Messages in this thread | | | Subject | Re: [PATCH] dma-mapping: Lift address space checks out of debug code | From | Robin Murphy <> | Date | Thu, 3 Oct 2019 10:42:45 +0100 |
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On 03/10/2019 00:58, Kees Cook wrote: > 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.
That's strictly true, it's just that many workloads can involve tens of thousands of "one time"s per second ;)
Overhead on the dma_map_* paths has shown to have a direct impact on throughput in such situations, hence various optimisation effort in IOVA allocation for IOMMU-based DMA ops, and the recent work to remove indirect calls entirely for the common dma-direct/SWIOTLB cases.
> 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...
I think it would be reasonable to pull the is_vmalloc_addr() check inline, as that probably covers 90+% of badness (especially given vmapped stacks), and as you say should be reliably cheap everywhere. Callers are certainly expected to use dma_mapping_error() and handle failure, so refusing to do a bogus mapping operation should be OK API-wise - ultimately if a driver goes ahead and uses DMA_MAPPING_ERROR as an address anyway, that's not likely to be any *more* catastrophic than if it did the same with whatever nonsense virt_to_phys() of a vmalloc address had returned.
Robin.
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