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

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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On Mon, Jul 29, 2019 at 02:02:54PM +0530, Souptick Joarder wrote:
> On Mon, Jul 29, 2019 at 1:35 PM Souptick Joarder <jrdr.linux@gmail.com> wrote:
> >
> > 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) ?

vma->vm_pgoff is used as index passed to gntdev_find_map_index. It's
basically (ab)using this parameter for "which grant reference to map".

> > 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.

Yes, that should work.

> > >  - 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.
> 
> typo.
> s/count > i / count > map->count

gntdev_find_map_index verifies it. Specifically, it looks for a map matching
both index and count.

> >
> > - 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 ?

gntdev uses vma->vm_pgoff to select which grant entry should be mapped.
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).

> > > 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 ?

Yes, 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


> >
> >
> > >
> > > > 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?

-- 
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?

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