On 03/10/17 13:55, David Woodhouse wrote: > On Thu, 2017-09-28 at 15:14 +0100, Robin Murphy wrote: >> The intel-iommu DMA ops fail to correctly handle scatterlists where >> sg->offset is greater than PAGE_SIZE - the IOVA allocation is computed >> appropriately based on the page-aligned portion of the offset, but the >> mapping is set up relative to sg->page, which means it fails to actually >> cover the whole buffer (and in the worst case doesn't cover it at all): >> >> (sg->dma_address + sg->dma_len) ----+ >> sg->dma_address ---------+ | >> iov_pfn------+ | | >> | | | >> v v v >> iova: a b c d e f >> |--------|--------|--------|--------|--------| >> <...calculated....> >> [_____mapped______] >> pfn: 0 1 2 3 4 5 >> |--------|--------|--------|--------|--------| >> ^ ^ ^ >> | | | >> sg->page ----+ | | >> sg->offset --------------+ | >> (sg->offset + sg->length) ----------+ > > I'd still dearly love to see some clear documentation of what it means > for sg->offset to be outside the page referenced by sg->page. I think the key is that for each SG segment, sg->page doesn't necessarily represent "a" page, but the first of one or more contiguous pages. Disregarding offsets for the moment, Here's a typical example of a 120KB buffer from the block layer as processed by iommu_dma_map_sg(): [ 16.092649] == initial (4) == [ 16.095591] 0: virt ffff800001372000 phys 0x0000000081372000 dma 0x0000000000000000 [ 16.095591] offset 0x00000000 length 0x0000e000 dma_len 0x00000000 [ 16.109541] 1: virt ffff800001380000 phys 0x0000000081380000 dma 0x0000000000000000 [ 16.109541] offset 0x00000000 length 0x0000d000 dma_len 0x00000000 [ 16.123491] 2: virt ffff80000138e000 phys 0x000000008138e000 dma 0x0000000000000000 [ 16.123491] offset 0x00000000 length 0x00002000 dma_len 0x00000000 [ 16.137440] 3: virt ffff800001390000 phys 0x0000000081390000 dma 0x0000000000000000 [ 16.137440] offset 0x00000000 length 0x00001000 dma_len 0x00000000 [ 16.216167] == final (2) == [ 16.219106] 0: virt ffff800001372000 phys 0x0000000081372000 dma 0x00000000ffb60000 [ 16.219106] offset 0x00000000 length 0x0000e000 dma_len 0x0000e000 [ 16.233056] 1: virt ffff800001380000 phys 0x0000000081380000 dma 0x00000000ffb70000 [ 16.233056] offset 0x00000000 length 0x0000d000 dma_len 0x00010000 i.e. segments of 14 pages, 13 pages, 2 pages and 1 page respectively (and we further merge the resulting DMA-contiguous segments on top of that). Now, there are indeed plenty of drivers and subsystems which do work on lists of explicitly single pages - anything doing some variant of "addr = kmap_atomic(sg_page(sg)) + sg->offset;" is easy to spot - but I don't think DMA API implementations are in a position to make any kind of assumption; nearly all of them just shut up and handle sg->length bytes from sg_phys(sg) without questioning the caller, and I reckon that's exactly what they should be doing. > Or is it really not "outside", and it's *only* valid for the offset to > be > PAGE_OFFSET when it's a huge page, so we can check that with a > BUG_ON() ? > > In particular, I'd like to know what is intended in the Xen PV case, > where there isn't a straight correspondence between pfn and mfn. Is the > out-of-range sg->offset intended to refer to the next *pfn* after sg- >> page, or to the next *mfn* after sg->page? Logically, it should mean the same thing as whatever a length of more than 1 page means to Xen - judging by blkif_queue_rw_req() at least, that seems to be a BUG_ON() in both cases. > I confess I've only followed this thread vaguely, but I haven't seen a > *coherent* explanation except in the huge page case (in which case I > want to see that BUG_ON in the patch) of why this isn't just totally > bogus. As I've said before, I'd certainly consider it a denormalised case, but not a bogus one, and certainly not something that is the DMA API's job to police. Having now audited every dma_map_ops::map_sg implementation I could find, the only ones not using sg_phys()/sg_virt() or some other construction immune to the absolute offset value (MIPS even explicitly normalises it) are intel-iommu and arch/frv, and the latter is clearly broken anyway as it ignores sg->length. Robin.
