Re: [PATCH] dma-direct: Fix potential NULL pointer dereference

[Thomas Tai]

On 2020-09-15 11:09 a.m., Christoph Hellwig wrote:
> On Tue, Sep 15, 2020 at 10:40:39AM -0400, Thomas Tai wrote:
> > > +++ b/include/linux/dma-direct.h
> > > @@ -62,9 +62,6 @@ static inline bool dma_capable(struct device *dev, dma_addr_t addr, size_t size,
> > >    {
> > >    	dma_addr_t end = addr + size - 1;
> > >    -	if (!dev->dma_mask)
> > > -		return false;
> > > -
> >
> > I am concerned that some drivers may rely on this NULL checking. Would you
> > think we can keep this checking and use the following WARN_ON_ONCE()?
>
> dma_capable is not a helper for drivers, but just for dma-direct
> and related code.  And this patch adds the checks for the three
> places how we call into the ->map* methods.

Hi Christoph,
I tried out the suggested changes, and it successfully warned the null 
pointer without panic. I notice that there are some places outside the 
dma-direct, which calls dma_capable().

https://elixir.bootlin.com/linux/v5.9-rc5/source/arch/x86/kernel/amd_gart_64.c#L187

https://elixir.bootlin.com/linux/v5.9-rc5/source/drivers/xen/swiotlb-xen.c#L387

Also, if I remove the null checking in dma_capable(), I may run into the 
risk of a null pointer dereference within the function.

@@ -62,9 +62,6 @@ static inline bool dma_capable(struct device *dev, 
dma_addr_t addr, size_t size,
 {
 	dma_addr_t end = addr + size - 1;

-	if (!dev->dma_mask)
-		return false;
-
 	if (is_ram && !IS_ENABLED(CONFIG_ARCH_DMA_ADDR_T_64BIT) &&
 	    min(addr, end) < phys_to_dma(dev, PFN_PHYS(min_low_pfn)))
 		return false;
	
	return end <= min_not_zero(*dev->dma_mask, dev->bus_dma_limit);
                                    ^
                                    |
                                    ** risk of a null dereference **
}


Given that the WARN_ON_ONCE already did the intended warning, would you 
be ok that I keep the null checking in dma_capable()?

Thank you,
Thomas