[Xen-devel] [PATCH] xen/gntdev.c: Replace vm_map_pages() with vm_map_pages_zero()

[Souptick Joarder <jrdr.linux@xxxxxxxxx>]

'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@xxxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Souptick Joarder <jrdr.linux@xxxxxxxxx>
Tested-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
---
 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);
                if (err)
                        goto out_put_map;
        } else {
-- 
1.9.1


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