diff options
authorJames Hogan <james.hogan@imgtec.com>2015-02-04 17:06:37 +0000
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2015-03-06 14:43:31 -0800
commit623347ab1c9885c235f90a5b9b75f8d4494b76c1 (patch)
parent645177f349bf6b276bb55d7d9b492b964ff066bd (diff)
KVM: MIPS: Don't leak FPU/DSP to guest
commit f798217dfd038af981a18bbe4bc57027a08bb182 upstream. 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 <james.hogan@imgtec.com> Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Ralf Baechle <ralf@linux-mips.org> Cc: Sanjay Lal <sanjayl@kymasys.com> Cc: Gleb Natapov <gleb@kernel.org> Cc: kvm@vger.kernel.org Cc: linux-mips@linux-mips.org Cc: <stable@vger.kernel.org> # v3.10+: 044f0f03eca0: MIPS: KVM: Deliver guest interrupts Cc: <stable@vger.kernel.org> # v3.10+ Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: James Hogan <james.hogan@imgtec.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2 files changed, 4 insertions, 4 deletions
diff --git a/arch/mips/kvm/kvm_locore.S b/arch/mips/kvm/kvm_locore.S
index bbace092ad0a..03a2db58b22d 100644
--- a/arch/mips/kvm/kvm_locore.S
+++ b/arch/mips/kvm/kvm_locore.S
@@ -428,7 +428,7 @@ __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
diff --git a/arch/mips/kvm/kvm_mips.c b/arch/mips/kvm/kvm_mips.c
index 28838f1a6c1a..897c605263f2 100644
--- a/arch/mips/kvm/kvm_mips.c
+++ b/arch/mips/kvm/kvm_mips.c
@@ -15,6 +15,7 @@
#include <linux/vmalloc.h>
#include <linux/fs.h>
#include <linux/bootmem.h>
+#include <asm/fpu.h>
#include <asm/page.h>
#include <asm/cacheflush.h>
#include <asm/mmu_context.h>
@@ -418,6 +419,8 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
vcpu->mmio_needed = 0;
+ lose_fpu(1);
/* Check if we have any exceptions/interrupts pending */
@@ -1021,9 +1024,6 @@ 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);