In the Linux kernel, the following vulnerability has been resolved:
tcp: take care of mixed splice()/sendmsg(MSG_ZEROCOPY) case
syzbot found that mixing sendpage() and sendmsg(MSG_ZEROCOPY) calls over the same TCP socket would again trigger the infamous warning in inet_sock_destruct()
WARN_ON(sk_forward_alloc_get(sk));
While Talal took into account a mix of regular copied data and MSG_ZEROCOPY one in the same skb, the sendpage() path has been forgotten.
We want the charging to happen for sendpage(), because pages could be coming from a pipe. What is missing is the downgrading of pure zerocopy status to make sure sk_forward_alloc will stay synced.
Add tcp_downgrade_zcopy_pure() helper so that we can use it from the two callers.