[Qemu-devel] [STABLE] [BUG] VNC mode can crash QEMU

[Stefan Weil]

Hello,

this scenario crashs the latest QEMU HEAD on Windows
(Linux users, please note that the bug is not Windows related,
so don't stop reading!):

* run qemu.exe -vnc :0
* connect using UltraVnc
* select fuzzy screen mode in UltraVnc

=> segfault of qemu.exe

The crash is caused by VNC protocols which are unsupported
by QEMU - in my case it was the fuzzy screen mode protocol.
These protocols trigger a call stack which releases the
VncState vs:

qemu_free(vs)
vnc_client_io_error(vs, ...)
vnc_client_error(vs, ...)
protocol_client_msg(vs, ...)
vnc_client_read
main_loop_wait
main_loop

The default handlers for unimplemented protocols in
protocol_client_msg call vnc_client_error which finally
calls qemu_free for the current VncState vs.

vs is then used in protocol_client_msg and vnc_client_read
although it is no longer valid. On Windows, this results
in a crash, for other host platforms, the result depends
on implementation details of the C library.

In any case, access to a data structure after a free()
is a bug.

The same bug seems to exist in the stable branch
(not tested, I only had a look into the code vnc.c).

I don't see a simple way to patch this, so I leave the
bug fixing to the VNC experts and the QEMU maintainers.

Regards

Stefan Weil