KVM: x86/mmu: do compare-and-exchange of gPTE via the user address

FNAME(cmpxchg_gpte) is an inefficient mess.  It is at least decent if it
can go through get_user_pages_fast(), but if it cannot then it tries to
use memremap(); that is not just terribly slow, it is also wrong because
it assumes that the VM_PFNMAP VMA is contiguous.

The right way to do it would be to do the same thing as
hva_to_pfn_remapped() does since commit add6a0cd1c5b ("KVM: MMU: try to
fix up page faults before giving up", 2016-07-05), using follow_pte()
and fixup_user_fault() to determine the correct address to use for
memremap().  To do this, one could for example extract hva_to_pfn()
for use outside virt/kvm/kvm_main.c.  But really there is no reason to
do that either, because there is already a perfectly valid address to
do the cmpxchg() on, only it is a userspace address.  That means doing
user_access_begin()/user_access_end() and writing the code in assembly
to handle exceptions correctly.  Worse, the guest PTE can be 8-byte
even on i686 so there is the extra complication of using cmpxchg8b to
account for.  But at least it is an efficient mess.

(Thanks to Linus for suggesting improvement on the inline assembly).

Reported-by: Qiuhao Li <qiuhao@sysec.org>
Reported-by: Gaoning Pan <pgn@zju.edu.cn>
Reported-by: Yongkang Jia <kangel@zju.edu.cn>
Reported-by: syzbot+6cde2282daa792c49ab8@syzkaller.appspotmail.com
Debugged-by: Tadeusz Struk <tadeusz.struk@linaro.org>
Tested-by: Maxim Levitsky <mlevitsk@redhat.com>
Cc: stable@vger.kernel.org
Fixes: bd53cb35a3e9 ("X86/KVM: Handle PFNs outside of kernel reach when touching GPTEs")
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
