From f798217dfd038af981a18bbe4bc57027a08bb182 Mon Sep 17 00:00:00 2001 From: James Hogan Date: Wed, 4 Feb 2015 17:06:37 +0000 Subject: [PATCH] KVM: MIPS: Don't leak FPU/DSP to guest The FPU and DSP are enabled via the CP0 Status CU1 and MX bits by kvm_mips_set_c0_status() on a guest exit, presumably in case there is active state that needs saving if pre-emption occurs. However neither of these bits are cleared again when returning to the guest. This effectively gives the guest access to the FPU/DSP hardware after the first guest exit even though it is not aware of its presence, allowing FP instructions in guest user code to intermittently actually execute instead of trapping into the guest OS for emulation. It will then read & manipulate the hardware FP registers which technically belong to the user process (e.g. QEMU), or are stale from another user process. It can also crash the guest OS by causing an FP exception, for which a guest exception handler won't have been registered. First lets save and disable the FPU (and MSA) state with lose_fpu(1) before entering the guest. This simplifies the problem, especially for when guest FPU/MSA support is added in the future, and prevents FR=1 FPU state being live when the FR bit gets cleared for the guest, which according to the architecture causes the contents of the FPU and vector registers to become UNPREDICTABLE. We can then safely remove the enabling of the FPU in kvm_mips_set_c0_status(), since there should never be any active FPU or MSA state to save at pre-emption, which should plug the FPU leak. DSP state is always live rather than being lazily restored, so for that it is simpler to just clear the MX bit again when re-entering the guest. Signed-off-by: James Hogan Cc: Paolo Bonzini Cc: Ralf Baechle Cc: Sanjay Lal Cc: Gleb Natapov Cc: kvm@vger.kernel.org Cc: linux-mips@linux-mips.org Cc: # v3.10+: 044f0f03eca0: MIPS: KVM: Deliver guest interrupts Cc: # v3.10+ Signed-off-by: Paolo Bonzini --- arch/mips/kvm/locore.S | 2 +- arch/mips/kvm/mips.c | 6 +++--- 2 files changed, 4 insertions(+), 4 deletions(-) diff --git a/arch/mips/kvm/locore.S b/arch/mips/kvm/locore.S index d7279c03c517..4a68b176d6e4 100644 --- a/arch/mips/kvm/locore.S +++ b/arch/mips/kvm/locore.S @@ -434,7 +434,7 @@ __kvm_mips_return_to_guest: /* Setup status register for running guest in UM */ .set at or v1, v1, (ST0_EXL | KSU_USER | ST0_IE) - and v1, v1, ~ST0_CU0 + and v1, v1, ~(ST0_CU0 | ST0_MX) .set noat mtc0 v1, CP0_STATUS ehb diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c index 9a28ea4f54f3..e97b90784031 100644 --- a/arch/mips/kvm/mips.c +++ b/arch/mips/kvm/mips.c @@ -15,6 +15,7 @@ #include #include #include +#include #include #include #include @@ -379,6 +380,8 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run) vcpu->mmio_needed = 0; } + lose_fpu(1); + local_irq_disable(); /* Check if we have any exceptions/interrupts pending */ kvm_mips_deliver_interrupts(vcpu, @@ -986,9 +989,6 @@ static void kvm_mips_set_c0_status(void) { uint32_t status = read_c0_status(); - if (cpu_has_fpu) - status |= (ST0_CU1); - if (cpu_has_dsp) status |= (ST0_MX); -- 2.39.5