Re: [PATCH] sh: mm: Fix unprotected access to struct device

[Thomas Petazzoni]

Hello,

On Tue, 17 Apr 2018 15:35:23 +0200, Jacopo Mondi wrote:
> With commit ce88313069c36eef80f21fd7 ("arch/sh: make the DMA mapping
> operations observe dev->dma_pfn_offset") the generic DMA allocation
> function on which the SH 'dma_alloc_coherent()' function relies on,
> access the 'dma_pfn_offset' field of struct device.
> 
> Unfortunately the 'dma_generic_alloc_coherent()' function is called from
> several places with a NULL struct device argument, halting the CPU
> during the boot process.
> 
> This patch fixes the issue protecting access to dev->dma_pfn_offset,
> with a trivial check for validity. It also passes a valid 'struct device'
> in the 'platform_resource_setup_memory' function which is the main user
> of 'dma_alloc_coherent()', and inserting a WARN_ON() check to make future
> (and existing) bogus users of this function they're should provide a valid
> 'struct device' whenever possible.
> 
> Fixes: ce88313069c36eef80f21fd7 ("arch/sh: make the DMA mapping operations observe dev->dma_pfn_offset")
> Signed-off-by: Jacopo Mondi <jacopo+renesas@xxxxxxxxxx>

I would have done two commits here, one to fix:

  dma_alloc_coherent(&pdev->dev, memsize, &dma_handle, GFP_KERNEL);

and one to switch to the WARN_ON + if(dev) model. But I don't really
care either way, so:

Reviewed-by: Thomas Petazzoni <thomas.petazzoni@xxxxxxxxxxx>

Note that even with the if (dev) check, you don't avoid all possible
regressions. For example, some parts of the sh_eth driver were passing
a non-NULL struct device, but it was the wrong struct device (the one
inside struct net_device, and not the one part of struct
platform_device). I fixed that for sh_eth, but there could be other
drivers doing bogus things.

Best regards,

Thomas
-- 
Thomas Petazzoni, CTO, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com