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Date:   Thu, 7 May 2020 12:13:10 +0200
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Takashi Iwai <tiwai@suse.de>
Cc:     butt3rflyh4ck <butterflyhuangxx@gmail.com>, security@kernel.org,
        syzkaller <syzkaller@googlegroups.com>, tiwai@suse.com,
        alsa-devel@alsa-project.org, linux-kernel@vger.kernel.org
Subject: Re: KASAN: use-after-free Write in snd_rawmidi_kernel_write1
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On Thu, May 07, 2020 at 11:56:22AM +0200, Takashi Iwai wrote:
> On Thu, 07 May 2020 10:23:02 +0200,
> Greg Kroah-Hartman wrote:
> > 
> > On Thu, May 07, 2020 at 04:04:25PM +0800, butt3rflyh4ck wrote:
> > > I report a bug (in linux-5.7-rc1) found by syzkaller.
> > > 
> > > kernel config: https://github.com/butterflyhack/syzkaller-fuzz/blob/master/v5.7.0-rc1.config
> > > reproducer: https://github.com/butterflyhack/syzkaller-fuzz/blob/master/repro.cprog
> > > 
> > > I test the reproducer in linux-5.7-rc4 and crash too.
> > 
> > Great, care to create a fix for this and send it to the proper
> > maintainers?  That's the best way to get it fixed, otherwise it just
> > goes in the file with the rest of the syzbot reports we are burried
> > under.
> 
> Don't worry, I already prepared a fix patch below :)
> 
> 
> thanks,
> 
> Takashi
> 
> -- 8< --
> From: Takashi Iwai <tiwai@suse.de>
> Subject: [PATCH] ALSA: rawmidi: Fix racy buffer resize under concurrent
>  accesses
> 
> The rawmidi core allows user to resize the runtime buffer via ioctl,
> and this may lead to UAF when performed during concurrent reads or
> writes.
> 
> This patch fixes the race by introducing a reference counter for the
> runtime buffer access and returns -EBUSY error when the resize is
> performed concurrently.
> 
> Reported-by: butt3rflyh4ck <butterflyhuangxx@gmail.com>
> Cc: <stable@vger.kernel.org>
> Link: https://lore.kernel.org/r/CAFcO6XMWpUVK_yzzCpp8_XP7+=oUpQvuBeCbMffEDkpe8jWrfg@mail.gmail.com
> Signed-off-by: Takashi Iwai <tiwai@suse.de>
> ---
>  include/sound/rawmidi.h |  1 +
>  sound/core/rawmidi.c    | 29 ++++++++++++++++++++++++++++-
>  2 files changed, 29 insertions(+), 1 deletion(-)
> 
> diff --git a/include/sound/rawmidi.h b/include/sound/rawmidi.h
> index a36b7227a15a..334842daa904 100644
> --- a/include/sound/rawmidi.h
> +++ b/include/sound/rawmidi.h
> @@ -61,6 +61,7 @@ struct snd_rawmidi_runtime {
>  	size_t avail_min;	/* min avail for wakeup */
>  	size_t avail;		/* max used buffer for wakeup */
>  	size_t xruns;		/* over/underruns counter */
> +	int buffer_ref;		/* buffer reference count */
>  	/* misc */
>  	spinlock_t lock;
>  	wait_queue_head_t sleep;
> diff --git a/sound/core/rawmidi.c b/sound/core/rawmidi.c
> index 20dd08e1f675..4185d9e81e3c 100644
> --- a/sound/core/rawmidi.c
> +++ b/sound/core/rawmidi.c
> @@ -120,6 +120,17 @@ static void snd_rawmidi_input_event_work(struct work_struct *work)
>  		runtime->event(runtime->substream);
>  }
>  
> +/* buffer refcount management: call with runtime->lock held */
> +static inline void snd_rawmidi_buffer_ref(struct snd_rawmidi_runtime *runtime)
> +{
> +	runtime->buffer_ref++;
> +}
> +
> +static inline void snd_rawmidi_buffer_unref(struct snd_rawmidi_runtime *runtime)
> +{
> +	runtime->buffer_ref--;
> +}

Why not use the reference count structure?

greg k-h