linux-kernel - Re: [RFC PATCH 2/6] virtio/console: Add a failback for unstealable pipe buffer

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Date:	Thu, 9 Aug 2012 15:44:47 +0530
From:	Amit Shah <amit.shah@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@...achi.com>,
	linux-kernel@...r.kernel.org,
	Anthony Liguori <anthony@...emonkey.ws>,
	Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...64.org>,
	"Franch Ch. Eigler" <fche@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Herbert Xu <herbert@...dor.hengli.com.au>,
	Ingo Molnar <mingo@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	virtualization@...ts.linux-foundation.org, qemu-devel@...gnu.org,
	yrl.pp-manager.tt@...achi.com,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [RFC PATCH 2/6] virtio/console: Add a failback for unstealable
 pipe buffer

On (Thu) 09 Aug 2012 [12:58:13], Avi Kivity wrote:
> On 08/09/2012 12:55 PM, Amit Shah wrote:
> > On (Thu) 09 Aug 2012 [18:24:58], Masami Hiramatsu wrote:
> >> (2012/08/09 18:03), Amit Shah wrote:
> >> > On (Tue) 24 Jul 2012 [11:37:18], Yoshihiro YUNOMAE wrote:
> >> >> From: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
> >> >>
> >> >> Add a failback memcpy path for unstealable pipe buffer.
> >> >> If buf->ops->steal() fails, virtio-serial tries to
> >> >> copy the page contents to an allocated page, instead
> >> >> of just failing splice().
> >> >>
> >> >> Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
> >> >> Cc: Amit Shah <amit.shah@...hat.com>
> >> >> Cc: Arnd Bergmann <arnd@...db.de>
> >> >> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> >> >> ---
> >> >>
> >> >>  drivers/char/virtio_console.c |   28 +++++++++++++++++++++++++---
> >> >>  1 files changed, 25 insertions(+), 3 deletions(-)
> >> >>
> >> >> diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
> >> >> index fe31b2f..911cb3e 100644
> >> >> --- a/drivers/char/virtio_console.c
> >> >> +++ b/drivers/char/virtio_console.c
> >> >> @@ -794,7 +794,7 @@ static int pipe_to_sg(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
> >> >>  			struct splice_desc *sd)
> >> >>  {
> >> >>  	struct sg_list *sgl = sd->u.data;
> >> >> -	unsigned int len = 0;
> >> >> +	unsigned int offset, len;
> >> >>  
> >> >>  	if (sgl->n == MAX_SPLICE_PAGES)
> >> >>  		return 0;
> >> >> @@ -807,9 +807,31 @@ static int pipe_to_sg(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
> >> >>  
> >> >>  		len = min(buf->len, sd->len);
> >> >>  		sg_set_page(&(sgl->sg[sgl->n]), buf->page, len, buf->offset);
> >> >> -		sgl->n++;
> >> >> -		sgl->len += len;
> >> >> +	} else {
> >> >> +		/* Failback to copying a page */
> >> >> +		struct page *page = alloc_page(GFP_KERNEL);
> >> > 
> >> > I prefer zeroing out the page.  If there's not enough data to be
> >> > filled in the page, the remaining data can be leaked to the host.
> >> 
> >> Yeah, it is really easy to fix that.
> >> But out of curiosity, would that be really a problem?
> >> I guess that host can access any guest page if need. If that
> >> is right, is that really insecure to leak randomly allocated
> >> unused page to the host?
> > 
> > I'm not sure if there is a way to really attack, but just something I
> > had thought about: the host kernel can access any guest page, that's
> > not something we can prevent.
> > 
> > However, if qemu is restricted from accessing guest pages, and the
> > guest shares this page with qemu for r/w purposes via the virtio
> > channel, a qemu exploit can expose guest data to host userspace.
> > 
> > I agree this is completely theoretical; can someone else with more
> > insight confirm or deny my apprehensions?
> 
> qemu can read and write any guest page (for the guest it controls).

OK, thanks for confirming -- no need to change this patch, then.

		Amit
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