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

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Date:   Tue, 17 Apr 2018 15:59:20 +0200
From:   jacopo mondi <jacopo@...ndi.org>
To:     Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Cc:     Jacopo Mondi <jacopo+renesas@...ndi.org>,
        ysato@...rs.sourceforge.jp, dalias@...c.org,
        thomas.petazzoni@...e-electrons.com, robin.murphy@....com,
        geert@...ux-m68k.org, linux-renesas-soc@...r.kernel.org,
        linux-sh@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sh: mm: Fix unprotected access to struct device

Hi Thomas,

On Tue, Apr 17, 2018 at 03:54:07PM +0200, Thomas Petazzoni wrote:
> 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@...ndi.org>
>
> 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:

I thought about doing the same, but as this commit is a fix to be
applied on top of v4.17-rc1, and it's likely being fast tracked as it
breaks SH architecture (at least SH7722) I thought it was good to keep
all of that in a single commit.

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

Thank you

> 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.

Well, not that much we can do here for other bogus users, right?

Thanks
   j

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

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