CVE-2021-47152

In the Linux kernel, the following vulnerability has been resolved:

mptcp: fix data stream corruption

Maxim reported several issues when forcing a TCP transparent proxy to use the MPTCP protocol for the inbound connections. He also provided a clean reproducer.

The problem boils down to 'mptcp_frag_can_collapse_to()' assuming that only MPTCP will use the given page_frag.

If others - e.g. the plain TCP protocol - allocate page fragments, we can end-up re-using already allocated memory for mptcp_data_frag.

Fix the issue ensuring that the to-be-expanded data fragment is located at the current page frag end.

v1 -> v2: - added missing fixes tag (Mat)