kvm: x86: do not leak guest xcr0 into host interrupt handlers

An interrupt handler that uses the fpu can kill a KVM VM, if it runs
under the following conditions:
 - the guest's xcr0 register is loaded on the cpu
 - the guest's fpu context is not loaded
 - the host is using eagerfpu

Note that the guest's xcr0 register and fpu context are not loaded as
part of the atomic world switch into "guest mode". They are loaded by
KVM while the cpu is still in "host mode".

Usage of the fpu in interrupt context is gated by irq_fpu_usable(). The
interrupt handler will look something like this:

if (irq_fpu_usable()) {
        kernel_fpu_begin();

        [... code that uses the fpu ...]

        kernel_fpu_end();
}

As long as the guest's fpu is not loaded and the host is using eager
fpu, irq_fpu_usable() returns true (interrupted_kernel_fpu_idle()
returns true). The interrupt handler proceeds to use the fpu with
the guest's xcr0 live.

kernel_fpu_begin() saves the current fpu context. If this uses
XSAVE[OPT], it may leave the xsave area in an undesirable state.
According to the SDM, during XSAVE bit i of XSTATE_BV is not modified
if bit i is 0 in xcr0. So it's possible that XSTATE_BV[i] == 1 and
xcr0[i] == 0 following an XSAVE.

kernel_fpu_end() restores the fpu context. Now if any bit i in
XSTATE_BV == 1 while xcr0[i] == 0, XRSTOR generates a #GP. The
fault is trapped and SIGSEGV is delivered to the current process.

Only pre-4.2 kernels appear to be vulnerable to this sequence of
events. Commit 653f52c3 ("kvm,x86: load guest FPU context more eagerly")
from 4.2 forces the guest's fpu to always be loaded on eagerfpu hosts.

This patch fixes the bug by keeping the host's xcr0 loaded outside
of the interrupts-disabled region where KVM switches into guest mode.

Cc: stable@vger.kernel.org
Suggested-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: David Matlack <dmatlack@google.com>
[Move load after goto cancel_injection. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>