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Subject: Re: Validating dma_mmap_coherent() parameters before calling (was Re:
 WARNING in memtype_reserve)
To:     Christoph Hellwig <hch@lst.de>
Cc:     Greg KH <gregkh@linuxfoundation.org>,
        Hillf Danton <hdanton@sina.com>,
        syzbot <syzbot+353be47c9ce21b68b7ed@syzkaller.appspotmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org, linux-usb@vger.kernel.org,
        syzkaller-bugs@googlegroups.com, x86@kernel.org
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From:   Jeremy Linton <jeremy.linton@arm.com>
Message-ID: <83544950-e946-b696-9cf4-c7df097332b4@arm.com>
Date:   Thu, 14 May 2020 06:16:10 -0500
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On 5/14/20 6:14 AM, Christoph Hellwig wrote:
> On Thu, May 14, 2020 at 06:10:03AM -0500, Jeremy Linton wrote:
>>> I only need to look at the commit for 3 seconds to tell you that it is
>>> completely buggy.  While using dma_mmap_coherent is fundamentally the
>>> right thing and absolutely required for dma_alloc_* allocations, USB
>>> also uses it's own local gen pool allocator or plain kmalloc for not
>>> DMA capable controller.  This need to use remap_pfn_range.  I'm pretty
>>> sure you hit one of those cases.
>>
>> ? The code path in question is usbdev_mmap() and the allocation is done ~13
>> lines lines before as a usb_alloc_coherent().
> 
> And did you take a look at how usb_alloc_coherent is implemented?  That
> should make it completely obvious that not all allocations come
> from dma_alloc_*.

No, your right, I noticed/remembered the usb_alloc vs dma_alloc 
difference right after sending that email, and was just about to say so.

  Sorry, you right.

> 
>> That sort of makes sense, except for the above, and the fact that I would
>> imagine the dma_mmap_coherent should be dealing with that case. I'm not
>> really clear about the details of the GCE usb device here, but my first
>> guess at this was the dma_pgprot() in dma_direct_mmap() is incorrectly
>> picking a pgprot...
> 
> No, dma_mmap_* / dma_direct_mmap has absolutely no business dealing
> with memory that did not come from the DMA allocator.
>