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

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

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