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[Qemu-devel] [PATCH 23/99] console: Avoid segfault in screendump

From: Michael Roth
Subject: [Qemu-devel] [PATCH 23/99] console: Avoid segfault in screendump
Date: Mon, 23 Jul 2018 15:16:32 -0500

From: Michal Privoznik <address@hidden>

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":"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 <address@hidden>
Reviewed-by: Thomas Huth <address@hidden>
Message-id: address@hidden
Signed-off-by: Gerd Hoffmann <address@hidden>
(cherry picked from commit 08d9864fa4e0c616e076ca8b225d39a7ecb189af)
Signed-off-by: Michael Roth <address@hidden>
 ui/console.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/ui/console.c b/ui/console.c
index 3fb2f4e09f..476bf901a4 100644
--- a/ui/console.c
+++ b/ui/console.c
@@ -370,6 +370,11 @@ void qmp_screendump(const char *filename, bool has_device, 
const char *device,
     surface = qemu_console_surface(con);
+    if (!surface) {
+        error_setg(errp, "no surface");
+        return;
+    }
     ppm_save(filename, surface, errp);

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