Executive brief
In the Linux kernel, the following vulnerability has been resolved:
ppp_synctty: ensure a writeable skb header
ppp_sync_txmunge() checks headroom before prepending the address and control bytes, but does not ensure that the skb header is writable. A received skb can reach this function through PPP channel bridging without passing through ppp_start_xmit(), which calls skb_cow_head().
For example, a PPPoE frame may share its buffer with a clone queued to an AF_PACKET socket. If it is bridged to a synchronous tty channel, the address/control bytes can overwrite data still visible to that socket.
Use skb_cow_head() to ensure both sufficient headroom and a writable header.