From 844a5fe219cf472060315971e15cbf97674a3324 Mon Sep 17 00:00:00 2001 From: Paolo Bonzini Date: Tue, 8 Mar 2016 12:13:39 +0100 Subject: [PATCH] KVM: MMU: fix ept=0/pte.u=1/pte.w=0/CR0.WP=0/CR4.SMEP=1/EFER.NX=0 combo Yes, all of these are needed. :) This is admittedly a bit odd, but kvm-unit-tests access.flat tests this if you run it with "-cpu host" and of course ept=0. KVM runs the guest with CR0.WP=1, so it must handle supervisor writes specially when pte.u=1/pte.w=0/CR0.WP=0. Such writes cause a fault when U=1 and W=0 in the SPTE, but they must succeed because CR0.WP=0. When KVM gets the fault, it sets U=0 and W=1 in the shadow PTE and restarts execution. This will still cause a user write to fault, while supervisor writes will succeed. User reads will fault spuriously now, and KVM will then flip U and W again in the SPTE (U=1, W=0). User reads will be enabled and supervisor writes disabled, going back to the originary situation where supervisor writes fault spuriously. When SMEP is in effect, however, U=0 will enable kernel execution of this page. To avoid this, KVM also sets NX=1 in the shadow PTE together with U=0. If the guest has not enabled NX, the result is a continuous stream of page faults due to the NX bit being reserved. The fix is to force EFER.NX=1 even if the CPU is taking care of the EFER switch. (All machines with SMEP have the CPU_LOAD_IA32_EFER vm-entry control, so they do not use user-return notifiers for EFER---if they did, EFER.NX would be forced to the same value as the host). There is another bug in the reserved bit check, which I've split to a separate patch for easier application to stable kernels. Cc: stable@vger.kernel.org Cc: Andy Lutomirski Reviewed-by: Xiao Guangrong Fixes: f6577a5fa15d82217ca73c74cd2dcbc0f6c781dd Signed-off-by: Paolo Bonzini --- Documentation/virtual/kvm/mmu.txt | 3 ++- arch/x86/kvm/vmx.c | 36 ++++++++++++++++++++----------- 2 files changed, 25 insertions(+), 14 deletions(-) diff --git a/Documentation/virtual/kvm/mmu.txt b/Documentation/virtual/kvm/mmu.txt index daf9c0f742d2..c81731096a43 100644 --- a/Documentation/virtual/kvm/mmu.txt +++ b/Documentation/virtual/kvm/mmu.txt @@ -358,7 +358,8 @@ In the first case there are two additional complications: - if CR4.SMEP is enabled: since we've turned the page into a kernel page, the kernel may now execute it. We handle this by also setting spte.nx. If we get a user fetch or read fault, we'll change spte.u=1 and - spte.nx=gpte.nx back. + spte.nx=gpte.nx back. For this to work, KVM forces EFER.NX to 1 when + shadow paging is in use. - if CR4.SMAP is disabled: since the page has been changed to a kernel page, it can not be reused when CR4.SMAP is enabled. We set CR4.SMAP && !CR0.WP into shadow page's role to avoid this case. Note, diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c index 6e51493ff4f9..9bd8f44baded 100644 --- a/arch/x86/kvm/vmx.c +++ b/arch/x86/kvm/vmx.c @@ -1857,26 +1857,31 @@ static void reload_tss(void) static bool update_transition_efer(struct vcpu_vmx *vmx, int efer_offset) { - u64 guest_efer; - u64 ignore_bits; + u64 guest_efer = vmx->vcpu.arch.efer; + u64 ignore_bits = 0; - guest_efer = vmx->vcpu.arch.efer; + if (!enable_ept) { + /* + * NX is needed to handle CR0.WP=1, CR4.SMEP=1. Testing + * host CPUID is more efficient than testing guest CPUID + * or CR4. Host SMEP is anyway a requirement for guest SMEP. + */ + if (boot_cpu_has(X86_FEATURE_SMEP)) + guest_efer |= EFER_NX; + else if (!(guest_efer & EFER_NX)) + ignore_bits |= EFER_NX; + } /* - * NX is emulated; LMA and LME handled by hardware; SCE meaningless - * outside long mode + * LMA and LME handled by hardware; SCE meaningless outside long mode. */ - ignore_bits = EFER_NX | EFER_SCE; + ignore_bits |= EFER_SCE; #ifdef CONFIG_X86_64 ignore_bits |= EFER_LMA | EFER_LME; /* SCE is meaningful only in long mode on Intel */ if (guest_efer & EFER_LMA) ignore_bits &= ~(u64)EFER_SCE; #endif - guest_efer &= ~ignore_bits; - guest_efer |= host_efer & ignore_bits; - vmx->guest_msrs[efer_offset].data = guest_efer; - vmx->guest_msrs[efer_offset].mask = ~ignore_bits; clear_atomic_switch_msr(vmx, MSR_EFER); @@ -1887,16 +1892,21 @@ static bool update_transition_efer(struct vcpu_vmx *vmx, int efer_offset) */ if (cpu_has_load_ia32_efer || (enable_ept && ((vmx->vcpu.arch.efer ^ host_efer) & EFER_NX))) { - guest_efer = vmx->vcpu.arch.efer; if (!(guest_efer & EFER_LMA)) guest_efer &= ~EFER_LME; if (guest_efer != host_efer) add_atomic_switch_msr(vmx, MSR_EFER, guest_efer, host_efer); return false; - } + } else { + guest_efer &= ~ignore_bits; + guest_efer |= host_efer & ignore_bits; - return true; + vmx->guest_msrs[efer_offset].data = guest_efer; + vmx->guest_msrs[efer_offset].mask = ~ignore_bits; + + return true; + } } static unsigned long segment_base(u16 selector) -- 2.39.5