mptcp: fix data stream corruption

Vulnerability Description

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)

References

• https://git.kernel.org/stable/c/3267a061096efc91eda52c2a0c61ba76e46e4b34
• https://git.kernel.org/stable/c/18e7f0580da15cac1e79d73683ada5a9e70980f8
• https://git.kernel.org/stable/c/29249eac5225429b898f278230a6ca2baa1ae154