Re: [PATCH 1/2] ALSA: seq: Don't allow resizing pool in use

[Nicolai Stange <nstange@suse.de>]

Takashi Iwai <tiwai@suse.de> writes:

> This is a fix for a (sort of) fallout in the recent commit
> d15d662e89fc ("ALSA: seq: Fix racy pool initializations") for
> CVE-2018-1000004.
> As the pool resize deletes the existing cells, it may lead to a race
> when another thread is writing concurrently, eventually resulting a
> UAF.
>
> A simple workaround is not to allow the pool resizing when the pool is
> in use.  It's an invalid behavior in anyway.
>
> Fixes: d15d662e89fc ("ALSA: seq: Fix racy pool initializations")
> Reported-by: 范龙飞 <long7573@126.com>
> Reported-by: Nicolai Stange <nstange@suse.de>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Takashi Iwai <tiwai@suse.de>
> ---
>  sound/core/seq/seq_clientmgr.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/sound/core/seq/seq_clientmgr.c b/sound/core/seq/seq_clientmgr.c
> index 04d4db44fae5..d41ce3ed62ca 100644
> --- a/sound/core/seq/seq_clientmgr.c
> +++ b/sound/core/seq/seq_clientmgr.c
> @@ -1838,6 +1838,9 @@ static int snd_seq_ioctl_set_client_pool(struct snd_seq_client *client,
>  	    (! snd_seq_write_pool_allocated(client) ||
>  	     info->output_pool != client->pool->size)) {
>  		if (snd_seq_write_pool_allocated(client)) {

Maybe I'm missing something, but doesn't this

> +			/* is the pool in use? */
> +			if (atomic_read(&client->pool->counter))
> +				return -EBUSY;



>  			/* remove all existing cells */
>  			snd_seq_pool_mark_closing(client->pool);

render this 

>  			snd_seq_queue_client_leave_cells(client->number);

useless (assuming the presence of [2/2] ("ALSA: seq: More protection for
concurrent write and ioctl races"))?

Thanks,

Nicolai

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