Commit b68f3cc7 authored by Sean Christopherson's avatar Sean Christopherson Committed by Paolo Bonzini
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KVM: x86: Always use 32-bit SMRAM save state for 32-bit kernels 



Invoking the 64-bit variation on a 32-bit kenrel will crash the guest,
trigger a WARN, and/or lead to a buffer overrun in the host, e.g.
rsm_load_state_64() writes r8-r15 unconditionally, but enum kvm_reg and
thus x86_emulate_ctxt._regs only define r8-r15 for CONFIG_X86_64.

KVM allows userspace to report long mode support via CPUID, even though
the guest is all but guaranteed to crash if it actually tries to enable
long mode.  But, a pure 32-bit guest that is ignorant of long mode will
happily plod along.

SMM complicates things as 64-bit CPUs use a different SMRAM save state
area.  KVM handles this correctly for 64-bit kernels, e.g. uses the
legacy save state map if userspace has hid long mode from the guest,
but doesn't fare well when userspace reports long mode support on a
32-bit host kernel (32-bit KVM doesn't support 64-bit guests).

Since the alternative is to crash the guest, e.g. by not loading state
or explicitly requesting shutdown, unconditionally use the legacy SMRAM
save state map for 32-bit KVM.  If a guest has managed to get far enough
to handle SMIs when running under a weird/buggy userspace hypervisor,
then don't deliberately crash the guest since there are no downsides
(from KVM's perspective) to allow it to continue running.

Fixes: 660a5d51 ("KVM: x86: save/load state on SMM switch")
Cc: stable@vger.kernel.org
Signed-off-by: default avatarSean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 8f4dc2e7

arch/x86/kvm/emulate.c

+10 −0
Original line number Diff line number Diff line
@@ -2331,12 +2331,16 @@ static int em_lseg(struct x86_emulate_ctxt *ctxt) 


static int emulator_has_longmode(struct x86_emulate_ctxt *ctxt)

{

#ifdef CONFIG_X86_64

	u32 eax, ebx, ecx, edx;



	eax = 0x80000001;

	ecx = 0;

	ctxt->ops->get_cpuid(ctxt, &eax, &ebx, &ecx, &edx, false);

	return edx & bit(X86_FEATURE_LM);

#else

	return false;

#endif

}



static void rsm_set_desc_flags(struct desc_struct *desc, u32 flags)
@@ -2372,6 +2376,7 @@ static int rsm_load_seg_32(struct x86_emulate_ctxt *ctxt, const char *smstate, 
	return X86EMUL_CONTINUE;

}



#ifdef CONFIG_X86_64

static int rsm_load_seg_64(struct x86_emulate_ctxt *ctxt, const char *smstate,

			   int n)

{
@@ -2391,6 +2396,7 @@ static int rsm_load_seg_64(struct x86_emulate_ctxt *ctxt, const char *smstate, 
	ctxt->ops->set_segment(ctxt, selector, &desc, base3, n);

	return X86EMUL_CONTINUE;

}

#endif



static int rsm_enter_protected_mode(struct x86_emulate_ctxt *ctxt,

				    u64 cr0, u64 cr3, u64 cr4)
@@ -2492,6 +2498,7 @@ static int rsm_load_state_32(struct x86_emulate_ctxt *ctxt, 
	return rsm_enter_protected_mode(ctxt, cr0, cr3, cr4);

}



#ifdef CONFIG_X86_64

static int rsm_load_state_64(struct x86_emulate_ctxt *ctxt,

			     const char *smstate)

{
@@ -2554,6 +2561,7 @@ static int rsm_load_state_64(struct x86_emulate_ctxt *ctxt, 


	return X86EMUL_CONTINUE;

}

#endif



static int em_rsm(struct x86_emulate_ctxt *ctxt)

{
@@ -2621,9 +2629,11 @@ static int em_rsm(struct x86_emulate_ctxt *ctxt) 
	if (ctxt->ops->pre_leave_smm(ctxt, buf))

		return X86EMUL_UNHANDLEABLE;



#ifdef CONFIG_X86_64

	if (emulator_has_longmode(ctxt))

		ret = rsm_load_state_64(ctxt, buf);

	else

#endif

		ret = rsm_load_state_32(ctxt, buf);



	if (ret != X86EMUL_CONTINUE) {

arch/x86/kvm/x86.c

+6 −4
Original line number Diff line number Diff line
@@ -7441,9 +7441,9 @@ static void enter_smm_save_state_32(struct kvm_vcpu *vcpu, char *buf) 
	put_smstate(u32, buf, 0x7ef8, vcpu->arch.smbase);

}



#ifdef CONFIG_X86_64

static void enter_smm_save_state_64(struct kvm_vcpu *vcpu, char *buf)

{

#ifdef CONFIG_X86_64

	struct desc_ptr dt;

	struct kvm_segment seg;

	unsigned long val;
@@ -7493,10 +7493,8 @@ static void enter_smm_save_state_64(struct kvm_vcpu *vcpu, char *buf) 


	for (i = 0; i < 6; i++)

		enter_smm_save_seg_64(vcpu, buf, i);

#else

	WARN_ON_ONCE(1);

#endif

}

#endif



static void enter_smm(struct kvm_vcpu *vcpu)

{
@@ -7507,9 +7505,11 @@ static void enter_smm(struct kvm_vcpu *vcpu) 


	trace_kvm_enter_smm(vcpu->vcpu_id, vcpu->arch.smbase, true);

	memset(buf, 0, 512);

#ifdef CONFIG_X86_64

	if (guest_cpuid_has(vcpu, X86_FEATURE_LM))

		enter_smm_save_state_64(vcpu, buf);

	else

#endif

		enter_smm_save_state_32(vcpu, buf);



	/*
@@ -7567,8 +7567,10 @@ static void enter_smm(struct kvm_vcpu *vcpu) 
	kvm_set_segment(vcpu, &ds, VCPU_SREG_GS);

	kvm_set_segment(vcpu, &ds, VCPU_SREG_SS);



#ifdef CONFIG_X86_64

	if (guest_cpuid_has(vcpu, X86_FEATURE_LM))

		kvm_x86_ops->set_efer(vcpu, 0);

#endif



	kvm_update_cpuid(vcpu);

	kvm_mmu_reset_context(vcpu);

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