Re: Validating dma_mmap_coherent() parameters before calling (was Re: WARNING in memtype_reserve)

Christoph Hellwig
2020-05-14T06:32:03Z

On Thu, May 14, 2020 at 08:27:50AM +0200, Greg KH wrote:
> On Thu, May 14, 2020 at 08:14:17AM +0200, Christoph Hellwig wrote:
> > Guys, can you please start formal thread on this? I have no
> > idea where this came from and what the rationale is. Btw, if the
> > pfn is crap in dma_direct_mmap then the dma_addr_t passed in is
> > crap, as it is derived from that. What is the caller, and how is
> > this triggered?
>
>
> Ok, to summarize, commit 2bef9aed6f0e ("usb: usbfs: correct kernel->user
> page attribute mismatch") changed a call from remap_pfn_range() to
> dma_mmap_coherent() for usb data buffers being sent from userspace.

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 logic should be something like:

if (hcd->localmem_pool || !hcd_uses_dma(hcd))
remap_pfn_range()
else
dma_mmap_coherent()