[Qemu-devel] [PATCH] vnc: clear vs->tlscreds after unparenting it

[Qemu-devel] [PATCH] vnc: clear vs->tlscreds after unparenting it

[Wolfgang Bumiller]

This pointer should be cleared in vnc_display_close()
otherwise a use-after-free can happen when when using the
old style 'x509' and 'tls' options rather than a persistent
tls-creds -object, by issuing monitor commands to change
the vnc server like so:

Start with: -vnc unix:test.socket,x509,tls
Then use the following monitor command:
  change vnc unix:test.socket

After this the pointer is still set but invalid and a crash
can be triggered for instance by issuing the same command a
second time which will try to object_unparent() the same
pointer again.
---
 ui/vnc.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/ui/vnc.c b/ui/vnc.c
index 09756cd..35843b5 100644
--- a/ui/vnc.c
+++ b/ui/vnc.c
@@ -3134,6 +3134,7 @@ static void vnc_display_close(VncDisplay *vs)
     vs->subauth = VNC_AUTH_INVALID;
     if (vs->tlscreds) {
         object_unparent(OBJECT(vs->tlscreds));
+        vs->tlscreds = NULL;
     }
     g_free(vs->tlsaclname);
     vs->tlsaclname = NULL;
-- 
2.1.4

Re: [Qemu-devel] [PATCH] vnc: clear vs->tlscreds after unparenting it

[Daniel P. Berrange]

On Tue, Jan 12, 2016 at 01:52:39PM +0100, Wolfgang Bumiller wrote:
> This pointer should be cleared in vnc_display_close()
> otherwise a use-after-free can happen when when using the
> old style 'x509' and 'tls' options rather than a persistent
> tls-creds -object, by issuing monitor commands to change
> the vnc server like so:
> 
> Start with: -vnc unix:test.socket,x509,tls
> Then use the following monitor command:
>   change vnc unix:test.socket
> 
> After this the pointer is still set but invalid and a crash
> can be triggered for instance by issuing the same command a
> second time which will try to object_unparent() the same
> pointer again.
> ---
>  ui/vnc.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/ui/vnc.c b/ui/vnc.c
> index 09756cd..35843b5 100644
> --- a/ui/vnc.c
> +++ b/ui/vnc.c
> @@ -3134,6 +3134,7 @@ static void vnc_display_close(VncDisplay *vs)
>      vs->subauth = VNC_AUTH_INVALID;
>      if (vs->tlscreds) {
>          object_unparent(OBJECT(vs->tlscreds));
> +        vs->tlscreds = NULL;
>      }
>      g_free(vs->tlsaclname);
>      vs->tlsaclname = NULL;

Reviewed-by: Daniel P. Berrange <berrange@redhat.com>


Regards,
Daniel
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Re: [Qemu-devel] [PATCH] vnc: clear vs->tlscreds after unparenting it

[Wolfgang Bumiller]

Ah I forgot to --signoff, sorry

> On January 12, 2016 at 1:52 PM Wolfgang Bumiller <w.bumiller@proxmox.com> wrote:
> 
> 
> This pointer should be cleared in vnc_display_close()
> otherwise a use-after-free can happen when when using the
> old style 'x509' and 'tls' options rather than a persistent
> tls-creds -object, by issuing monitor commands to change
> the vnc server like so:
> 
> Start with: -vnc unix:test.socket,x509,tls
> Then use the following monitor command:
>   change vnc unix:test.socket
> 
> After this the pointer is still set but invalid and a crash
> can be triggered for instance by issuing the same command a
> second time which will try to object_unparent() the same
> pointer again.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

> ---
>  ui/vnc.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/ui/vnc.c b/ui/vnc.c
> index 09756cd..35843b5 100644
> --- a/ui/vnc.c
> +++ b/ui/vnc.c
> @@ -3134,6 +3134,7 @@ static void vnc_display_close(VncDisplay *vs)
>      vs->subauth = VNC_AUTH_INVALID;
>      if (vs->tlscreds) {
>          object_unparent(OBJECT(vs->tlscreds));
> +        vs->tlscreds = NULL;
>      }
>      g_free(vs->tlsaclname);
>      vs->tlsaclname = NULL;
> -- 
> 2.1.4

Re: [Qemu-devel] [PATCH] vnc: clear vs->tlscreds after unparenting it

[Gerd Hoffmann]

On Di, 2016-01-12 at 13:52 +0100, Wolfgang Bumiller wrote:
> This pointer should be cleared in vnc_display_close()
> otherwise a use-after-free can happen when when using the
> old style 'x509' and 'tls' options rather than a persistent
> tls-creds -object, by issuing monitor commands to change
> the vnc server like so:
> 
> Start with: -vnc unix:test.socket,x509,tls
> Then use the following monitor command:
>   change vnc unix:test.socket
> 
> After this the pointer is still set but invalid and a crash
> can be triggered for instance by issuing the same command a
> second time which will try to object_unparent() the same
> pointer again.

Added to patch queue.

thanks,
  Gerd