Re: [PATCH 3/6] macvtap: zerocopy: validate vector length beforepinning user pages

[Jason Wang]

Hi:
On 04/16/2012 02:53 PM, Eric Dumazet wrote:
> On Mon, 2012-04-16 at 14:08 +0800, Jason Wang wrote:
> > Currently we do not validate the vector length before calling
> > get_user_pages_fast(), host stack would be easily overflowed by
> > malicious guest driver who give us a descriptor with length greater
> > than MAX_SKB_FRAGS. Solve this problem by checking the free entries
> > before trying to pin user pages.
> >
> > Signed-off-by: Jason Wang<jasowang@xxxxxxxxxx>
> > ---
> >   drivers/net/macvtap.c |    2 ++
> >   1 files changed, 2 insertions(+), 0 deletions(-)
> >
> > diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
> > index 7cb2684..d197a78 100644
> > --- a/drivers/net/macvtap.c
> > +++ b/drivers/net/macvtap.c
> > @@ -529,6 +529,8 @@ static int zerocopy_sg_from_iovec(struct sk_buff *skb, const struct iovec *from,
> >   		}
> >   		base = (unsigned long)from->iov_base + offset;
> >   		size = ((base&  ~PAGE_MASK) + len + ~PAGE_MASK)>>  PAGE_SHIFT;
> > +		if (i + size>= MAX_SKB_FRAGS)
> > +			return -EFAULT;
> >   		num_pages = get_user_pages_fast(base, size, 0,&page[i]);
> >   		if ((num_pages != size) ||
> >   		    (num_pages>  MAX_SKB_FRAGS - skb_shinfo(skb)->nr_frags))
> Hi Jason
>
> Why is -EFAULT the right error code ?

E2BIG or is there any error code you prefer?
> Also, why not removing the "(num_pages>  MAX_SKB_FRAGS -
> skb_shinfo(skb)->nr_frags)" test done few lines after ?

Yes, it can be removed.
> Also, if get_user_pages_fast() returns truncated result (size - 1 for
> example), we apparently dont free the references on pages.

It's a bug of this patch, thanks.
> The comment applies only on pages that were added in skb frags.

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