Re: [PATCH] xhci: fix null pointer deref for xhci_urb_enqueue

[Mathias Nyman]

On 18.11.2023 12.19, Kuen-Han Tsai wrote:
> Hi Mathias
>
> > >        if (usb_endpoint_xfer_isoc(&urb->ep->desc))
> > > @@ -1552,8 +1561,10 @@ static int xhci_urb_enqueue(struct usb_hcd *hcd, struct urb *urb, gfp_t mem_flag
> > >                num_tds = 1;
> > >
> > >        urb_priv = kzalloc(struct_size(urb_priv, td, num_tds), mem_flags);
> > kzalloc with spinlock held, should preferably be moved outside lock, otherwise should use GFP_ATOMIC
>
> Thanks for pointing this out. I realize this patch is incorrect and it
> is non-ideal to include many codes unrelated to xhci->devs[slot_id]
> within the lock.
>
> > xhci_check_maxpacket() called here can't be called with spinlock held
>
> It appears that xhci_check_maxpacket() might potentially lead to a
> deadlock later if a spinlock is held. Is this the concern you were
> referring to? If not, please let me know if there are any other
> potential issues that I may have missed, thanks!

xhci_check_maxpacket() will allocate memory, wait for completion, and use the same lock,
so there are several issues here.

I actually think we shouldn't call xhci_check_maxpacket() at all while queuing urbs.

usb core knows when there was max packet size mismatch during enumeration.
I think we should add a hook to the hcd that usb core can call in these cases

Thanks
Mathias