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

[Robin Murphy <robin.murphy@arm.com>]

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.