tcp: take care of mixed splice()/sendmsg(MSG_ZEROCOPY) case

Vulnerability Description

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.

References

• https://git.kernel.org/stable/c/47f3860c4931175f112f28dcac66eacca9b1040f
• https://git.kernel.org/stable/c/f8d9d938514f46c4892aff6bfe32f425e84d81cc