Re: [Linux kernel bug] general protection fault in disable_store

[Sam Sun]

On Wed, Apr 17, 2024 at 12:35 AM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, Apr 16, 2024 at 05:05:55PM +0800, Sam Sun wrote:
> > On Mon, Apr 15, 2024 at 10:47 PM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > Actually, I've got a completely different patch which I think will fix
> > > the problem you encountered.  Instead of using mutual exclusion to
> > > avoid the race, it prevents the two routines from being called at the
> > > same time so the race can't occur in the first place.  It also should
> > > guarantee the usb_hub_to_struct_hub() doesn't return NULL when
> > > disable_store() calls it.
> > >
> > > Can you try the patch below, instead of (not along with) the first
> > > patch?  Thanks.
> > >
> > > Alan Stern
>
> > I tried this patch and it worked. I agree this patch is better and it
> > avoids introducing new locks.
>
> It turns out that patch is no good.  The reason is mentioned in the
> changelog for commit 543d7784b07f ("USB: fix race between hub_disconnect
> and recursively_mark_NOTATTACHED"); it says that the port devices have to
> be removed _after_ maxchild has been set to 0.
>

I checked the commit you mentioned. Maybe your first fix is all we
need to fix the problem? At least no race would occur for
hdev->maxchild and usb_set_intfdata().

> So okay, here's a third attempt to fix the problem.  This doesn't try to
> avoid the race or anything like that.  Instead it just adds checks for
> usb_hub_to_struct_hub() returning NULL.  That should be enough to prevent
> the invalid pointer dereference you encountered.
>
> This should be tested by itself, without either of the first two patches.
>
> Alan Stern
>
>
>
> Index: usb-devel/drivers/usb/core/port.c
> ===================================================================
> --- usb-devel.orig/drivers/usb/core/port.c
> +++ usb-devel/drivers/usb/core/port.c
> @@ -51,13 +51,15 @@ static ssize_t disable_show(struct devic
>         struct usb_port *port_dev = to_usb_port(dev);
>         struct usb_device *hdev = to_usb_device(dev->parent->parent);
>         struct usb_hub *hub = usb_hub_to_struct_hub(hdev);
> -       struct usb_interface *intf = to_usb_interface(hub->intfdev);
> +       struct usb_interface *intf = to_usb_interface(dev->parent);
>         int port1 = port_dev->portnum;
>         u16 portstatus, unused;
>         bool disabled;
>         int rc;
>         struct kernfs_node *kn;
>
> +       if (!hub)
> +               return -ENODEV;
>         hub_get(hub);
>         rc = usb_autopm_get_interface(intf);
>         if (rc < 0)
> @@ -101,12 +103,14 @@ static ssize_t disable_store(struct devi
>         struct usb_port *port_dev = to_usb_port(dev);
>         struct usb_device *hdev = to_usb_device(dev->parent->parent);
>         struct usb_hub *hub = usb_hub_to_struct_hub(hdev);
> -       struct usb_interface *intf = to_usb_interface(hub->intfdev);
> +       struct usb_interface *intf = to_usb_interface(dev->parent);
>         int port1 = port_dev->portnum;
>         bool disabled;
>         int rc;
>         struct kernfs_node *kn;
>
> +       if (!hub)
> +               return -ENODEV;
>         rc = kstrtobool(buf, &disabled);
>         if (rc)
>                 return rc;
>

I applied this patch and it also can fix the warning. I am not sure
which one is better.

Best,
Yue