Re: aconnect occasionally causes kernel oopses

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Hi,

To which kernel version should I apply it?
Because:

folkert@oensoens:~/linux-5.11.0$ patch -p1 < ~/alsa-patch.diff 
patching file sound/core/seq/seq_ports.c
patching file sound/core/seq/seq_ports.h
patching file sound/core/seq/seq_ports.c
Hunk #1 succeeded at 526 (offset 12 lines).
Hunk #2 succeeded at 538 (offset 12 lines).
Hunk #3 succeeded at 547 with fuzz 2 (offset 12 lines).
Hunk #4 FAILED at 602.
1 out of 4 hunks FAILED -- saving rejects to file sound/core/seq/seq_ports.c.rej

On Tue, Aug 03, 2021 at 08:55:36AM +0200, Takashi Iwai wrote:
> On Mon, 02 Aug 2021 21:53:49 +0200,
> folkert wrote:
> > 
> > > > [ kernel bug if repeatingly aconnect'ing midi devices ]
> > > > 
> > > > > Does this happen if you do reconnect of kernel sequencer client?
> > > > > You can use snd-virmidi as well as snd-dummy.
> > > > > I'm asking it because it'll simplify the test a lot, which will be
> > > > > almost self-contained.
> > > > 
> > > > Like this?
> > > > 
> > > > root@lappiemctopface:~# aplaymidi -l
> > > >  Port    Client name                      Port name
> > > >  14:0    Midi Through                     Midi Through Port-0
> > > >  20:0    Virtual Raw MIDI 1-0             VirMIDI 1-0
> > > >  21:0    Virtual Raw MIDI 1-1             VirMIDI 1-1
> > > >  22:0    Virtual Raw MIDI 1-2             VirMIDI 1-2
> > > >  23:0    Virtual Raw MIDI 1-3             VirMIDI 1-3
> > > > 128:0    rtpmidi lappiemctopface          Network
> > > > 128:1    rtpmidi lappiemctopface          metronoom
> > > > 128:2    rtpmidi lappiemctopface          AppleMidi2IPMidiBridge
> > > > 128:3    rtpmidi lappiemctopface          oensoens
> > > > 130:0    FLUID Synth (11462)              Synth input port (11462:0)
> > > > 
> > > > and then:
> > > > 
> > > > root@lappiemctopface:~# cat test.sh 
> > > > while true
> > > > do
> > > > 	aconnect 20:0 21:0
> > > > 	aconnect -d 20:0 21:0
> > > > done
> > > > 
> > > > root@lappiemctopface:~# for i in `seq 0 3` ; do (./test.sh &) ; done
> > > > 
> > > > This hard locks-up my laptop: it doesn't even respond to capslock (led
> > > > on/off) anymore nor the ctrl+prtscr+alt+b combination.
> > > 
> > > Thanks, that's really helpful!
> > > I see the possible race now.
> > 
> > > Could you try the quick fix below?
> > 
> > Something may have changed:
> > 
> > folkert@oensoens:~$ aplaymidi -l
> > ALSA lib seq_hw.c:466:(snd_seq_hw_open) open /dev/snd/seq failed: Permission denied
> > Cannot open sequencer - Permission denied
> > 
> > ???
> 
> Hrm, that's unexpected.
> 
> Meanwhile, I reconsidered the fix and a better idea came after the
> sleep.  Below is the new fix patch.  Could you give it a try instead
> of the previous one?
> 
> 
> thanks,
> 
> Takashi
> 
> -- 8< --
> From: Takashi Iwai <tiwai@xxxxxxx>
> Subject: [PATCH] ALSA: seq: Fix racy deletion of subscriber
> 
> It turned out that the current implementation of the port subscription
> is racy.  The subscription contains two linked lists, and we have to
> add to or delete from both lists.  Since both connection and
> disconnection procedures perform the same order for those two lists
> (i.e. src list, then dest list), when a deletion happens during a
> connection procedure, the src list may be deleted before the dest list
> addition completes, and this may lead to a use-after-free or an Oops,
> even though the access to both lists are protected via mutex.
> 
> The simple workaround for this race is to change the access order for
> the disconnection, namely, dest list, then src list.  This assures
> that the connection has been established when disconnecting, and also
> the concurrent deletion can be avoided.
> 
> Reported-by: folkert <folkert@xxxxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Link: https://lore.kernel.org/r/20210801182754.GP890690@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
> ---
>  sound/core/seq/seq_ports.c | 39 ++++++++++++++++++++++++++------------
>  1 file changed, 27 insertions(+), 12 deletions(-)
> 
> diff --git a/sound/core/seq/seq_ports.c b/sound/core/seq/seq_ports.c
> index b9c2ce2b8d5a..84d78630463e 100644
> --- a/sound/core/seq/seq_ports.c
> +++ b/sound/core/seq/seq_ports.c
> @@ -514,10 +514,11 @@ static int check_and_subscribe_port(struct snd_seq_client *client,
>  	return err;
>  }
>  
> -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)
> +/* called with grp->list_mutex held */
> +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;
>  	struct list_head *list;
> @@ -525,7 +526,6 @@ static void delete_and_unsubscribe_port(struct snd_seq_client *client,
>  
>  	grp = is_src ? &port->c_src : &port->c_dest;
>  	list = is_src ? &subs->src_list : &subs->dest_list;
> -	down_write(&grp->list_mutex);
>  	write_lock_irq(&grp->list_lock);
>  	empty = list_empty(list);
>  	if (!empty)
> @@ -535,6 +535,18 @@ static void delete_and_unsubscribe_port(struct snd_seq_client *client,
>  
>  	if (!empty)
>  		unsubscribe_port(client, port, grp, &subs->info, ack);
> +}
> +
> +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);
> +	__delete_and_unsubscribe_port(client, port, subs, is_src, ack);
>  	up_write(&grp->list_mutex);
>  }
>  
> @@ -590,27 +602,30 @@ int snd_seq_port_disconnect(struct snd_seq_client *connector,
>  			    struct snd_seq_client_port *dest_port,
>  			    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;
>  
> -	down_write(&src->list_mutex);
> +	/* always start from deleting the dest port for avoiding concurrent
> +	 * deletions
> +	 */
> +	down_write(&dest->list_mutex);
>  	/* look for the connection */
> -	list_for_each_entry(subs, &src->list_head, src_list) {
> +	list_for_each_entry(subs, &dest->list_head, dest_list) {
>  		if (match_subs_info(info, &subs->info)) {
> -			atomic_dec(&subs->ref_count); /* mark as not ready */
> +			__delete_and_unsubscribe_port(dest_client, dest_port,
> +						      subs, false,
> +						      connector->number != dest_client->number);
>  			err = 0;
>  			break;
>  		}
>  	}
> -	up_write(&src->list_mutex);
> +	up_write(&dest->list_mutex);
>  	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;
>  }
> -- 
> 2.26.2


Folkert van Heusden

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