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

[Amit Shah]

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 <address@hidden>
> >> >>
> >> >> 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 <address@hidden>
> >> >> Cc: Amit Shah <address@hidden>
> >> >> Cc: Arnd Bergmann <address@hidden>
> >> >> Cc: Greg Kroah-Hartman <address@hidden>
> >> >> ---
> >> >>
> >> >>  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