macvtap: always pass ethernet header in linear

The stack expects link layer headers in the skb linear section.
Macvtap can create skbs with llheader in frags in edge cases:
when (IFF_VNET_HDR is off or vnet_hdr.hdr_len < ETH_HLEN) and
prepad + len > PAGE_SIZE and vnet_hdr.flags has no or bad csum.

Add checks to ensure linear is always at least ETH_HLEN.
At this point, len is already ensured to be >= ETH_HLEN.

For backwards compatiblity, rounds up short vnet_hdr.hdr_len.
This differs from tap and packet, which return an error.

Fixes b9fb9ee07e67 ("macvtap: add GSO/csum offload support")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 3 deletions

diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
index d636d051fac8..95394edd1ed5 100644
--- a/drivers/net/macvtap.c
+++ b/drivers/net/macvtap.c
@@ -760,6 +760,8 @@ static ssize_t macvtap_get_user(struct macvtap_queue *q, struct msghdr *m,
                         macvtap16_to_cpu(q, vnet_hdr.hdr_len) : GOODCOPY_LEN;
                 if (copylen > good_linear)
                         copylen = good_linear;
+                else if (copylen < ETH_HLEN)
+                        copylen = ETH_HLEN;
                 linear = copylen;
                 i = *from;
                 iov_iter_advance(&i, copylen);
@@ -769,10 +771,11 @@ static ssize_t macvtap_get_user(struct macvtap_queue *q, struct msghdr *m,
 
         if (!zerocopy) {
                 copylen = len;
-                if (macvtap16_to_cpu(q, vnet_hdr.hdr_len) > good_linear)
+                linear = macvtap16_to_cpu(q, vnet_hdr.hdr_len);
+                if (linear > good_linear)
                         linear = good_linear;
-                else
-                        linear = macvtap16_to_cpu(q, vnet_hdr.hdr_len);
+                else if (linear < ETH_HLEN)
+                        linear = ETH_HLEN;
         }
 
         skb = macvtap_alloc_skb(&q->sk, MACVTAP_RESERVE, copylen,