Re: [PATCH] xen/gntdev.c: Replace vm_map_pages() with vm_map_pages_zero()

Juergen Gross
2019-07-31T06:15:41Z

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