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On AMD CPUs, SYSRET can return with a valid SS descriptor with
with the hidden attributes set to an unusable state.  Make sure
the kernel doesn't let this happen.  This detects an
as-yet-unfixed regression.

Note that the 64-bit version of this test fails on AMD CPUs on
all kernel versions, although the issue in the 64-bit case is
much less severe than in the 32-bit case.
Reported-by: Brian Gerst <brgerst@gmail.com>
Tested-by: Denys Vlasenko <dvlasenk@redhat.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Tests: e7d6eefa ("x86/vdso32/syscall.S: Do not load __USER32_DS to %ss")
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@in...