staging: most: net: fix buffer overflow

If the length of the socket buffer is 0xFFFFFFFF (max size for an
unsigned int), then payload_len becomes 0xFFFFFFF1 after subtracting 14
(ETH_HLEN).  Then, mdp_len is set to payload_len + 16 (MDP_HDR_LEN)
which overflows and results in a value of 2.  These values for
payload_len and mdp_len will pass current buffer size checks.

This patch checks if derived from skb->len sum may overflow.

The check is based on the following idea:

For any `unsigned V1, V2` and derived `unsigned SUM = V1 + V2`,
`V1 + V2` overflows iif `SUM < V1`.

Reported-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrey Shvetsov <andrey.shvetsov@k2l.de>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200116172238.6046-1-andrey.shvetsov@microchip.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/staging/most/net/net.c b/drivers/staging/most/net/net.c
index 8218c9a06cb5d3e6fdd5c0707ca42b253dd06397..5547e36e09de1f3572d4f3478f225b43070cfe4a 100644 (file)
--- a/drivers/staging/most/net/net.c
+++ b/drivers/staging/most/net/net.c
@@ -82,6 +82,11 @@ static int skb_to_mamac(const struct sk_buff *skb, struct mbo *mbo)
        unsigned int payload_len = skb->len - ETH_HLEN;
        unsigned int mdp_len = payload_len + MDP_HDR_LEN;
 
+       if (mdp_len < skb->len) {
+               pr_err("drop: too large packet! (%u)\n", skb->len);
+               return -EINVAL;
+       }
+
        if (mbo->buffer_length < mdp_len) {
                pr_err("drop: too small buffer! (%d for %d)\n",
                       mbo->buffer_length, mdp_len);
@@ -129,6 +134,11 @@ static int skb_to_mep(const struct sk_buff *skb, struct mbo *mbo)
        u8 *buff = mbo->virt_address;
        unsigned int mep_len = skb->len + MEP_HDR_LEN;
 
+       if (mep_len < skb->len) {
+               pr_err("drop: too large packet! (%u)\n", skb->len);
+               return -EINVAL;
+       }
+
        if (mbo->buffer_length < mep_len) {
                pr_err("drop: too small buffer! (%d for %d)\n",
                       mbo->buffer_length, mep_len);