Re: [PATCH 3/3] vhost: fix get_user_pages_fast error handling

Gleb Natapov
2010-02-23T17:43:34Z

On Tue, Feb 23, 2010 at 07:39:08PM +0200, Michael S. Tsirkin wrote:
> On Tue, Feb 23, 2010 at 07:39:52PM +0200, Gleb Natapov wrote:
> > On Tue, Feb 23, 2010 at 07:32:58PM +0200, Michael S. Tsirkin wrote:
> > > On Tue, Feb 23, 2010 at 07:34:34PM +0200, Gleb Natapov wrote:
> > > > On Tue, Feb 23, 2010 at 06:57:58PM +0200, Michael S. Tsirkin wrote:
> > > > > get_user_pages_fast returns number of pages on success, negative value
> > > > > on failure, but never 0. Fix vhost code to match this logic.
> > > > >
> > > > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > > > > ---
> > > > > drivers/vhost/vhost.c | 3 ++-
> > > > > 1 files changed, 2 insertions(+), 1 deletions(-)
> > > > >
> > > > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > > > index d4f8fdf..d003504 100644
> > > > > --- a/drivers/vhost/vhost.c
> > > > > +++ b/drivers/vhost/vhost.c
> > > > > @@ -646,8 +646,9 @@ static int set_bit_to_user(int nr, void __user *addr)
> > > > > int bit = nr + (log % PAGE_SIZE) * 8;
> > > > > int r;
> > > > > r = get_user_pages_fast(log, 1, 1, &page);
> > > > > - if (r)
> > > > > + if (r < 0)
> > > > > return r;
> > > > > + BUG_ON(r != 1);
> > > > Can't this be easily triggered from user space?
> > >
> > > I think no. get_user_pages_fast always returns number of pages
> > > pinned (in this case always 1) or an error (< 0).
> > > Anything else is a kernel bug.
> > >
> > But what if page is unmapped from userspace?
>
> Then we get -EFAULT
>
Ah correct.

> > > > > base = kmap_atomic(page, KM_USER0);
> > > > > set_bit(bit, base);
> > > > > kunmap_atomic(base, KM_USER0);
> > > > > --
> > > > > 1.7.0.18.g0d53a5
> > > >
> > > > --
> > > > Gleb.
> >
> > --
> > Gleb.

--
Gleb.