linux-kernel - Re: [PATCH 3/3] vhost: fix get_user_pages

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Message-ID: <20100223173908.GB25338@redhat.com>
Date:	Tue, 23 Feb 2010 19:39:08 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>, kvm@...r.kernel.org,
	virtualization@...ts.osdl.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net>,
	markmc@...hat.com, herbert.xu@...hat.com, quintela@...hat.com,
	dlaor@...hat.com, avi@...hat.com
Subject: Re: [PATCH 3/3] vhost: fix get_user_pages_fast error handling

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@...hat.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

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