Re: [PATCH v8 1/1] audio/jack: fix use after free segfault

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Freitag, 21. August 2020 15:45:54 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>
> ---
>  audio/jackaudio.c | 55 ++++++++++++++++++++++++++++++++++++-----------
>  1 file changed, 42 insertions(+), 13 deletions(-)
> 
> diff --git a/audio/jackaudio.c b/audio/jackaudio.c
> index 72ed7c4929..572ebea208 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 +64,8 @@ typedef struct QJackClient {
>      QJackState      state;
>      jack_client_t  *client;
>      jack_nframes_t  freq;
> +    QemuMutex       shutdown_lock;
> +    QEMUBH         *shutdown_bh;
> 
>      struct QJack   *j;
>      int             nchannels;
> @@ -306,21 +309,27 @@ 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;
> +    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) {
> @@ -489,15 +498,18 @@ 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;
>      jo->c.opt       = dev->u.jack.out;
> 
> +    jo->c.shutdown_bh = qemu_bh_new(qjack_shutdown_bh, &jo->c);
> +    qemu_mutex_init(&jo->c.shutdown_lock);
> +
>      int ret = qjack_client_init(&jo->c);
>      if (ret != 0) {
> +        qemu_bh_delete(jo->c.shutdown_bh);
> +        qemu_mutex_destroy(&jo->c.shutdown_lock);
>          return ret;
>      }
> 
> @@ -525,15 +537,18 @@ 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;
>      ji->c.opt       = dev->u.jack.in;
> 
> +    ji->c.shutdown_bh = qemu_bh_new(qjack_shutdown_bh, &ji->c);
> +    qemu_mutex_init(&ji->c.shutdown_lock);
> +
>      int ret = qjack_client_init(&ji->c);
>      if (ret != 0) {
> +        qemu_bh_delete(ji->c.shutdown_bh);
> +        qemu_mutex_destroy(&ji->c.shutdown_lock);
>          return ret;
>      }
> 
> @@ -555,7 +570,7 @@ static int qjack_init_in(HWVoiceIn *hw, struct
> audsettings *as, return 0;
>  }
> 
> -static void qjack_client_fini(QJackClient *c)
> +static void qjack_client_fini_locked(QJackClient *c)
>  {
>      switch (c->state) {
>      case QJACK_STATE_RUNNING:
> @@ -564,28 +579,42 @@ static void qjack_client_fini(QJackClient *c)
> 
>      case QJACK_STATE_SHUTDOWN:
>          jack_client_close(c->client);
> +        c->client = NULL;
> +
> +        qjack_buffer_free(&c->fifo);
> +        g_free(c->port);
> +
> +        c->state = QJACK_STATE_DISCONNECTED;
>          /* fallthrough */
> 
>      case QJACK_STATE_DISCONNECTED:
>          break;
>      }
> +}
> 
> -    qjack_buffer_free(&c->fifo);
> -    g_free(c->port);
> -
> -    c->state = QJACK_STATE_DISCONNECTED;
> +static void qjack_client_fini(QJackClient *c)
> +{
> +    qemu_mutex_lock(&c->shutdown_lock);
> +    qjack_client_fini_locked(c);
> +    qemu_mutex_unlock(&c->shutdown_lock);
>  }
> 
>  static void qjack_fini_out(HWVoiceOut *hw)
>  {
>      QJackOut *jo = (QJackOut *)hw;
>      qjack_client_fini(&jo->c);
> +
> +    qemu_bh_delete(jo->c.shutdown_bh);

Paolo wrapped that qemu_bh_delete() call inside the lock as well. So I guess 
it makes a difference for the BH API?

> +    qemu_mutex_destroy(&jo->c.shutdown_lock);
>  }

Hmmm, is this qemu_mutex_destroy() safe at this point?

> 
>  static void qjack_fini_in(HWVoiceIn *hw)
>  {
>      QJackIn *ji = (QJackIn *)hw;
>      qjack_client_fini(&ji->c);
> +
> +    qemu_bh_delete(ji->c.shutdown_bh);
> +    qemu_mutex_destroy(&ji->c.shutdown_lock);
>  }
> 
>  static void qjack_enable_out(HWVoiceOut *hw, bool enable)

Best regards,
Christian Schoenebeck