Comment 0 for bug 1775137

Upstream's Spectre v1 mitigation prevents speculation on a user controlled pointer. This part of the Spectre v1 patchset was never backported to 4.4 (for unknown reasons) so Xenial is lacking it as well. All the other stable upstream kernels include it, so add it to Xenial. Specifically, the following patches are needed:

c7f631cb07e7 x86/get_user: Use pointer masking to limit speculation
304ec1b05031 x86/uaccess: Use __uaccess_begin_nospec() and uaccess_try_nospec
b5c4ae4f3532 x86/usercopy: Replace open coded stac/clac with __uaccess_{begin, end}
b3bbfb3fb5d2 x86: Introduce __uaccess_begin_nospec() and uaccess_try_nospec