Subject: Re: [PATCH] xhci-mtk: Fix NULL pointer dereference with xhci_irq() for shared_hcd

From: Macpaul Lin <[email-masked]>
Date: Sat, 1 Feb 2020 19:20:39 +0800

On Fri, 2020-01-31 at 16:50 +0200, Mathias Nyman wrote:
> On 17.1.2020 9.41, Macpaul Lin wrote:
> > According to NULL pointer fix: https://tinyurl.com/uqft5ra
> > xhci: Fix NULL pointer dereference with xhci_irq() for shared_hcd
> > The similar issue has also been found in QC activities in Mediatek.
> > 
> > Here quote the description from the referenced patch as follows.
> > "Commit ("f068090426ea xhci: Fix leaking USB3 shared_hcd
> > at xhci removal") sets xhci_shared_hcd to NULL without
> > stopping xhci host. This results into a race condition
> > where shared_hcd (super speed roothub) related interrupts
> > are being handled with xhci_irq happens when the
> > xhci_plat_remove is called and shared_hcd is set to NULL.
> > Fix this by setting the shared_hcd to NULL only after the
> > controller is halted and no interrupts are generated."
> > 
> > Signed-off-by: Sriharsha Allenki <[email-masked]>
> > Signed-off-by: Macpaul Lin <[email-masked]>
> > ---
> >   drivers/usb/host/xhci-mtk.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/usb/host/xhci-mtk.c b/drivers/usb/host/xhci-mtk.c
> > index b18a6baef204..c227c67f5dc5 100644
> > --- a/drivers/usb/host/xhci-mtk.c
> > +++ b/drivers/usb/host/xhci-mtk.c
> > @@ -593,11 +593,11 @@ static int xhci_mtk_remove(struct platform_device *dev)
> >   	struct usb_hcd  *shared_hcd = xhci->shared_hcd;
> >   
> >   	usb_remove_hcd(shared_hcd);
> > -	xhci->shared_hcd = NULL;
> >   	device_init_wakeup(&dev->dev, false);
> >   
> >   	usb_remove_hcd(hcd);
> >   	usb_put_hcd(shared_hcd);
> > +	xhci->shared_hcd = NULL;
> >   	usb_put_hcd(hcd);
> >   	xhci_mtk_sch_exit(mtk);
> >   	xhci_mtk_clks_disable(mtk);
> > 
> 
> Could you share details of the NULL pointer dereference, (backtrace).

This bug was found by our QA staff while doing 500 times plug-in and
plug-out devices. The backtrace I have was recorded by QA and I didn't
reproduce this issue on my own environment. However, after applied this
patch the issue seems resolve. Here is the backtrace:

Exception Class: Kernel (KE)
PC is at [<ffffff8008cccbc0>] xhci_irq+0x728/0x2364
LR is at [<ffffff8008ccc788>] xhci_irq+0x2f0/0x2364

Current Executing Process:
[iptables, 859][netdagent, 770]

Backtrace:
[<ffffff80080ead58>] __atomic_notifier_call_chain+0xa8/0x130   
[<ffffff80080eb6d4>] notify_die+0x84/0xac      
[<ffffff800808e874>] die+0x1d8/0x3b8   
[<ffffff80080a89b0>] __do_kernel_fault+0x178/0x188     
[<ffffff80080a81b4>] do_page_fault+0x44/0x3b0  
[<ffffff80080a811c>] do_translation_fault+0x44/0x98    
[<ffffff8008080e08>] do_mem_abort+0x4c/0x128   
[<ffffff80080832d0>] el1_da+0x24/0x3c  
[<ffffff8008cccbc0>] xhci_irq+0x728/0x2364     
[<ffffff8008c98804>] usb_hcd_irq+0x2c/0x44     
[<ffffff8008179bb0>] __handle_irq_event_percpu+0x26c/0x4a4     
[<ffffff8008179ec8>] handle_irq_event+0x5c/0xd0
[<ffffff800817e3c0>] handle_fasteoi_irq+0x10c/0x1e0    
[<ffffff80081787b0>] __handle_domain_irq+0x32c/0x738   
[<ffffff800808159c>] gic_handle_irq+0x174/0x1c4
[<ffffff8008083cf8>] el0_irq_naked+0x50/0x5c   
[<ffffffffffffffff>] 0xffffffffffffffff

> The USB3 hcd is already removed when xhci->shared_hcd is set to NULL.
> We might want to add some checks to make sure we are not using the removed
> hcd anymore in that codepath anymore.
> 
> -Mathias

Thanks,
Macpaul Lin

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