Re: [PATCH v3 1/2] dma-mapping: Add vmap checks to dma_map_single()

[Robin Murphy]

On 2019-10-11 6:02 am, Greg Kroah-Hartman wrote:
> On Thu, Oct 10, 2019 at 03:28:28PM -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..ff4e91c66f44 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 (unlikely(is_vmalloc_addr(ptr))) {
> > +		dev_warn_once(dev, "bad map: %zu bytes in vmalloc\n", size);
>
> Can we get a bit better error text here?  In USB we were at least giving
> people a hint as to what went wrong, "bad map" might not really make
> that much sense to a USB developer as to what they needed to do to fix
> this issue.

Agreed, something along the lines of "dma_map_single of vmalloc'ed 
buffer..." might be clearer. Also I'd be inclined to use dev_WARN_ONCE() 
to include a diagnostically-useful backtrace, as the existing USB code 
would.

Thanks,
Robin.