Re: [PATCH 1/2] Fix the segment fault when calling yank_register_instance

[Marc-André Lureau]

Hi

On Tue, Mar 16, 2021 at 6:46 PM Li Zhang <li.zhang@cloud.ionos.com> wrote:

Hi Marc-André,

By looking into chardev and yank_register_function logic,  this old chardev is registered according to the chardev label. 

So it's been in yank_instance_list. yank instance only has a chardev label, and the new chardev's label is the same as the old chardev.

So it doesn't need to register it again when changing the chardev backend. Otherwise, it will report  duplicated yank instances. 

I think the chardev logic has no problems. And it works with yank functions. 

The previous instance is being removed with yank_unregister_instance() during object_unparent(). The new instance is not registered.

That scenario deserves tests. (it's a shame there are no tests for yank ...)

 

Thanks

Li  

On Mon, Mar 15, 2021 at 7:51 PM Marc-André Lureau <marcandre.lureau@gmail.com> wrote:

Hi

On Mon, Mar 15, 2021 at 9:22 PM Li Zhang <zhlcindy@gmail.com> wrote:

From: Li Zhang <li.zhang@cloud.ionos.com>

When executing the QMP commands "chardev-change" to change the
backend device to socket, it will cause a segment fault because
it assumes chr->label as non-NULL in function yank_register_instance.
The function qmp_chardev_change calls chardev_new, which label
is NULL when creating a new chardev. The label will be passed to
yank_register_instance which causes a segment fault. The callchain
is as the following:
        chardev_new ->
            qemu_char_open ->
                cc->open ->
                qmp_chardev_open_socket ->
                    yank_register_instance

Signed-off-by: Li Zhang <li.zhang@cloud.ionos.com>
---
 chardev/char-socket.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/chardev/char-socket.c b/chardev/char-socket.c
index c8bced76b7..26d5172682 100644
--- a/chardev/char-socket.c
+++ b/chardev/char-socket.c
@@ -1421,10 +1421,12 @@ static void qmp_chardev_open_socket(Chardev *chr,
         qemu_chr_set_feature(chr, QEMU_CHAR_FEATURE_FD_PASS);
     }

-    if (!yank_register_instance(CHARDEV_YANK_INSTANCE(chr->label), errp)) {
-        return;
+    if (chr->label) {
+        if (!yank_register_instance(CHARDEV_YANK_INSTANCE(chr->label), errp)) {
+            return;
+        }
+        s->registered_yank = true;
     }
-    s->registered_yank = true;

     /* be isn't opened until we get a connection */
     *be_opened = false

Looks wrong to me, the new chardev will get the same label, and it should still be possible to call the yank functions then. The registration logic needs to be reworked during chardev-change.

--

Marc-André Lureau

--

Marc-André Lureau