[SRU B/D/E/F] CVE-2020-10757

[Thadeu Lima de Souza Cascardo]

[Impact/Description]
 User can control PTE value to read/write anywhere, when "mremap" a
 hugepage mmaped DAX file to a mmaped anonymous memory region

[Test case]
Mounted a DAX filesystem with an emulated PMEM device and ran reproducer
as in oss-sec message.
 https://www.openwall.com/lists/oss-security/2020/06/04/4

Tested on Bionic, Eoan and Focal kernels.

[Regression potential]
Could mostly break DAX, though the test case cover some of it.