Re: [PATCH v4 1/2] dma-mapping: Add vmap checks to dma_map_single()

[Kees Cook]

On Wed, Oct 30, 2019 at 07:09:21PM +0100, Christoph Hellwig wrote:
> On Wed, Oct 30, 2019 at 10:18:49AM +0100, Greg Kroah-Hartman wrote:
> > On Tue, Oct 29, 2019 at 02:34:22PM -0700, Kees Cook wrote:
> > > As we've seen from USB and other areas[1], we need to always do runtime
> > > checks for DMA operating on memory regions that might be remapped. This
> > > adds vmap checks (similar to those already in USB but missing in other
> > > places) into dma_map_single() so all callers benefit from the checking.
> > > 
> > > [1] https://git.kernel.org/linus/3840c5b78803b2b6cc1ff820100a74a092c40cbb
> > > 
> > > Suggested-by: Laura Abbott <labbott@xxxxxxxxxx>
> > > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > > ---
> > >  include/linux/dma-mapping.h | 6 ++++++
> > >  1 file changed, 6 insertions(+)
> > > 
> > > diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> > > index 4a1c4fca475a..54de3c496407 100644
> > > --- a/include/linux/dma-mapping.h
> > > +++ b/include/linux/dma-mapping.h
> > > @@ -583,6 +583,12 @@ static inline unsigned long dma_get_merge_boundary(struct device *dev)
> > >  static inline dma_addr_t dma_map_single_attrs(struct device *dev, void *ptr,
> > >  		size_t size, enum dma_data_direction dir, unsigned long attrs)
> > >  {
> > > +	/* DMA must never operate on areas that might be remapped. */
> > > +	if (dev_WARN_ONCE(dev, is_vmalloc_addr(ptr),
> > > +			  "wanted %zu bytes mapped in vmalloc\n", size)) {
> > > +		return DMA_MAPPING_ERROR;
> > > +	}
> > 
> > That's a very odd error string, I know if I saw it for the first time, I
> > would have no idea what it meant.  The USB message at least gives you a
> > bit more context as to what went wrong and how to fix it.
> > 
> > How about something like "Memory is not DMA capabable, please fix the
> > allocation of it to be correct", or "non-dma-able memory was attempted
> > to be mapped, but this is impossible to to" or something else.
> 
> I've fixed the message to "rejecting DMA map of vmalloc memory" and
> applied the patch.

Great; thank you!

-- 
Kees Cook