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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 23 Feb 2010 19:32:58 +0200

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 <[email-masked]>
> > ---
> >  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.

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