console: Avoid segfault in screendump

After f771c5440e04626f1 it is possible to select device and
head which to take screendump from. And even though we check if
provided head number falls within range, it may still happen that
the console has no surface yet leading to SIGSEGV:

  qemu.git $ ./x86_64-softmmu/qemu-system-x86_64 \
    -qmp stdio \
    -device virtio-vga,id=video0,max_outputs=4

  {"execute":"qmp_capabilities"}
  {"execute":"screendump", "arguments":{"filename":"/tmp/screen.ppm", "device":"video0", "head":1}}
  Segmentation fault

 #0  0x00005628249dda88 in ppm_save (filename=0x56282826cbc0 "/tmp/screen.ppm", ds=0x0, errp=0x7fff52a6fae0) at ui/console.c:304
 #1  0x00005628249ddd9b in qmp_screendump (filename=0x56282826cbc0 "/tmp/screen.ppm", has_device=true, device=0x5628276902d0 "video0", has_head=true, head=1, errp=0x7fff52a6fae0) at ui/console.c:375
 #2  0x00005628247740df in qmp_marshal_screendump (args=0x562828265e00, ret=0x7fff52a6fb68, errp=0x7fff52a6fb60) at qapi/qapi-commands-ui.c:110

Here, @ds from frame #0 (or @surface from frame #1) is
dereferenced at the very beginning of ppm_save(). And because
it's NULL crash happens.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: cb05bb1909daa6ba62145c0194aafa05a14ed3d1.1526569138.git.mprivozn@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
(cherry picked from commit 08d9864fa4e0c616e076ca8b225d39a7ecb189af)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

diff --git a/ui/console.c b/ui/console.c
index c4c95abed7efe37cb48d2cea05e8580f508fe9db..96272b5c454e56c138e8e5375e4a95cd3f15c7c5 100644 (file)
--- a/ui/console.c
+++ b/ui/console.c
@@ -354,6 +354,11 @@ void qmp_screendump(const char *filename, Error **errp)
 
     graphic_hw_update(con);
     surface = qemu_console_surface(con);
+    if (!surface) {
+        error_setg(errp, "no surface");
+        return;
+    }
+
     ppm_save(filename, surface, errp);
 }