kernel: Local privilege escalation in XFRM framework（CVE-2017-7184）

A security issue was reported by ZDI, on behalf of Chaitin Security
Research Lab, against the Linux kernel in Ubuntu. It also affected the
upstream kernel.

Chaitin Security Research Lab discovered that xfrm_replay_verify_len(),
as called by xfrm_new_ae(), did not verify that the user-specified
replay_window was within the replay state buffer.

This allowed for out-of-bounds reads and writes of kernel memory.
Chaitin Security showed that this can lead to local privilege escalation
by using user namespaces in order to configure XFRM. XFRM configuration
requires CAP_NET_ADMIN so this issue is mitigated in kernels which do
not enable user namespaces by default.

Fixes:

• https://git.kernel.org/linus/677e806da4d916052585301785d847c3b3e6186a
• https://git.kernel.org/linus/f843ee6dd019bcece3e74e76ad9df0155655d0df