Commit 46a704f8 ("KVM: PPC: Book3S HV: Preserve userspace HTM
state properly", 2017-06-15) added code which assumes that the kernel
is able to handle a TM (transactional memory) unavailable interrupt
from userspace by reloading the TM-related registers and enabling TM
for the process.  That ability was added in the 4.9 kernel; earlier
kernel versions simply panic on getting the TM unavailable interrupt.

Since commit 46a704f8 has been backported to the 4.4 stable tree
as commit 824b9506, 4.4.75 and subsequent versions are vulnerable
to a userspace-triggerable panic.

This patch fixes the problem by explicitly reloading the TM-related
registers before returning to userspace, rather than disabling TM
for the process.

Commit 46a704f8 also failed to enable TM for the kernel, leading
to a TM unavailable interrupt in the kernel, causing an oops.  This
fixes that problem too, by enabling TM before accessing the TM
registers.  That problem is fixed upstream by the patch "KVM: PPC:
Book3S HV: Enable TM before accessing TM registers".

Fixes: 824b9506 ("KVM: PPC: Book3S HV: Preserve userspace HTM state properly")
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>