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

From: Jason Wang
Date: Mon Apr 16 2012 - 04:21:43 EST

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.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at