Re: [PATCH] dma-mapping: Lift address space checks out of debug code

[Kees Cook <keescook@chromium.org>]

On Wed, Oct 02, 2019 at 10:15:43PM +0100, Robin Murphy wrote:
> Hi Kees,
> 
> On 2019-10-02 9:46 pm, Kees Cook wrote:
> > As we've seen from USB and other areas, we need to always do runtime
> > checks for DMA operating on memory regions that might be remapped. This
> > consolidates the (existing!) checks and makes them on by default. A
> > warning will be triggered for any drivers still using DMA on the stack
> > (as has been seen in a few recent reports).
> > 
> > Suggested-by: Laura Abbott <labbott@redhat.com>
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> >   include/linux/dma-debug.h   |  8 --------
> >   include/linux/dma-mapping.h |  8 +++++++-
> >   kernel/dma/debug.c          | 16 ----------------
> >   3 files changed, 7 insertions(+), 25 deletions(-)
> > 
> > diff --git a/include/linux/dma-debug.h b/include/linux/dma-debug.h
> > index 4208f94d93f7..2af9765d9af7 100644
> > --- a/include/linux/dma-debug.h
> > +++ b/include/linux/dma-debug.h
> > @@ -18,9 +18,6 @@ struct bus_type;
> >   extern void dma_debug_add_bus(struct bus_type *bus);
> > -extern void debug_dma_map_single(struct device *dev, const void *addr,
> > -				 unsigned long len);
> > -
> >   extern void debug_dma_map_page(struct device *dev, struct page *page,
> >   			       size_t offset, size_t size,
> >   			       int direction, dma_addr_t dma_addr);
> > @@ -75,11 +72,6 @@ static inline void dma_debug_add_bus(struct bus_type *bus)
> >   {
> >   }
> > -static inline void debug_dma_map_single(struct device *dev, const void *addr,
> > -					unsigned long len)
> > -{
> > -}
> > -
> >   static inline void debug_dma_map_page(struct device *dev, struct page *page,
> >   				      size_t offset, size_t size,
> >   				      int direction, dma_addr_t dma_addr)
> > diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> > index 4a1c4fca475a..2d6b8382eab1 100644
> > --- a/include/linux/dma-mapping.h
> > +++ b/include/linux/dma-mapping.h
> > @@ -583,7 +583,13 @@ 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)
> >   {
> > -	debug_dma_map_single(dev, ptr, size);
> > +	/* DMA must never operate on stack or other remappable places. */
> > +	WARN_ONCE(is_vmalloc_addr(ptr) || !virt_addr_valid(ptr),
> 
> This stands to absolutely cripple I/O performance on arm64, because every
> valid call will end up going off and scanning the memblock list, which is
> not something we want on a fastpath in non-debug configurations. We'd need a
> much better solution to the "pfn_valid() vs. EFI no-map" problem before this
> might be viable.

Ah! Interesting. I didn't realize this was fast-path (I don't know the
DMA code at all). I thought it was more of a "one time setup" before
actual DMA activity started.

Regardless, is_vmalloc_addr() is extremely light (a bounds check), and is the
most important part of this as far as catching stack-based DMA attempts.
I thought virt_addr_valid() was cheap too, but I see it's much heavier on
arm64.

I just went to compare what the existing USB check does, and it happens
immediately before its call to dma_map_single(). Both checks are simple
bounds checks, so it shouldn't be an issue:

			if (is_vmalloc_addr(urb->setup_packet)) {
				WARN_ONCE(1, "setup packet is not dma capable\n");
				return -EAGAIN;
			} else if (object_is_on_stack(urb->setup_packet)) {
				WARN_ONCE(1, "setup packet is on stack\n");
				return -EAGAIN;
			}

			urb->setup_dma = dma_map_single(
					hcd->self.sysdev,
					urb->setup_packet,
					sizeof(struct usb_ctrlrequest),


In the USB case, it'll actually refuse to do the operation. Should
dma_map_single() similarly fail? I could push these checks down into
dma_map_single(), which would be a no-change on behavior for USB and
gain the checks on all other callers...

-- 
Kees Cook
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