[Xen-changelog] [xen stable-4.2] tools: libxl: Explicitly disable graphics backends on qemu cmdline

[patchbot@xxxxxxx]

commit 5bec01c19839e150e489dd04376c65f961830c86
Author:     Ian Campbell <ian.campbell@xxxxxxxxxx>
AuthorDate: Fri Feb 20 14:41:09 2015 +0000
Commit:     Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
CommitDate: Thu Mar 12 13:48:11 2015 +0000

    tools: libxl: Explicitly disable graphics backends on qemu cmdline
    
    By default qemu will try to create some sort of backend for the
    emulated VGA device, either SDL or VNC.
    
    However when the user specifies sdl=0 and vnc=0 in their configuration
    libxl was not explicitly disabling either backend, which could lead to
    one unexpectedly running.
    
    If either sdl=1 or vnc=1 is configured then both before and after this
    change only the backends which are explicitly enabled are configured,
    i.e. this issue only occurs when all backends are supposed to have
    been disabled.
    
    This affects qemu-xen and qemu-xen-traditional differently.
    
    If qemu-xen was compiled with SDL support then this would result in an
    SDL window being opened if $DISPLAY is valid, or a failure to start
    the guest if not. Passing "-display none" to qemu before any further
    -sdl options disables this default behaviour and ensures that SDL is
    only started if the libxl configuration demands it.
    
    If qemu-xen was compiled without SDL support then qemu would instead
    start a VNC server listening on ::1 (IPv6 localhost) or 127.0.0.1
    (IPv4 localhost) with IPv6 preferred if available. Explicitly pass
    "-vnc none" when vnc is not enabled in the libxl configuration to
    remove this possibility.
    
    qemu-xen-traditional would never start a vnc backend unless asked.
    However by default it will start an SDL backend, the way to disable
    this is to pass a -vnc option. In other words passing "-vnc none" will
    disable both vnc and sdl by default. sdl can then be reenabled if
    configured by subsequent use of the -sdl option.
    
    Tested with both qemu-xen and qemu-xen-traditional built with SDL
    support and:
        xl cr # defaults
        xl cr sdl=0 vnc=0
        xl cr sdl=1 vnc=0
        xl cr sdl=0 vnc=1
        xl cr sdl=0 vnc=0 vga=\"none\"
        xl cr sdl=0 vnc=0 nographic=1
    with both valid and invalid $DISPLAY.
    
    This is XSA-119 / CVE-2015-2152.
    
    Reported-by: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
    Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
    Acked-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
---
 tools/libxl/libxl_dm.c |   23 +++++++++++++++++++++--
 1 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/tools/libxl/libxl_dm.c b/tools/libxl/libxl_dm.c
index 88a4894..6a1b91f 100644
--- a/tools/libxl/libxl_dm.c
+++ b/tools/libxl/libxl_dm.c
@@ -147,7 +147,15 @@ static char ** 
libxl__build_device_model_args_old(libxl__gc *gc,
         if (libxl_defbool_val(vnc->findunused)) {
             flexarray_append(dm_args, "-vncunused");
         }
-    }
+    } else
+        /*
+         * VNC is not enabled by default by qemu-xen-traditional,
+         * however passing -vnc none causes SDL to not be
+         * (unexpectedly) enabled by default. This is overridden by
+         * explicitly passing -sdl below as required.
+         */
+        flexarray_append_pair(dm_args, "-vnc", "none");
+
     if (sdl) {
         flexarray_append(dm_args, "-sdl");
         if (!libxl_defbool_val(sdl->opengl)) {
@@ -394,7 +402,18 @@ static char ** 
libxl__build_device_model_args_new(libxl__gc *gc,
             vncarg = libxl__sprintf(gc, "%s,to=99", vncarg);
         }
         flexarray_append(dm_args, vncarg);
-    }
+    } else
+        /*
+         * Ensure that by default no vnc server is created.
+         */
+        flexarray_append_pair(dm_args, "-vnc", "none");
+
+    /*
+     * Ensure that by default no display backend is created. Further
+     * options given below might then enable more.
+     */
+    flexarray_append_pair(dm_args, "-display", "none");
+
     if (sdl) {
         flexarray_append(dm_args, "-sdl");
         /* XXX sdl->{display,xauthority} into $DISPLAY/$XAUTHORITY */
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.2

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