CVE-2022-48819

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(MSGZEROCOPY) calls over the same TCP socket would again trigger the infamous warning in inetsock_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 skforwardalloc will stay synced.

Add tcpdowngradezcopy_pure() helper so that we can use it from the two callers.