Re: amd5536udc interrupts bug

[Robert Hancock <hancockr@shaw.ca>]

Vadim Lobanov wrote:
> Hello,
> 
> From perusing the code and playing with the module, it seems to me that 
> the amd5536udc driver's handling of interrupts is currently "bustificated".
> 
> The long story:
> 
> During the amd5536udc initialization sequence within udc_pci_probe(), 
> the code attempts to request a shared irq for the device thusly:
> 	request_irq(pdev->irq, udc_irq, IRQF_SHARED, name, dev)
> where 'dev' is the internal state structure. By the time this call is made, 
> the 'dev' structure is still not fully initialized and contains blank/zero 
> data for many of the fields, in particular both 'dev->lock' and 'dev->regs' 
> which are both clearly used within the udc_irq() handler. Those get 
> initialized a bit later, namely inside the udc_probe() call at the bottom of 
> udc_pci_probe().
> 
> It is my understanding that a handler for a shared interrupt can be 
> invoked at any time after the corresponding request_irq() call is made, 
> simply because some other device on the same interrupt may already 
> be active. This leaves us with a very noticeable race condition, where 
> udc_irq() can be invoked with uninitialized 'dev' data.

Yes, your analysis appears correct.

> 
> In particular, this effect is very noticeable when I try modprobing the 
> amd5536udc driver on a kernel that is built with CONFIG_DEBUG_SHIRQ 
> set. Given that the debug config option forces an invocation of the irq 
> handler from within the request_irq() function, the immediate effect is a 
> masterful kernel NULL-dereference OOPS within udc_irq().
> 
> The simple fix may be to say that amd5536udc does not support shared 
> irqs, and to remove the IRQF_SHARED flag from the request_irq() call. A 

That will bust any other hardware that tries to share the interrupt. If 
a driver requests the interrupt without IRQF_SHARED, nothing else can 
request that interrupt line.

> more complicated fix may be to try to shuffle all the code around to 
> make sure that 'dev' is fully initialized before we request the irq, but I 
> don't understand the code well enough (yet) to comfortably do this.

Yeah, that's the proper fix.

> 
> Comments? Thoughts?
> 
> On a side note, it occurs to me that the CONFIG_DEBUG_SHIRQ option 
> went into the kernel a bit less than two years ago, and that it exposes a 
> very immediate and reproducible OOPS in this driver. Does this mean 
> that noone uses the 5536 UDC functionality with any recent kernels? 
> Should I be worried? :)

Presumably nobody uses it with CONFIG_DEBUG_SHIRQ, that option wouldn't 
normally be used on non-debug kernels..

> 
> -- Vadim Lobanov
>