Re: [Xen-devel] [PATCH v4 8/9] xen/gntdev.c: Convert to use vm_map_pages()

[Souptick Joarder <jrdr.linux@gmail.com>]

On Sun, Jul 28, 2019 at 11:36 PM Marek Marczykowski-Górecki
<marmarek@invisiblethingslab.com> wrote:
>
> On Fri, Feb 15, 2019 at 08:18:31AM +0530, Souptick Joarder wrote:
> > Convert to use vm_map_pages() to map range of kernel
> > memory to user vma.
> >
> > map->count is passed to vm_map_pages() and internal API
> > verify map->count against count ( count = vma_pages(vma))
> > for page array boundary overrun condition.
>
> This commit breaks gntdev driver. If vma->vm_pgoff > 0, vm_map_pages
> will:
>  - use map->pages starting at vma->vm_pgoff instead of 0

The actual code ignores vma->vm_pgoff > 0 scenario and mapped
the entire map->pages[i]. Why the entire map->pages[i] needs to be mapped
if vma->vm_pgoff > 0 (in original code) ?

are you referring to set vma->vm_pgoff = 0 irrespective of value passed
from user space ? If yes, using vm_map_pages_zero() is an alternate
option.


>  - verify map->count against vma_pages()+vma->vm_pgoff instead of just
>    vma_pages().

In original code ->

diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index 559d4b7f807d..469dfbd6cf90 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -1084,7 +1084,7 @@ static int gntdev_mmap(struct file *flip, struct
vm_area_struct *vma)
int index = vma->vm_pgoff;
int count = vma_pages(vma);

Count is user passed value.

struct gntdev_grant_map *map;
- int i, err = -EINVAL;
+ int err = -EINVAL;
if ((vma->vm_flags & VM_WRITE) && !(vma->vm_flags & VM_SHARED))
return -EINVAL;
@@ -1145,12 +1145,9 @@ static int gntdev_mmap(struct file *flip,
struct vm_area_struct *vma)
goto out_put_map;
if (!use_ptemod) {
- for (i = 0; i < count; i++) {
- err = vm_insert_page(vma, vma->vm_start + i*PAGE_SIZE,
- map->pages[i]);

and when count > i , we end up with trying to map memory outside
boundary of map->pages[i], which was not correct.

- if (err)
- goto out_put_map;
- }
+ err = vm_map_pages(vma, map->pages, map->count);
+ if (err)
+ goto out_put_map;

With this commit, inside __vm_map_pages(), we have addressed this scenario.

+static int __vm_map_pages(struct vm_area_struct *vma, struct page **pages,
+ unsigned long num, unsigned long offset)
+{
+ unsigned long count = vma_pages(vma);
+ unsigned long uaddr = vma->vm_start;
+ int ret, i;
+
+ /* Fail if the user requested offset is beyond the end of the object */
+ if (offset > num)
+ return -ENXIO;
+
+ /* Fail if the user requested size exceeds available object size */
+ if (count > num - offset)
+ return -ENXIO;

By checking count > num -offset. (considering vma->vm_pgoff != 0 as well).
So we will never cross the boundary of map->pages[i].


>
> In practice, this breaks using a single gntdev FD for mapping multiple
> grants.

How ?

>
> It looks like vm_map_pages() is not a good fit for this code and IMO it
> should be reverted.

Did you hit any issue around this code in real time ?


>
> > Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
> > Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> > ---
> >  drivers/xen/gntdev.c | 11 ++++-------
> >  1 file changed, 4 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> > index 5efc5ee..5d64262 100644
> > --- a/drivers/xen/gntdev.c
> > +++ b/drivers/xen/gntdev.c
> > @@ -1084,7 +1084,7 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma)
> >       int index = vma->vm_pgoff;
> >       int count = vma_pages(vma);
> >       struct gntdev_grant_map *map;
> > -     int i, err = -EINVAL;
> > +     int err = -EINVAL;
> >
> >       if ((vma->vm_flags & VM_WRITE) && !(vma->vm_flags & VM_SHARED))
> >               return -EINVAL;
> > @@ -1145,12 +1145,9 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma)
> >               goto out_put_map;
> >
> >       if (!use_ptemod) {
> > -             for (i = 0; i < count; i++) {
> > -                     err = vm_insert_page(vma, vma->vm_start + i*PAGE_SIZE,
> > -                             map->pages[i]);
> > -                     if (err)
> > -                             goto out_put_map;
> > -             }
> > +             err = vm_map_pages(vma, map->pages, map->count);
> > +             if (err)
> > +                     goto out_put_map;
> >       } else {
> >  #ifdef CONFIG_X86
> >               /*
>
> --
> Best Regards,
> Marek Marczykowski-Górecki
> Invisible Things Lab
> A: Because it messes up the order in which people normally read text.
> Q: Why is top-posting such a bad thing?