On 7/30/19 2:34 PM, 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.
Cc: stable@vger.kernel.org # v5.2+ Fixes: df9bde015a72 ("xen/gntdev.c: convert to use vm_map_pages()")
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
Reviewed-by: Boris Ostrovsky boris.ostrovsky@oracle.com
drivers/xen/gntdev.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c index 4c339c7..a446a72 100644 --- a/drivers/xen/gntdev.c +++ b/drivers/xen/gntdev.c @@ -1143,7 +1143,7 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma) goto out_put_map; if (!use_ptemod) {
err = vm_map_pages(vma, map->pages, map->count);
err = vm_map_pages_zero(vma, map->pages, map->count);