tcp: take care of mixed splice()/sendmsg(MSG_ZEROCOPY) case
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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.
Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.
No remediation guidance has been written for this CVE yet. What the record does show:
SSVC signals action urgency (what to patch first), separate from CVSS which measures severity.
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