[PATCH][net-next] iavf: fix dereference of null rx_buffer pointer

[Colin King <colin.king@canonical.com>]

From: Colin Ian King <colin.king@canonical.com>

A recent commit efa14c3985828d ("iavf: allow null RX descriptors") added
a null pointer sanity check on rx_buffer, however, rx_buffer is being
dereferenced before that check, which implies a null pointer dereference
bug can potentially occur.  Fix this by only dereferencing rx_buffer
until after the null pointer check.

Addresses-Coverity: ("Dereference before null check")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
---
 drivers/net/ethernet/intel/iavf/iavf_txrx.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/intel/iavf/iavf_txrx.c b/drivers/net/ethernet/intel/iavf/iavf_txrx.c
index 1cde1601bc32..0cca1b589b56 100644
--- a/drivers/net/ethernet/intel/iavf/iavf_txrx.c
+++ b/drivers/net/ethernet/intel/iavf/iavf_txrx.c
@@ -1296,7 +1296,7 @@ static struct sk_buff *iavf_construct_skb(struct iavf_ring *rx_ring,
 					  struct iavf_rx_buffer *rx_buffer,
 					  unsigned int size)
 {
-	void *va = page_address(rx_buffer->page) + rx_buffer->page_offset;
+	void *va;
 #if (PAGE_SIZE < 8192)
 	unsigned int truesize = iavf_rx_pg_size(rx_ring) / 2;
 #else
@@ -1308,6 +1308,7 @@ static struct sk_buff *iavf_construct_skb(struct iavf_ring *rx_ring,
 	if (!rx_buffer)
 		return NULL;
 	/* prefetch first cache line of first page */
+	va = page_address(rx_buffer->page) + rx_buffer->page_offset;
 	prefetch(va);
 #if L1_CACHE_BYTES < 128
 	prefetch(va + L1_CACHE_BYTES);
@@ -1362,7 +1363,7 @@ static struct sk_buff *iavf_build_skb(struct iavf_ring *rx_ring,
 				      struct iavf_rx_buffer *rx_buffer,
 				      unsigned int size)
 {
-	void *va = page_address(rx_buffer->page) + rx_buffer->page_offset;
+	void *va;
 #if (PAGE_SIZE < 8192)
 	unsigned int truesize = iavf_rx_pg_size(rx_ring) / 2;
 #else
@@ -1374,6 +1375,7 @@ static struct sk_buff *iavf_build_skb(struct iavf_ring *rx_ring,
 	if (!rx_buffer)
 		return NULL;
 	/* prefetch first cache line of first page */
+	va = page_address(rx_buffer->page) + rx_buffer->page_offset;
 	prefetch(va);
 #if L1_CACHE_BYTES < 128
 	prefetch(va + L1_CACHE_BYTES);
-- 
2.20.1