On 22.03.23 02:37, Mathias Krause wrote: > Guests like grsecurity that make heavy use of CR0.WP to implement kernel > level W^X will suffer from the implied VMEXITs. > > With EPT there is no need to intercept a guest change of CR0.WP, so > simply make it a guest owned bit if we can do so. > > This implies that a read of a guest's CR0.WP bit might need a VMREAD. > However, the only potentially affected user seems to be kvm_init_mmu() > which is a heavy operation to begin with. But also most callers already > cache the full value of CR0 anyway, so no additional VMREAD is needed. > The only exception is nested_vmx_load_cr3(). > > This change is VMX-specific, as SVM has no such fine grained control > register intercept control. Just a heads up! We did more tests, especially with the backports we did internally already, and ran into a bug when running a nested guest on an ESXi host. Setup is like: ESXi (L0) -> Linux (L1) -> Linux (L2) The Linux system, especially the kernel, is the same for L1 and L2. It's a grsecurity kernel, so makes use of toggling CR0.WP at runtime. The bug we see is that when L2 disables CR0.WP and tries to write to an r/o memory region (implicitly to the r/o GDT via LTR in our use case), this triggers a fault (EPT violation?) that gets ignored by L1, as, apparently, everything is fine from its point of view. I suspect the L0 VMM to be at fault here, as the VMCS structures look good, IMO. Here is a dump of vmx->loaded_vmcs in handle_triple_fault(): […] VMX: TRIPLE FAULT! […] VMCS ffff8883a9f18000, last attempted VM-entry on CPU 8 […] *** Guest State *** […] CR0: actual=0x0000000080040033, shadow=0x0000000080050033, gh_mask=fffffffffffefff7 CR0 in the L2 VM has CR0.WP disabled. However, it is set in the shadow CR0 but masked out via CR0_MASK, so should be read as the guest's value, according to the SDM. I also tried masking the shadow CR0 value in vmx_set_cr0(), but that makes no difference. […] CR4: actual=0x00000000003220f0, shadow=0x00000000003208b0, gh_mask=fffffffffffff871 […] CR3 = 0x0000000002684000 […] PDPTR0 = 0x0000000007d39001 PDPTR1 = 0x00000000033b5001 […] PDPTR2 = 0x000000000238c001 PDPTR3 = 0x0000000001c54001 […] RSP = 0xfffffe8040087f50 RIP = 0xffffffff8105d435 […] RFLAGS=0x00010006 DR7 = 0x0000000000000400 […] Sysenter RSP=0000000000000000 CS:RIP=0000:0000000000000000 […] CS: sel=0x0010, attr=0x0a09b, limit=0xffffffff, base=0x0000000000000000 […] DS: sel=0x0000, attr=0x1c001, limit=0xffffffff, base=0x0000000000000000 […] SS: sel=0x0000, attr=0x1c001, limit=0xffffffff, base=0x0000000000000000 […] ES: sel=0x0000, attr=0x1c001, limit=0xffffffff, base=0x0000000000000000 […] FS: sel=0x0000, attr=0x1c001, limit=0xffffffff, base=0x0000000000000000 […] GS: sel=0x0000, attr=0x1c001, limit=0xffffffff, base=0xffff888232e00000 […] GDTR: limit=0x00000097, base=0xfffffe0000201000 […] LDTR: sel=0x0000, attr=0x00082, limit=0x0000ffff, base=0x0000000000000000 […] IDTR: limit=0x000001ff, base=0xffffffff84004000 […] TR: sel=0x0000, attr=0x0008b, limit=0x0000ffff, base=0x0000000000000000 […] EFER= 0x0000000000000d01 (effective) […] PAT = 0x0007040600070406 […] DebugCtl = 0x0000000000000000 DebugExceptions = 0x0000000000000000 […] Interruptibility = 00000000 ActivityState = 00000000 […] *** Host State *** […] RIP = 0xffffffff86d28db6 RSP = 0xfffffe8040927d50 […] CS=0010 SS=0018 DS=0000 ES=0000 FS=0000 GS=0000 TR=0040 […] FSBase=0000639563fce700 GSBase=ffff88881a800000 TRBase=fffffe0001003000 […] GDTBase=fffffe0001001000 IDTBase=fffffe0000000000 […] CR0=0000000080050033 CR3=00000000026a0004 CR4=00000000007626f0 The "host" (which is our L1 VMM, I guess) has CR0.WP enabled and that is what I think confuses ESXi to enforce the read-only property to the L2 guest as well -- for unknown reasons so far. […] Sysenter RSP=fffffe0001003000 CS:RIP=0010:ffffffff810031a0 […] PAT = 0x0407050600070106 […] *** Control State *** […] PinBased=0000003f CPUBased=b5a06dfa SecondaryExec=001034ee […] EntryControls=000053ff ExitControls=000befff […] ExceptionBitmap=00060042 PFECmask=00000000 PFECmatch=00000000 […] VMEntry: intr_info=00000000 errcode=00000003 ilen=00000000 […] VMExit: intr_info=00000000 errcode=00000000 ilen=00000000 […] reason=00000002 qualification=0000000000000000 […] IDTVectoring: info=00000000 errcode=00000000 […] TSC Offset = 0xffffad7a1057f4cc […] TPR Threshold = 0x00 […] virt-APIC addr = 0x000000015716b000 […] EPT pointer = 0x0000000583c1e05e […] PLE Gap=00000080 Window=00001000 […] Virtual processor ID = 0x0002 I tried to reproduce the bug with different KVM based L0 VMMs (with and without this series; vanilla and grsecurity kernels) but no luck. That's why I'm suspecting a ESXi bug. I'm leaning to make CR0.WP guest owned only iff we're running on bare metal or the VMM is KVM to not play whack-a-mole for all the VMMs that might have similar bugs. (Will try to test a few others here as well.) However, that would prevent them from getting the performance gain, so I'd rather have this fixed / worked around in KVM instead. Any ideas how to investigate this further? Thanks, Mathias PS: ...should have left the chicken bit of v3 to be able to disable the feature by a module parameter ;) > > Suggested-and-co-developed-by: Sean Christopherson <seanjc@xxxxxxxxxx> > Signed-off-by: Mathias Krause <minipli@xxxxxxxxxxxxxx> > --- > arch/x86/kvm/kvm_cache_regs.h | 2 +- > arch/x86/kvm/vmx/nested.c | 4 ++-- > arch/x86/kvm/vmx/vmx.c | 2 +- > arch/x86/kvm/vmx/vmx.h | 18 ++++++++++++++++++ > 4 files changed, 22 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/kvm/kvm_cache_regs.h b/arch/x86/kvm/kvm_cache_regs.h > index 4c91f626c058..e50d353b5c1c 100644 > --- a/arch/x86/kvm/kvm_cache_regs.h > +++ b/arch/x86/kvm/kvm_cache_regs.h > @@ -4,7 +4,7 @@ > > #include <linux/kvm_host.h> > > -#define KVM_POSSIBLE_CR0_GUEST_BITS X86_CR0_TS > +#define KVM_POSSIBLE_CR0_GUEST_BITS (X86_CR0_TS | X86_CR0_WP) > #define KVM_POSSIBLE_CR4_GUEST_BITS \ > (X86_CR4_PVI | X86_CR4_DE | X86_CR4_PCE | X86_CR4_OSFXSR \ > | X86_CR4_OSXMMEXCPT | X86_CR4_PGE | X86_CR4_TSD | X86_CR4_FSGSBASE) > diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c > index f63b28f46a71..61d940fc91ba 100644 > --- a/arch/x86/kvm/vmx/nested.c > +++ b/arch/x86/kvm/vmx/nested.c > @@ -4481,7 +4481,7 @@ static void load_vmcs12_host_state(struct kvm_vcpu *vcpu, > * CR0_GUEST_HOST_MASK is already set in the original vmcs01 > * (KVM doesn't change it); > */ > - vcpu->arch.cr0_guest_owned_bits = KVM_POSSIBLE_CR0_GUEST_BITS; > + vcpu->arch.cr0_guest_owned_bits = vmx_l1_guest_owned_cr0_bits(); > vmx_set_cr0(vcpu, vmcs12->host_cr0); > > /* Same as above - no reason to call set_cr4_guest_host_mask(). */ > @@ -4632,7 +4632,7 @@ static void nested_vmx_restore_host_state(struct kvm_vcpu *vcpu) > */ > vmx_set_efer(vcpu, nested_vmx_get_vmcs01_guest_efer(vmx)); > > - vcpu->arch.cr0_guest_owned_bits = KVM_POSSIBLE_CR0_GUEST_BITS; > + vcpu->arch.cr0_guest_owned_bits = vmx_l1_guest_owned_cr0_bits(); > vmx_set_cr0(vcpu, vmcs_readl(CR0_READ_SHADOW)); > > vcpu->arch.cr4_guest_owned_bits = ~vmcs_readl(CR4_GUEST_HOST_MASK); > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c > index 8fc1a0c7856f..e501f6864a72 100644 > --- a/arch/x86/kvm/vmx/vmx.c > +++ b/arch/x86/kvm/vmx/vmx.c > @@ -4790,7 +4790,7 @@ static void init_vmcs(struct vcpu_vmx *vmx) > /* 22.2.1, 20.8.1 */ > vm_entry_controls_set(vmx, vmx_vmentry_ctrl()); > > - vmx->vcpu.arch.cr0_guest_owned_bits = KVM_POSSIBLE_CR0_GUEST_BITS; > + vmx->vcpu.arch.cr0_guest_owned_bits = vmx_l1_guest_owned_cr0_bits(); > vmcs_writel(CR0_GUEST_HOST_MASK, ~vmx->vcpu.arch.cr0_guest_owned_bits); > > set_cr4_guest_host_mask(vmx); > diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h > index 2acdc54bc34b..423e9d3c9c40 100644 > --- a/arch/x86/kvm/vmx/vmx.h > +++ b/arch/x86/kvm/vmx/vmx.h > @@ -640,6 +640,24 @@ BUILD_CONTROLS_SHADOW(tertiary_exec, TERTIARY_VM_EXEC_CONTROL, 64) > (1 << VCPU_EXREG_EXIT_INFO_1) | \ > (1 << VCPU_EXREG_EXIT_INFO_2)) > > +static inline unsigned long vmx_l1_guest_owned_cr0_bits(void) > +{ > + unsigned long bits = KVM_POSSIBLE_CR0_GUEST_BITS; > + > + /* > + * CR0.WP needs to be intercepted when KVM is shadowing legacy paging > + * in order to construct shadow PTEs with the correct protections. > + * Note! CR0.WP technically can be passed through to the guest if > + * paging is disabled, but checking CR0.PG would generate a cyclical > + * dependency of sorts due to forcing the caller to ensure CR0 holds > + * the correct value prior to determining which CR0 bits can be owned > + * by L1. Keep it simple and limit the optimization to EPT. > + */ > + if (!enable_ept) > + bits &= ~X86_CR0_WP; > + return bits; > +} > + > static __always_inline struct kvm_vmx *to_kvm_vmx(struct kvm *kvm) > { > return container_of(kvm, struct kvm_vmx, kvm);