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    SubjectRe: [PATCH] xen/gntdev.c: Replace vm_map_pages() with vm_map_pages_zero()
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    On 30.07.19 20:34, Souptick Joarder wrote:
    > 'commit df9bde015a72 ("xen/gntdev.c: convert to use vm_map_pages()")'
    > breaks gntdev driver. If vma->vm_pgoff > 0, vm_map_pages()
    > will:
    > - use map->pages starting at vma->vm_pgoff instead of 0
    > - verify map->count against vma_pages()+vma->vm_pgoff instead of just
    > vma_pages().
    >
    > In practice, this breaks using a single gntdev FD for mapping multiple
    > grants.
    >
    > relevant strace output:
    > [pid 857] ioctl(7, IOCTL_GNTDEV_MAP_GRANT_REF, 0x7ffd3407b6d0) = 0
    > [pid 857] mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, 7, 0) =
    > 0x777f1211b000
    > [pid 857] ioctl(7, IOCTL_GNTDEV_SET_UNMAP_NOTIFY, 0x7ffd3407b710) = 0
    > [pid 857] ioctl(7, IOCTL_GNTDEV_MAP_GRANT_REF, 0x7ffd3407b6d0) = 0
    > [pid 857] mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, 7,
    > 0x1000) = -1 ENXIO (No such device or address)
    >
    > details here:
    > https://github.com/QubesOS/qubes-issues/issues/5199
    >
    > The reason is -> ( copying Marek's word from discussion)
    >
    > vma->vm_pgoff is used as index passed to gntdev_find_map_index. It's
    > basically using this parameter for "which grant reference to map".
    > map struct returned by gntdev_find_map_index() describes just the pages
    > to be mapped. Specifically map->pages[0] should be mapped at
    > vma->vm_start, not vma->vm_start+vma->vm_pgoff*PAGE_SIZE.
    >
    > When trying to map grant with index (aka vma->vm_pgoff) > 1,
    > __vm_map_pages() will refuse to map it because it will expect map->count
    > to be at least vma_pages(vma)+vma->vm_pgoff, while it is exactly
    > vma_pages(vma).
    >
    > Converting vm_map_pages() to use vm_map_pages_zero() will fix the
    > problem.
    >
    > Marek has tested and confirmed the same.
    >
    > Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
    > Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
    > Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>

    Pushed to xen/tip.git for-linus-5.3a


    Juergen

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