Re: [Qemu-devel] [PATCH v2] vnc: fix segfault in closed connection handling

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Jan 31, 2018 at 04:15:37PM +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 (obj=obj@entry=0x0)
> 1  qio_channel_readv_full (ioc=0x0, iov=0x7ffe55277180 ...
> 2  qio_channel_read (ioc=<optimized out> ...
> 3  vnc_client_read_buf (vs=vs@entry=0x55625f3c6000, ...
> 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 (context=context@entry=0x556251568a50)
> 9  glib_pollfds_poll ()
> 10 os_host_main_loop_wait (timeout=<optimized out>)
> 11 main_loop_wait (nonblocking=nonblocking@entry=0)
> 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.
> 
> I suggest to check ioc_tag in vnc_disconnect_finish to prevent such
> an occurrence.

I think the root cause here is some codepath failing to check the
'vs->disconnecting' flag. eg vnc_write() checks that before it
calls qio_channel_add_watch. I suspect somewhere else in code needs
such a check

> 
> Signed-off-by: Klim Kireev <klim.kireev@virtuozzo.com>
> ---
> Changelog:
> v2: Apply the backtrace
> 
>  ui/vnc.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/ui/vnc.c b/ui/vnc.c
> index 33b087221f..b8bf0180cb 100644
> --- a/ui/vnc.c
> +++ b/ui/vnc.c
> @@ -1270,6 +1270,10 @@ void vnc_disconnect_finish(VncState *vs)
>      }
>      g_free(vs->lossy_rect);
>  
> +    if (vs->ioc_tag) {
> +        g_source_remove(vs->ioc_tag);
> +        vs->ioc_tag = 0;
> +    }
>      object_unref(OBJECT(vs->ioc));
>      vs->ioc = NULL;
>      object_unref(OBJECT(vs->sioc));

Regards,
Daniel
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