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@redhat.com>
---
 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
@@ -527,6 +527,8 @@ static int zerocopy_sg_from_iovec(struct sk_buff *skb, const struct iovec *from,
 		}
 		base = (unsigned long)from->iov_base + offset1;
 		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))