Re: [PATCH v5 1/1] audio/jack: fix use after free segfault

[Christian Schoenebeck]

On Mittwoch, 19. August 2020 08:29:39 CEST Geoffrey McRae wrote:
> This change registers a bottom handler to close the JACK client
> connection when a server shutdown signal is recieved. Without this
> libjack2 attempts to "clean up" old clients and causes a use after free
> segfault.
> 
> Signed-off-by: Geoffrey McRae <geoff@hostfission.com>
> ---

Looks much better now, but ...

>  audio/jackaudio.c | 29 ++++++++++++++++++++---------
>  1 file changed, 20 insertions(+), 9 deletions(-)
> 
> diff --git a/audio/jackaudio.c b/audio/jackaudio.c
> index 72ed7c4929..b0da5cd00b 100644
> --- a/audio/jackaudio.c
> +++ b/audio/jackaudio.c
> @@ -25,6 +25,7 @@
>  #include "qemu/osdep.h"
>  #include "qemu/module.h"
>  #include "qemu/atomic.h"
> +#include "qemu/main-loop.h"
>  #include "qemu-common.h"
>  #include "audio.h"
> 
> @@ -63,6 +64,7 @@ typedef struct QJackClient {
>      QJackState      state;
>      jack_client_t  *client;
>      jack_nframes_t  freq;
> +    QEMUBH         *shutdown_bh;
> 
>      struct QJack   *j;
>      int             nchannels;
> @@ -306,21 +308,27 @@ static int qjack_xrun(void *arg)
>      return 0;
>  }
> 
> +static void qjack_shutdown_bh(void *opaque)
> +{
> +    QJackClient *c = (QJackClient *)opaque;
> +    qjack_client_fini(c);
> +}
> +
>  static void qjack_shutdown(void *arg)
>  {
>      QJackClient *c = (QJackClient *)arg;
> -    c->state = QJACK_STATE_SHUTDOWN;
> +    c->state       = QJACK_STATE_SHUTDOWN;

White space changes are not much embraced on high traffic projects BTW.

> +    qemu_bh_schedule(c->shutdown_bh);
>  }
> 
>  static void qjack_client_recover(QJackClient *c)
>  {
> -    if (c->state == QJACK_STATE_SHUTDOWN) {
> -        qjack_client_fini(c);
> +    if (c->state != QJACK_STATE_DISCONNECTED) {
> +        return;
>      }
> 
>      /* packets is used simply to throttle this */
> -    if (c->state == QJACK_STATE_DISCONNECTED &&
> -        c->packets % 100 == 0) {
> +    if (c->packets % 100 == 0) {
> 
>          /* if enabled then attempt to recover */
>          if (c->enabled) {
> @@ -417,6 +425,10 @@ static int qjack_client_init(QJackClient *c)
>          options |= JackServerName;
>      }
> 
> +    if (!c->shutdown_bh) {
> +        c->shutdown_bh = qemu_bh_new(qjack_shutdown_bh, c);
> +    }
> +

Where is qemu_bh_delete() ?

>      c->client = jack_client_open(client_name, options, &status,
>        c->opt->server_name);
> 
> @@ -489,8 +501,6 @@ static int qjack_init_out(HWVoiceOut *hw, struct
> audsettings *as, QJackOut *jo  = (QJackOut *)hw;
>      Audiodev *dev = (Audiodev *)drv_opaque;
> 
> -    qjack_client_fini(&jo->c);
> -
>      jo->c.out       = true;
>      jo->c.enabled   = false;
>      jo->c.nchannels = as->nchannels;
> @@ -525,8 +535,6 @@ static int qjack_init_in(HWVoiceIn *hw, struct
> audsettings *as, QJackIn  *ji  = (QJackIn *)hw;
>      Audiodev *dev = (Audiodev *)drv_opaque;
> 
> -    qjack_client_fini(&ji->c);
> -
>      ji->c.out       = false;
>      ji->c.enabled   = false;
>      ji->c.nchannels = as->nchannels;
> @@ -557,6 +565,8 @@ static int qjack_init_in(HWVoiceIn *hw, struct
> audsettings *as,
> 
>  static void qjack_client_fini(QJackClient *c)
>  {
> +    qemu_bh_cancel(c->shutdown_bh);
> +

Looks like a potential race. Quote from the API doc of qemu_bh_cancel():

        "While cancellation itself is also wait-free and thread-safe, it can of 
        
        course race with the loop that executes bottom halves unless you are 
        holding the iothread mutex.  This makes it mostly useless if you are 
not 
        holding the mutex."

>      switch (c->state) {
>      case QJACK_STATE_RUNNING:
>          jack_deactivate(c->client);
> @@ -564,6 +574,7 @@ static void qjack_client_fini(QJackClient *c)
> 
>      case QJACK_STATE_SHUTDOWN:
>          jack_client_close(c->client);
> +        c->client = NULL;
>          /* fallthrough */
> 
>      case QJACK_STATE_DISCONNECTED:

Best regards,
Christian Schoenebeck