Re: [Qemu-devel] [PATCH] vnc: fix segfault in closed connection handling

[Daniel P . Berrangé]

On Wed, Feb 07, 2018 at 12:48:44PM +0300, Klim Kireev wrote:
> On one of our client's node, due to trying to read from closed ioc,
> a segmentation fault occured. Corresponding backtrace:
> 
> 0  object_get_class (address@hidden)
> 1  qio_channel_readv_full (ioc=0x0, iov=0x7ffe55277180 ...
> 2  qio_channel_read (ioc=<optimized out> ...
> 3  vnc_client_read_buf (address@hidden, ...
> 4  vnc_client_read_plain (vs=0x55625f3c6000)
> 5  vnc_client_read (vs=0x55625f3c6000)
> 6  vnc_client_io (ioc=<optimized out>, condition=G_IO_IN, ...
> 7  g_main_dispatch (context=0x556251568a50)
> 8  g_main_context_dispatch (address@hidden)
> 9  glib_pollfds_poll ()
> 10 os_host_main_loop_wait (timeout=<optimized out>)
> 11 main_loop_wait (address@hidden)
> 12 main_loop () at vl.c:1909
> 13 main (argc=<optimized out>, argv=<optimized out>, ...
> 
> Having analyzed the coredump, I understood that the reason is that
> ioc_tag is reset on vnc_disconnect_start and ioc is cleaned
> in vnc_disconnect_finish. Between these two events due to some
> reasons the ioc_tag was set again and after vnc_disconnect_finish
> the handler is running with freed ioc,
> which led to the segmentation fault.
> 
> The patch checks vs->disconnecting in places where we call
> qio_channel_add_watch and resets handler if disconnecting == TRUE
> to prevent such an occurrence.
> 
> Signed-off-by: Klim Kireev <address@hidden>
> ---
> Changelog:
> v2: Attach the backtrace
> 
> v3: Change checks
> 
>  ui/vnc-jobs.c |  6 ++++--
>  ui/vnc.c      | 15 ++++++++++++++-
>  2 files changed, 18 insertions(+), 3 deletions(-)

Reviewed-by: Daniel P. Berrangé <address@hidden>


> 
> diff --git a/ui/vnc-jobs.c b/ui/vnc-jobs.c
> index e326679dd0..868dddef4b 100644
> --- a/ui/vnc-jobs.c
> +++ b/ui/vnc-jobs.c
> @@ -148,8 +148,10 @@ void vnc_jobs_consume_buffer(VncState *vs)
>              if (vs->ioc_tag) {
>                  g_source_remove(vs->ioc_tag);
>              }
> -            vs->ioc_tag = qio_channel_add_watch(
> -                vs->ioc, G_IO_IN | G_IO_OUT, vnc_client_io, vs, NULL);
> +            if (vs->disconnecting == FALSE) {
> +                vs->ioc_tag = qio_channel_add_watch(
> +                    vs->ioc, G_IO_IN | G_IO_OUT, vnc_client_io, vs, NULL);
> +            }
>          }
>          buffer_move(&vs->output, &vs->jobs_buffer);
>  
> diff --git a/ui/vnc.c b/ui/vnc.c
> index 93731accb6..67ccc8160f 100644
> --- a/ui/vnc.c
> +++ b/ui/vnc.c
> @@ -1536,12 +1536,19 @@ gboolean vnc_client_io(QIOChannel *ioc G_GNUC_UNUSED,
>      VncState *vs = opaque;
>      if (condition & G_IO_IN) {
>          if (vnc_client_read(vs) < 0) {
> -            return TRUE;
> +            goto end;
>          }
>      }
>      if (condition & G_IO_OUT) {
>          vnc_client_write(vs);
>      }
> +end:
> +    if (vs->disconnecting) {
> +        if (vs->ioc_tag != 0) {
> +            g_source_remove(vs->ioc_tag);
> +        }
> +        vs->ioc_tag = 0;
> +    }
>      return TRUE;
>  }
>  
> @@ -1630,6 +1637,12 @@ void vnc_flush(VncState *vs)
>      if (vs->ioc != NULL && vs->output.offset) {
>          vnc_client_write_locked(vs);
>      }
> +    if (vs->disconnecting) {
> +        if (vs->ioc_tag != 0) {
> +            g_source_remove(vs->ioc_tag);
> +        }
> +        vs->ioc_tag = 0;
> +    }
>      vnc_unlock_output(vs);
>  }
>  
> -- 
> 2.14.3
> 

Regards,
Daniel
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