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From:   Souptick Joarder <jrdr.linux@gmail.com>
Date:   Tue, 30 Jul 2019 11:33:47 +0530
Message-ID: <CAFqt6zZN+6r6wYJY+f15JAjj8dY+o30w_+EWH9Vy2kUXCKSBog@mail.gmail.com>
Subject: Re: [Xen-devel] [PATCH v4 8/9] xen/gntdev.c: Convert to use vm_map_pages()
To:     =?UTF-8?Q?Marek_Marczykowski=2DG=C3=B3recki?= 
        <marmarek@invisiblethingslab.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>,
        Michal Hocko <mhocko@suse.com>,
        Juergen Gross <jgross@suse.com>,
        Russell King - ARM Linux <linux@armlinux.org.uk>,
        robin.murphy@arm.com, xen-devel@lists.xenproject.org,
        linux-kernel@vger.kernel.org, Linux-MM <linux-mm@kvack.org>
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On Mon, Jul 29, 2019 at 7:06 PM Marek Marczykowski-G=C3=B3recki
<marmarek@invisiblethingslab.com> wrote:
>
> 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-G=C3=B3recki
> > > <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 =3D vma_pages(vma))
> > > > > for page array boundary overrun condition.
> > > >
> > > > This commit breaks gntdev driver. If vma->vm_pgoff > 0, vm_map_page=
s
> > > > 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 ma=
pped
> > > 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 =3D 0 irrespective of value pa=
ssed
> > > from user space ? If yes, using vm_map_pages_zero() is an alternate
> > > option.
>
> Yes, that should work.

I prefer to use vm_map_pages_zero() to resolve both the issues. Alternative=
ly
the patch can be reverted as you suggested. Let me know you opinion and wai=
t
for feedback from others.

Boris, would you like to give any feedback ?

>
> > > >  - verify map->count against vma_pages()+vma->vm_pgoff instead of j=
ust
> > > >    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, struc=
t
> > > vm_area_struct *vma)
> > > int index =3D vma->vm_pgoff;
> > > int count =3D vma_pages(vma);
> > >
> > > Count is user passed value.
> > >
> > > struct gntdev_grant_map *map;
> > > - int i, err =3D -EINVAL;
> > > + int err =3D -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 =3D 0; i < count; i++) {
> > > - err =3D 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 match=
ing
> both index and count.
>
> > >
> > > - if (err)
> > > - goto out_put_map;
> > > - }
> > > + err =3D 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 sce=
nario.
> > >
> > > +static int __vm_map_pages(struct vm_area_struct *vma, struct page **=
pages,
> > > + unsigned long num, unsigned long offset)
> > > +{
> > > + unsigned long count =3D vma_pages(vma);
> > > + unsigned long uaddr =3D vma->vm_start;
> > > + int ret, i;
> > > +
> > > + /* Fail if the user requested offset is beyond the end of the objec=
t */
> > > + 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 !=3D 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 multi=
ple
> > > > 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 IM=
O 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) =3D 0
> [pid   857] mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, 7, 0) =3D =
0x777f1211b000
> [pid   857] ioctl(7, IOCTL_GNTDEV_SET_UNMAP_NOTIFY, 0x7ffd3407b710) =3D 0
> [pid   857] ioctl(7, IOCTL_GNTDEV_MAP_GRANT_REF, 0x7ffd3407b6d0) =3D 0
> [pid   857] mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, 7, 0x1000)=
 =3D -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, s=
truct vm_area_struct *vma)
> > > > >       int index =3D vma->vm_pgoff;
> > > > >       int count =3D vma_pages(vma);
> > > > >       struct gntdev_grant_map *map;
> > > > > -     int i, err =3D -EINVAL;
> > > > > +     int err =3D -EINVAL;
> > > > >
> > > > >       if ((vma->vm_flags & VM_WRITE) && !(vma->vm_flags & VM_SHAR=
ED))
> > > > >               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 =3D 0; i < count; i++) {
> > > > > -                     err =3D vm_insert_page(vma, vma->vm_start +=
 i*PAGE_SIZE,
> > > > > -                             map->pages[i]);
> > > > > -                     if (err)
> > > > > -                             goto out_put_map;
> > > > > -             }
> > > > > +             err =3D vm_map_pages(vma, map->pages, map->count);
> > > > > +             if (err)
> > > > > +                     goto out_put_map;
> > > > >       } else {
> > > > >  #ifdef CONFIG_X86
> > > > >               /*
> > > >
> > > > --
> > > > Best Regards,
> > > > Marek Marczykowski-G=C3=B3recki
> > > > Invisible Things Lab
> > > > A: Because it messes up the order in which people normally read tex=
t.
> > > > Q: Why is top-posting such a bad thing?
>
> --
> Best Regards,
> Marek Marczykowski-G=C3=B3recki
> 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?