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    SubjectRe: [PATCH] iommu/vt-d: Fix scatterlist offset handling
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    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.

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