From: Nicholas Piggin npiggin@gmail.com
[ Upstream commit 5088eb4092df12d701af8e0e92860b7186365279 ]
The host CTRL (runlatch) value is not restored after guest exit. The host CTRL should always be 1 except in CPU idle code, so this can result in the host running with runlatch clear, and potentially switching to a different vCPU which then runs with runlatch clear as well.
This has little effect on P9 machines, CTRL is only responsible for some PMU counter logic in the host and so other than corner cases of software relying on that, or explicitly reading the runlatch value (Linux does not appear to be affected but it's possible non-Linux guests could be), there should be no execution correctness problem, though it could be used as a covert channel between guests.
There may be microcontrollers, firmware or monitoring tools that sample the runlatch value out-of-band, however since the register is writable by guests, these values would (should) not be relied upon for correct operation of the host, so suboptimal performance or incorrect reporting should be the worst problem.
Fixes: 95a6432ce9038 ("KVM: PPC: Book3S HV: Streamlined guest entry/exit path on P9 for radix guests") Signed-off-by: Nicholas Piggin npiggin@gmail.com Signed-off-by: Michael Ellerman mpe@ellerman.id.au Link: https://lore.kernel.org/r/20210412014845.1517916-2-npiggin@gmail.com Signed-off-by: Sasha Levin sashal@kernel.org --- arch/powerpc/kvm/book3s_hv.c | 3 +++ 1 file changed, 3 insertions(+)
diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c index 6f612d240392..138556cb559d 100644 --- a/arch/powerpc/kvm/book3s_hv.c +++ b/arch/powerpc/kvm/book3s_hv.c @@ -3709,7 +3709,10 @@ static int kvmhv_p9_guest_entry(struct kvm_vcpu *vcpu, u64 time_limit, vcpu->arch.dec_expires = dec + tb; vcpu->cpu = -1; vcpu->arch.thread_cpu = -1; + /* Save guest CTRL register, set runlatch to 1 */ vcpu->arch.ctrl = mfspr(SPRN_CTRLF); + if (!(vcpu->arch.ctrl & 1)) + mtspr(SPRN_CTRLT, vcpu->arch.ctrl | 1);
vcpu->arch.iamr = mfspr(SPRN_IAMR); vcpu->arch.pspb = mfspr(SPRN_PSPB);