[Qemu-devel] [PATCH] chardev: fix pty_chr_timer

[Qemu-devel] [PATCH] chardev: fix pty_chr_timer

[Gerd Hoffmann]

pty_chr_timer first calls pty_chr_update_read_handler(), then clears
timer_tag (because it is a one-shot timer).   This is the wrong order
though.  pty_chr_update_read_handler might re-arm time timer, and the
new timer_tag gets overwitten in that case.

This leads to crashes when unplugging a pty chardev:  pty_chr_close
thinks no timer is running -> timer isn't canceled -> pty_chr_timer gets
called with stale CharDevState -> BOOM.

This patch fixes the ordering.
Kill the pointless goto while being at it.

https://bugzilla.redhat.com/show_bug.cgi?id=994414

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
 qemu-char.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/qemu-char.c b/qemu-char.c
index 1be1cf6..1621fbd 100644
--- a/qemu-char.c
+++ b/qemu-char.c
@@ -1026,15 +1026,11 @@ static gboolean pty_chr_timer(gpointer opaque)
     struct CharDriverState *chr = opaque;
     PtyCharDriver *s = chr->opaque;
 
-    if (s->connected) {
-        goto out;
-    }
-
-    /* Next poll ... */
-    pty_chr_update_read_handler(chr);
-
-out:
     s->timer_tag = 0;
+    if (!s->connected) {
+        /* Next poll ... */
+        pty_chr_update_read_handler(chr);
+    }
     return FALSE;
 }
 
-- 
1.8.3.1

Re: [Qemu-devel] [PATCH] chardev: fix pty_chr_timer

[Laszlo Ersek]

On 08/22/13 11:57, Gerd Hoffmann wrote:
> pty_chr_timer first calls pty_chr_update_read_handler(), then clears
> timer_tag (because it is a one-shot timer).   This is the wrong order
> though.  pty_chr_update_read_handler might re-arm time timer, and the
> new timer_tag gets overwitten in that case.
> 
> This leads to crashes when unplugging a pty chardev:  pty_chr_close
> thinks no timer is running -> timer isn't canceled -> pty_chr_timer gets
> called with stale CharDevState -> BOOM.
> 
> This patch fixes the ordering.
> Kill the pointless goto while being at it.
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=994414
> 
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> ---
>  qemu-char.c | 12 ++++--------
>  1 file changed, 4 insertions(+), 8 deletions(-)
> 
> diff --git a/qemu-char.c b/qemu-char.c
> index 1be1cf6..1621fbd 100644
> --- a/qemu-char.c
> +++ b/qemu-char.c
> @@ -1026,15 +1026,11 @@ static gboolean pty_chr_timer(gpointer opaque)
>      struct CharDriverState *chr = opaque;
>      PtyCharDriver *s = chr->opaque;
>  
> -    if (s->connected) {
> -        goto out;
> -    }
> -
> -    /* Next poll ... */
> -    pty_chr_update_read_handler(chr);
> -
> -out:
>      s->timer_tag = 0;
> +    if (!s->connected) {
> +        /* Next poll ... */
> +        pty_chr_update_read_handler(chr);
> +    }
>      return FALSE;
>  }

pty_chr_timer()
  s->timer_tag = 0;
  pty_chr_update_read_handler() // s->connected == 0
    pty_chr_state(..., 1)       // G_IO_HUP is clear
      not calling: g_source_remove(s->timer_tag)
      s->connected = 1;
      qemu_chr_be_generic_open()
      s->fd_tag = io_add_watch_poll()

So, in this order, s->timer_tag is not removed in pty_chr_state().

But that shouldn't be necessary anyway, since pty_chr_timer() returns
FALSE, and its associated tag (s->timer_tag, see pty_chr_rearm_timer())
is removed anyway.

Seems OK to me.

Reviewed-by: Laszlo Ersek <lersek@redhat.com>

Re: [Qemu-devel] [PATCH] chardev: fix pty_chr_timer

[Gerd Hoffmann]

On Do, 2013-08-22 at 11:57 +0200, Gerd Hoffmann wrote:
> pty_chr_timer first calls pty_chr_update_read_handler(), then clears
> timer_tag (because it is a one-shot timer).   This is the wrong order
> though.  pty_chr_update_read_handler might re-arm time timer, and the
> new timer_tag gets overwitten in that case.
> 
> This leads to crashes when unplugging a pty chardev:  pty_chr_close
> thinks no timer is running -> timer isn't canceled -> pty_chr_timer gets
> called with stale CharDevState -> BOOM.
> 
> This patch fixes the ordering.
> Kill the pointless goto while being at it.

Ping?

> 
> https://bugzilla.redhat.com/show_bug.cgi?id=994414
> 
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> ---
>  qemu-char.c | 12 ++++--------
>  1 file changed, 4 insertions(+), 8 deletions(-)
> 
> diff --git a/qemu-char.c b/qemu-char.c
> index 1be1cf6..1621fbd 100644
> --- a/qemu-char.c
> +++ b/qemu-char.c
> @@ -1026,15 +1026,11 @@ static gboolean pty_chr_timer(gpointer opaque)
>      struct CharDriverState *chr = opaque;
>      PtyCharDriver *s = chr->opaque;
>  
> -    if (s->connected) {
> -        goto out;
> -    }
> -
> -    /* Next poll ... */
> -    pty_chr_update_read_handler(chr);
> -
> -out:
>      s->timer_tag = 0;
> +    if (!s->connected) {
> +        /* Next poll ... */
> +        pty_chr_update_read_handler(chr);
> +    }
>      return FALSE;
>  }
>