {
subs = list_entry(p, struct snd_seq_subscribers, dest_list);
else
aport = get_client_port(&subs->info.sender, &c);
}
list_del(p);
unsubscribe_port(client, port, grp, &subs->info, 0);
delete_and_unsubscribe_port(client, port, subs, is_src, false);

if (!aport) {
/* looks like the connected port is being deleted.
* we decrease the counter, and when both ports are deleted
* remove the subscriber info
*/
if (atomic_dec_and_test(&subs->ref_count))
kfree(subs);
} else {
/* ok we got the connected port */
struct snd_seq_port_subs_info *agrp;
agrp = (grptype == SRC_LIST) ? &aport->c_dest : &aport->c_src;
down_write(&agrp->list_mutex);
if (grptype == SRC_LIST)
list_del(&subs->dest_list);
else
list_del(&subs->src_list);
up_write(&agrp->list_mutex);
unsubscribe_port(c, aport, agrp, &subs->info, 1);
kfree(subs);
snd_seq_port_unlock(aport);
snd_seq_client_unlock(c);
continue;
}

/* ok we got the connected port */
delete_and_unsubscribe_port(c, aport, subs, !is_src, true);
kfree(subs);
snd_seq_port_unlock(aport);
snd_seq_client_unlock(c);
}
}

Expand All @@ -262,8 +263,8 @@ static int port_delete(struct snd_seq_client *client,
snd_use_lock_sync(&port->use_lock);

/* clear subscribers info */
clear_subscriber_list(client, port, &port->c_src, SRC_LIST);
clear_subscriber_list(client, port, &port->c_dest, DEST_LIST);
clear_subscriber_list(client, port, &port->c_src, true);
clear_subscriber_list(client, port, &port->c_dest, false);

if (port->private_free)
port->private_free(port->private_data);
Expand Down Expand Up @@ -479,85 +480,120 @@ static int match_subs_info(struct snd_seq_port_subscribe *r,
return 0;
}


/* connect two ports */
int snd_seq_port_connect(struct snd_seq_client *connector,
struct snd_seq_client *src_client,
struct snd_seq_client_port *src_port,
struct snd_seq_client *dest_client,
struct snd_seq_client_port *dest_port,
struct snd_seq_port_subscribe *info)
static int check_and_subscribe_port(struct snd_seq_client *client,
struct snd_seq_client_port *port,
struct snd_seq_subscribers *subs,
bool is_src, bool exclusive, bool ack)
{
struct snd_seq_port_subs_info *src = &src_port->c_src;
struct snd_seq_port_subs_info *dest = &dest_port->c_dest;
struct snd_seq_subscribers *subs, *s;
int err, src_called = 0;
unsigned long flags;
int exclusive;
struct snd_seq_port_subs_info *grp;
struct list_head *p;
struct snd_seq_subscribers *s;
int err;

subs = kzalloc(sizeof(*subs), GFP_KERNEL);
if (! subs)
return -ENOMEM;

subs->info = *info;
atomic_set(&subs->ref_count, 2);

down_write(&src->list_mutex);
down_write_nested(&dest->list_mutex, SINGLE_DEPTH_NESTING);

exclusive = info->flags & SNDRV_SEQ_PORT_SUBS_EXCLUSIVE ? 1 : 0;
grp = is_src ? &port->c_src : &port->c_dest;
err = -EBUSY;
down_write(&grp->list_mutex);
if (exclusive) {
if (! list_empty(&src->list_head) || ! list_empty(&dest->list_head))
if (!list_empty(&grp->list_head))
goto __error;
} else {
if (src->exclusive || dest->exclusive)
if (grp->exclusive)
goto __error;
/* check whether already exists */
list_for_each_entry(s, &src->list_head, src_list) {
if (match_subs_info(info, &s->info))
goto __error;
}
list_for_each_entry(s, &dest->list_head, dest_list) {
if (match_subs_info(info, &s->info))
list_for_each(p, &grp->list_head) {
s = get_subscriber(p, is_src);
if (match_subs_info(&subs->info, &s->info))
goto __error;
}
}

if ((err = subscribe_port(src_client, src_port, src, info,
connector->number != src_client->number)) < 0)
goto __error;
src_called = 1;

if ((err = subscribe_port(dest_client, dest_port, dest, info,
connector->number != dest_client->number)) < 0)
err = subscribe_port(client, port, grp, &subs->info, ack);
if (err < 0) {
grp->exclusive = 0;
goto __error;
}

/* add to list */
write_lock_irqsave(&src->list_lock, flags);
// write_lock(&dest->list_lock); // no other lock yet
list_add_tail(&subs->src_list, &src->list_head);
list_add_tail(&subs->dest_list, &dest->list_head);
// write_unlock(&dest->list_lock); // no other lock yet
write_unlock_irqrestore(&src->list_lock, flags);
write_lock_irq(&grp->list_lock);
if (is_src)
list_add_tail(&subs->src_list, &grp->list_head);
else
list_add_tail(&subs->dest_list, &grp->list_head);
grp->exclusive = exclusive;
atomic_inc(&subs->ref_count);
write_unlock_irq(&grp->list_lock);
err = 0;

__error:
up_write(&grp->list_mutex);
return err;
}

src->exclusive = dest->exclusive = exclusive;
static void delete_and_unsubscribe_port(struct snd_seq_client *client,
struct snd_seq_client_port *port,
struct snd_seq_subscribers *subs,
bool is_src, bool ack)
{
struct snd_seq_port_subs_info *grp;

grp = is_src ? &port->c_src : &port->c_dest;
down_write(&grp->list_mutex);
write_lock_irq(&grp->list_lock);
if (is_src)
list_del(&subs->src_list);
else
list_del(&subs->dest_list);
grp->exclusive = 0;
write_unlock_irq(&grp->list_lock);
up_write(&grp->list_mutex);

unsubscribe_port(client, port, grp, &subs->info, ack);
}

/* connect two ports */
int snd_seq_port_connect(struct snd_seq_client *connector,
struct snd_seq_client *src_client,
struct snd_seq_client_port *src_port,
struct snd_seq_client *dest_client,
struct snd_seq_client_port *dest_port,
struct snd_seq_port_subscribe *info)
{
struct snd_seq_subscribers *subs;
bool exclusive;
int err;

subs = kzalloc(sizeof(*subs), GFP_KERNEL);
if (!subs)
return -ENOMEM;

subs->info = *info;
atomic_set(&subs->ref_count, 0);
INIT_LIST_HEAD(&subs->src_list);
INIT_LIST_HEAD(&subs->dest_list);

exclusive = !!(info->flags & SNDRV_SEQ_PORT_SUBS_EXCLUSIVE);

err = check_and_subscribe_port(src_client, src_port, subs, true,
exclusive,
connector->number != src_client->number);
if (err < 0)
goto error;
err = check_and_subscribe_port(dest_client, dest_port, subs, false,
exclusive,
connector->number != dest_client->number);
if (err < 0)
goto error_dest;

up_write(&dest->list_mutex);
up_write(&src->list_mutex);
return 0;

__error:
if (src_called)
unsubscribe_port(src_client, src_port, src, info,
connector->number != src_client->number);
error_dest:
delete_and_unsubscribe_port(src_client, src_port, subs, true,
connector->number != src_client->number);
error:
kfree(subs);
up_write(&dest->list_mutex);
up_write(&src->list_mutex);
return err;
}


/* remove the connection */
int snd_seq_port_disconnect(struct snd_seq_client *connector,
struct snd_seq_client *src_client,
Expand All @@ -567,37 +603,28 @@ int snd_seq_port_disconnect(struct snd_seq_client *connector,
struct snd_seq_port_subscribe *info)
{
struct snd_seq_port_subs_info *src = &src_port->c_src;
struct snd_seq_port_subs_info *dest = &dest_port->c_dest;
struct snd_seq_subscribers *subs;
int err = -ENOENT;
unsigned long flags;

down_write(&src->list_mutex);
down_write_nested(&dest->list_mutex, SINGLE_DEPTH_NESTING);

/* look for the connection */
list_for_each_entry(subs, &src->list_head, src_list) {
if (match_subs_info(info, &subs->info)) {
write_lock_irqsave(&src->list_lock, flags);
// write_lock(&dest->list_lock); // no lock yet
list_del(&subs->src_list);
list_del(&subs->dest_list);
// write_unlock(&dest->list_lock);
write_unlock_irqrestore(&src->list_lock, flags);
src->exclusive = dest->exclusive = 0;
unsubscribe_port(src_client, src_port, src, info,
connector->number != src_client->number);
unsubscribe_port(dest_client, dest_port, dest, info,
connector->number != dest_client->number);
kfree(subs);
atomic_dec(&subs->ref_count); /* mark as not ready */
err = 0;
break;
}
}

up_write(&dest->list_mutex);
up_write(&src->list_mutex);
return err;
if (err < 0)
return err;

delete_and_unsubscribe_port(src_client, src_port, subs, true,
connector->number != src_client->number);
delete_and_unsubscribe_port(dest_client, dest_port, subs, false,
connector->number != dest_client->number);
kfree(subs);
return 0;
}


Expand Down

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