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

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Date:   Tue, 28 Nov 2023 15:57:51 +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 20.11.2023 17.33, Mathias Nyman wrote:
> 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

I moved the max packet checks away from xhci_urb_enqueue() and fixed up the locking.

I can't trigger the original issue, but I tested it by setting incorrect initial max packet
size values.

If you have the chance to test this with your setup I'd appreciate it.

patches found here:
git://git.kernel.org/pub/scm/linux/kernel/git/mnyman/xhci.git fix_urb_enqueue_locking
https://git.kernel.org/pub/scm/linux/kernel/git/mnyman/xhci.git/log/?h=fix_urb_enqueue_locking

I'll add them to this thread as well

thanks
Mathias