• Souptick Joarder's avatar
    xen/gntdev.c: Replace vm_map_pages() with vm_map_pages_zero() · 8d1502f6
    Souptick Joarder authored
    'commit df9bde01 ("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.
    
    Cc: stable@vger.kernel.org # v5.2+
    Fixes: df9bde01 ("xen/gntdev.c: convert to use vm_map_pages()")
    Reported-by: default avatarMarek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
    Signed-off-by: default avatarSouptick Joarder <jrdr.linux@gmail.com>
    Tested-by: default avatarMarek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
    Reviewed-by: default avatarBoris Ostrovsky <boris.ostrovsky@oracle.com>
    Signed-off-by: default avatarJuergen Gross <jgross@suse.com>
    8d1502f6
gntdev.c 29.6 KB