linux-kernel - Re: [PATCH] xhci: fix null pointer deref for xhci_urb

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Date:   Mon, 20 Nov 2023 17:33:23 +0200
From:   Mathias Nyman <mathias.nyman@...ux.intel.com>
To:     Kuen-Han Tsai <khtsai@...gle.com>
Cc:     mathias.nyman@...el.com, gregkh@...uxfoundation.org,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        alan Stern <stern@...land.harvard.edu>
Subject: Re: [PATCH] xhci: fix null pointer deref for xhci_urb_enqueue

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