[v2] KVM: nVMX: Consult only the "basic" exit reason when routing nested exit

Commit Message

Sean Christopherson June 8, 2020, 7:18 p.m. UTC

Consult only the basic exit reason, i.e. bits 15:0 of vmcs.EXIT_REASON,
when determining whether a nested VM-Exit should be reflected into L1 or
handled by KVM in L0.

For better or worse, the switch statements nested_vmx_l0_wants_exit()
and nested_vmx_l1_wants_exit() default to reflecting the VM-Exit into L1
for any nested VM-Exit without dedicated logic.  Because the case
statements only contain the basic exit reason, any VM-Exit with modifier
bits set will be reflected to L1, even if KVM intended to handle it in
L0.

Practically speaking, this only affects EXIT_REASON_MCE_DURING_VMENTRY,
i.e. a #MC that occurs on nested VM-Enter would be incorrectly routed to
L1, as "failed VM-Entry" is the only modifier that KVM can currently
encounter.  The SMM modifiers will never be generated as KVM doesn't
support/employ a SMI Transfer Monitor.  Ditto for "exit from enclave",
as KVM doesn't yet support virtualizing SGX, i.e. it's impossible to
enter an enclave in a KVM guest (L1 or L2).

Note, the original version of this fix[*] is functionally equivalent and
far more suited to backporting as the affected code was refactored since
the original patch was posted.

[*] https://lkml.kernel.org/r/[e-mail]

Fixes: 644d711aa0e1 ("KVM: nVMX: Deciding if L0 or L1 should handle an L2 exit")
Cc: Jim Mattson <[e-mail]>
Cc: Xiaoyao Li <[e-mail]>
Cc: [e-mail]
Cc: Oliver Upton <[e-mail]>
Cc: Krish Sadhukhan <[e-mail]>
Cc: Miaohe Lin <[e-mail]>
Signed-off-by: Sean Christopherson <[e-mail]>
---

Another wounded soldier. 

Oliver, Krish, and Miaohe all provided reviews for v1, but I didn't feel
comfortable adding the tags to v2 because this is far from a straight
rebase.

v2: Rebased to kvm/queue, commit fb7333dfd812 ("KVM: SVM: fix calls ...").

 arch/x86/kvm/vmx/nested.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Comments

Oliver Upton June 8, 2020, 7:38 p.m. UTC | #1

On Mon, Jun 8, 2020 at 12:19 PM Sean Christopherson
<[e-mail]> wrote:
>
> Consult only the basic exit reason, i.e. bits 15:0 of vmcs.EXIT_REASON,
> when determining whether a nested VM-Exit should be reflected into L1 or
> handled by KVM in L0.
>
> For better or worse, the switch statements nested_vmx_l0_wants_exit()
> and nested_vmx_l1_wants_exit() default to reflecting the VM-Exit into L1
> for any nested VM-Exit without dedicated logic.  Because the case
> statements only contain the basic exit reason, any VM-Exit with modifier
> bits set will be reflected to L1, even if KVM intended to handle it in
> L0.
>
> Practically speaking, this only affects EXIT_REASON_MCE_DURING_VMENTRY,
> i.e. a #MC that occurs on nested VM-Enter would be incorrectly routed to
> L1, as "failed VM-Entry" is the only modifier that KVM can currently
> encounter.  The SMM modifiers will never be generated as KVM doesn't
> support/employ a SMI Transfer Monitor.  Ditto for "exit from enclave",
> as KVM doesn't yet support virtualizing SGX, i.e. it's impossible to
> enter an enclave in a KVM guest (L1 or L2).
>
> Note, the original version of this fix[*] is functionally equivalent and
> far more suited to backporting as the affected code was refactored since
> the original patch was posted.
>
> [*] https://lkml.kernel.org/r/[e-mail]
>
> Fixes: 644d711aa0e1 ("KVM: nVMX: Deciding if L0 or L1 should handle an L2 exit")
> Cc: Jim Mattson <[e-mail]>
> Cc: Xiaoyao Li <[e-mail]>
> Cc: [e-mail]
> Cc: Oliver Upton <[e-mail]>
> Cc: Krish Sadhukhan <[e-mail]>
> Cc: Miaohe Lin <[e-mail]>
> Signed-off-by: Sean Christopherson <[e-mail]>
Reviewed-by: Oliver Upton <[e-mail]>
> ---
>
> Another wounded soldier.
>
> Oliver, Krish, and Miaohe all provided reviews for v1, but I didn't feel
> comfortable adding the tags to v2 because this is far from a straight
> rebase.
>
> v2: Rebased to kvm/queue, commit fb7333dfd812 ("KVM: SVM: fix calls ...").
>
>  arch/x86/kvm/vmx/nested.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index bcb50724be38..adb11b504d5c 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -5672,7 +5672,7 @@ static bool nested_vmx_l0_wants_exit(struct kvm_vcpu *vcpu, u32 exit_reason)
>  {
>         u32 intr_info;
>
> -       switch (exit_reason) {
> +       switch ((u16)exit_reason) {
>         case EXIT_REASON_EXCEPTION_NMI:
>                 intr_info = vmx_get_intr_info(vcpu);
>                 if (is_nmi(intr_info))
> @@ -5733,7 +5733,7 @@ static bool nested_vmx_l1_wants_exit(struct kvm_vcpu *vcpu, u32 exit_reason)
>         struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
>         u32 intr_info;
>
> -       switch (exit_reason) {
> +       switch ((u16)exit_reason) {
>         case EXIT_REASON_EXCEPTION_NMI:
>                 intr_info = vmx_get_intr_info(vcpu);
>                 if (is_nmi(intr_info))
> --
> 2.26.0
>

Paolo Bonzini June 9, 2020, 10:18 a.m. UTC | #2

On 08/06/20 21:18, Sean Christopherson wrote:
> Consult only the basic exit reason, i.e. bits 15:0 of vmcs.EXIT_REASON,
> when determining whether a nested VM-Exit should be reflected into L1 or
> handled by KVM in L0.
> 
> For better or worse, the switch statements nested_vmx_l0_wants_exit()
> and nested_vmx_l1_wants_exit() default to reflecting the VM-Exit into L1
> for any nested VM-Exit without dedicated logic.  Because the case
> statements only contain the basic exit reason, any VM-Exit with modifier
> bits set will be reflected to L1, even if KVM intended to handle it in
> L0.
> 
> Practically speaking, this only affects EXIT_REASON_MCE_DURING_VMENTRY,
> i.e. a #MC that occurs on nested VM-Enter would be incorrectly routed to
> L1, as "failed VM-Entry" is the only modifier that KVM can currently
> encounter.  The SMM modifiers will never be generated as KVM doesn't
> support/employ a SMI Transfer Monitor.  Ditto for "exit from enclave",
> as KVM doesn't yet support virtualizing SGX, i.e. it's impossible to
> enter an enclave in a KVM guest (L1 or L2).
> 
> Note, the original version of this fix[*] is functionally equivalent and
> far more suited to backporting as the affected code was refactored since
> the original patch was posted.
> 
> [*] https://lkml.kernel.org/r/[e-mail]
> 
> Fixes: 644d711aa0e1 ("KVM: nVMX: Deciding if L0 or L1 should handle an L2 exit")
> Cc: Jim Mattson <[e-mail]>
> Cc: Xiaoyao Li <[e-mail]>
> Cc: [e-mail]
> Cc: Oliver Upton <[e-mail]>
> Cc: Krish Sadhukhan <[e-mail]>
> Cc: Miaohe Lin <[e-mail]>
> Signed-off-by: Sean Christopherson <[e-mail]>
> ---
> 
> Another wounded soldier. 
> 
> Oliver, Krish, and Miaohe all provided reviews for v1, but I didn't feel
> comfortable adding the tags to v2 because this is far from a straight
> rebase.
> 
> v2: Rebased to kvm/queue, commit fb7333dfd812 ("KVM: SVM: fix calls ...").
> 
>  arch/x86/kvm/vmx/nested.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index bcb50724be38..adb11b504d5c 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -5672,7 +5672,7 @@ static bool nested_vmx_l0_wants_exit(struct kvm_vcpu *vcpu, u32 exit_reason)
>  {
>  	u32 intr_info;
>  
> -	switch (exit_reason) {
> +	switch ((u16)exit_reason) {
>  	case EXIT_REASON_EXCEPTION_NMI:
>  		intr_info = vmx_get_intr_info(vcpu);
>  		if (is_nmi(intr_info))
> @@ -5733,7 +5733,7 @@ static bool nested_vmx_l1_wants_exit(struct kvm_vcpu *vcpu, u32 exit_reason)
>  	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
>  	u32 intr_info;
>  
> -	switch (exit_reason) {
> +	switch ((u16)exit_reason) {
>  	case EXIT_REASON_EXCEPTION_NMI:
>  		intr_info = vmx_get_intr_info(vcpu);
>  		if (is_nmi(intr_info))
> 

Thanks, queued with git magic (committed v1 based on v5.7, merged into
kvm/queue, added the delta to v2 in the merge commit) so that stable
branches can cherry-pick cleanly.

Paolo

Patch

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index bcb50724be38..adb11b504d5c 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -5672,7 +5672,7 @@  static bool nested_vmx_l0_wants_exit(struct kvm_vcpu *vcpu, u32 exit_reason)
 {
 	u32 intr_info;
 
-	switch (exit_reason) {
+	switch ((u16)exit_reason) {
 	case EXIT_REASON_EXCEPTION_NMI:
 		intr_info = vmx_get_intr_info(vcpu);
 		if (is_nmi(intr_info))
@@ -5733,7 +5733,7 @@  static bool nested_vmx_l1_wants_exit(struct kvm_vcpu *vcpu, u32 exit_reason)
 	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
 	u32 intr_info;
 
-	switch (exit_reason) {
+	switch ((u16)exit_reason) {
 	case EXIT_REASON_EXCEPTION_NMI:
 		intr_info = vmx_get_intr_info(vcpu);
 		if (is_nmi(intr_info))