Messages in this thread | | | Subject | Re: [PATCH] dma-mapping: Lift address space checks out of debug code | From | Robin Murphy <> | Date | Fri, 4 Oct 2019 19:50:54 +0100 |
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On 03/10/2019 22:38, Kees Cook wrote: > On Thu, Oct 03, 2019 at 10:42:45AM +0100, Robin Murphy wrote: >> 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. > > What do you think about the object_is_on_stack() check? That does a > dereference through "current" to find the stack bounds...
I guess it depends what the aim is - is it just to bail out of operations which have near-zero chance of working correctly and every chance of going catastrophically wrong, or to lay down strict argument checking for the API in general? (for cache-coherent devices, or if the caller is careful to ensure the appropriate alignment, DMA from a non-virtually-mapped stack can be *technically* fine, it's just banned in general because those necessary assumptions can be tricky to meet and aren't at all portable).
Robin.
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