* [Bug 216033] New: KVM VMX nested virtualization: VMXON does not check guest CR0 against IA32_VMX_CR0_FIXED0
@ 2022-05-26 3:54 bugzilla-daemon
2022-05-26 16:18 ` [Bug 216033] " bugzilla-daemon
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0 siblings, 5 replies; 6+ messages in thread
From: bugzilla-daemon @ 2022-05-26 3:54 UTC (permalink / raw)
To: kvm
https://bugzilla.kernel.org/show_bug.cgi?id=216033
Bug ID: 216033
Summary: KVM VMX nested virtualization: VMXON does not check
guest CR0 against IA32_VMX_CR0_FIXED0
Product: Virtualization
Version: unspecified
Kernel Version: 5.17.8-200.fc35.x86_64
Hardware: Intel
OS: Linux
Tree: Mainline
Status: NEW
Severity: normal
Priority: P1
Component: kvm
Assignee: virtualization_kvm@kernel-bugs.osdl.org
Reporter: ercli@ucdavis.edu
Regression: No
Created attachment 301050
--> https://bugzilla.kernel.org/attachment.cgi?id=301050&action=edit
Guest hypervisor to reproduce this bug (xz compressed)
CPU model I am running: Intel(R) Core(TM) i7-4510U CPU @ 2.00GHz
Host kernel version: 5.17.8-200.fc35.x86_64
Host kernel arch: x86_64
Guest: a hypervisor I wrote myself, 32-bits, compressed and attached as
c.img.xz.
QEMU command line: qemu-system-x86_64 -m 512M -smp 1 -cpu Haswell,vmx=yes
-enable-kvm -serial stdio -drive media=disk,file=c.img,index=1
The problem does not go away if using -machine kernel_irqchip=off
Since the guest is a hypervisor, -accel tcg cannot be used (TCG does not
support nested virtualization)
Actual behavior: serial port shows:
...
CR0 = 0x0000000080000015
CR0 fixed0 = 0x0000000080000021
CR0 fixed1 = 0x00000000ffffffff
VMXON succeeds
Expected behaivor: serial port shows:
...
CR0 = 0x0000000080000015
CR0 fixed0 = 0x0000000080000021
CR0 fixed1 = 0x00000000ffffffff
[00]: unhandled exception 13 (0xd), halting!
[00]: error code: 0x00000000
[00]: state dump follows...
[00] CS:EIP ...
...
Explanation:
When the guest hypervisor starts VMX using the VMXON instruction, the guest
hypervisor's CR0 is not legal. IA32_VMX_CR0_FIXED0 = 0x0000000080000021. The
0x20 bit in this MSR is 1, which indicates that the 0x20 bit in CR0 must be 1
when executing VMXON. However, my hypervisor uses CR0 = 0x80000015 (the 0x20
bit is 0).
According to SDM 29.3, if "the values of CR0 and CR4 are not supported in VMX
operation", then a general protection exception (#GP(0)) should be raised. This
happens on real hardware, but not on KVM.
The relevant code in my hypervisor is:
https://github.com/lxylxy123456/uberxmhf/blob/770bdaa7afce560b9f46348bee5a05e2c680de06/xmhf/src/xmhf-core/xmhf-runtime/xmhf-startup/lhv-vmx.c#L250
The pseudo code is
print the hypervisor's CR0 to serial port
print MSR value IA32_VMX_CR0_FIXED0 to serial port
print MSR value IA32_VMX_CR0_FIXED1 to serial port
sleep for 3 seconds
run VMXON instruction
If succeed, write "VMXON succeeds" to serial port.
If VMXON receives an exception, write exception details to serial port ("[00]:
unhandled exception...")
To fix this bug, handle_vmon() in arch/x86/kvm/vmx/nested.c needs to be
updated. The check to CR0 and CR4 against IA32_VMX_CR0_FIXED0 etc need to be
added.
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* [Bug 216033] KVM VMX nested virtualization: VMXON does not check guest CR0 against IA32_VMX_CR0_FIXED0
2022-05-26 3:54 [Bug 216033] New: KVM VMX nested virtualization: VMXON does not check guest CR0 against IA32_VMX_CR0_FIXED0 bugzilla-daemon
@ 2022-05-26 16:18 ` bugzilla-daemon
2022-09-02 18:39 ` bugzilla-daemon
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: bugzilla-daemon @ 2022-05-26 16:18 UTC (permalink / raw)
To: kvm
https://bugzilla.kernel.org/show_bug.cgi?id=216033
Sean Christopherson (seanjc@google.com) changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |seanjc@google.com
--- Comment #1 from Sean Christopherson (seanjc@google.com) ---
Ugh, KVM is comically wrong. It _deliberately_ avoids checking CR0/CR4 with a
comment saying that "most faulting conditions have already been checked by
hardware", but the SDM pseudocode makes it very clear that only the (CR0.PE =
0) or (CR4.VMXE = 0) or (RFLAGS.VM = 1) or (IA32_EFER.LMA = 1 and CS.L = 0)
checks are performed before the VM-Exit occurs.
/*
* The Intel VMX Instruction Reference lists a bunch of bits that are
* prerequisite to running VMXON, most notably cr4.VMXE must be set to
* 1 (see vmx_is_valid_cr4() for when we allow the guest to set this).
* Otherwise, we should fail with #UD. But most faulting conditions
* have already been checked by hardware, prior to the VM-exit for
* VMXON. We do test guest cr4.VMXE because processor CR4 always has
* that bit set to 1 in non-root mode.
*/
if (!kvm_read_cr4_bits(vcpu, X86_CR4_VMXE)) {
kvm_queue_exception(vcpu, UD_VECTOR);
return 1;
}
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* [Bug 216033] KVM VMX nested virtualization: VMXON does not check guest CR0 against IA32_VMX_CR0_FIXED0
2022-05-26 3:54 [Bug 216033] New: KVM VMX nested virtualization: VMXON does not check guest CR0 against IA32_VMX_CR0_FIXED0 bugzilla-daemon
2022-05-26 16:18 ` [Bug 216033] " bugzilla-daemon
@ 2022-09-02 18:39 ` bugzilla-daemon
2022-09-02 19:00 ` bugzilla-daemon
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: bugzilla-daemon @ 2022-09-02 18:39 UTC (permalink / raw)
To: kvm
https://bugzilla.kernel.org/show_bug.cgi?id=216033
--- Comment #2 from Eric Li (ercli@ucdavis.edu) ---
@Sean Christopherson Thanks for submitting the fix to this bug in
https://lore.kernel.org/lkml/20220607213604.3346000-4-seanjc@google.com/ .
However, I recently tested this fix and the behavior is not as expected.
According to Intel's SDM, VMXON may generate 2 types of exceptions:
IF (register operand) or (CR0.PE = 0) or (CR4.VMXE = 0) or ...
THEN #UD;
ELSIF not in VMX operation
THEN
IF (CPL > 0) or (in A20M mode) or
(the values of CR0 and CR4 are not supported in VMX operation ...
THEN #GP(0);
For example, when CR4 value is incorrect, different exceptions may be generated
depending on which bit is incorrect. If CR4.VMXE = 0, #UD should be generated.
Otherwise, #GP(0) should be generated. However, after the fix, #UD is always
generated.
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* [Bug 216033] KVM VMX nested virtualization: VMXON does not check guest CR0 against IA32_VMX_CR0_FIXED0
2022-05-26 3:54 [Bug 216033] New: KVM VMX nested virtualization: VMXON does not check guest CR0 against IA32_VMX_CR0_FIXED0 bugzilla-daemon
2022-05-26 16:18 ` [Bug 216033] " bugzilla-daemon
2022-09-02 18:39 ` bugzilla-daemon
@ 2022-09-02 19:00 ` bugzilla-daemon
2022-09-02 19:45 ` bugzilla-daemon
2022-09-02 20:57 ` bugzilla-daemon
4 siblings, 0 replies; 6+ messages in thread
From: bugzilla-daemon @ 2022-09-02 19:00 UTC (permalink / raw)
To: kvm
https://bugzilla.kernel.org/show_bug.cgi?id=216033
--- Comment #3 from Sean Christopherson (seanjc@google.com) ---
On Fri, Sep 02, 2022, bugzilla-daemon@kernel.org wrote:
> https://bugzilla.kernel.org/show_bug.cgi?id=216033
>
> --- Comment #2 from Eric Li (ercli@ucdavis.edu) ---
> @Sean Christopherson Thanks for submitting the fix to this bug in
> https://lore.kernel.org/lkml/20220607213604.3346000-4-seanjc@google.com/ .
> However, I recently tested this fix and the behavior is not as expected.
>
> According to Intel's SDM, VMXON may generate 2 types of exceptions:
>
> IF (register operand) or (CR0.PE = 0) or (CR4.VMXE = 0) or ...
> THEN #UD;
> ELSIF not in VMX operation
> THEN
> IF (CPL > 0) or (in A20M mode) or
> (the values of CR0 and CR4 are not supported in VMX operation ...
> THEN #GP(0);
>
> For example, when CR4 value is incorrect, different exceptions may be
> generated
> depending on which bit is incorrect. If CR4.VMXE = 0, #UD should be
> generated.
> Otherwise, #GP(0) should be generated. However, after the fix, #UD is always
> generated.
/facepalm
All that and I overlooked that the other CR0/CR4 checks take a #GP.
On the bright side, it does mean I can blame Jim at least a little bit for
commit
70f3aac964ae ("kvm: nVMX: Remove superfluous VMX instruction fault checks").
Untested, but this should do the trick.
---
arch/x86/kvm/vmx/nested.c | 23 +++++++++++++++--------
1 file changed, 15 insertions(+), 8 deletions(-)
diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index ddd4367d4826..86ee2ab8a497 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -4936,25 +4936,32 @@ static int handle_vmxon(struct kvm_vcpu *vcpu)
| FEAT_CTL_VMX_ENABLED_OUTSIDE_SMX;
/*
- * Note, KVM cannot rely on hardware to perform the CR0/CR4 #UD checks
- * that have higher priority than VM-Exit (see Intel SDM's pseudocode
- * for VMXON), as KVM must load valid CR0/CR4 values into hardware
while
- * running the guest, i.e. KVM needs to check the _guest_ values.
+ * Note, KVM cannot rely on hardware to perform the CR0.PE and CR4.VMXE
+ * #UD checks that have higher priority than VM-Exit (see Intel SDM's
+ * pseudocode for VMXON), as KVM must load valid CR0/CR4 values into
+ * hardware while running the guest, i.e. KVM needs to check the
_guest_
+ * values.
*
* Rely on hardware for the other two pre-VM-Exit checks, !VM86 and
* !COMPATIBILITY modes. KVM may run the guest in VM86 to emulate Real
* Mode, but KVM will never take the guest out of those modes.
*/
+ if (!kvm_read_cr4_bits(vcpu, X86_CR4_VMXE) ||
+ !kvm_read_cr0_bits(vcpu, X86_CR0_PE)) {
+ kvm_queue_exception(vcpu, UD_VECTOR);
+ return 1;
+ }
+
+ /*
+ * All other checks that are lower priority than VM-Exit must be
+ * checked manually, including the other CR0/CR4 reserved bit checks.
+ */
if (!nested_host_cr0_valid(vcpu, kvm_read_cr0(vcpu)) ||
!nested_host_cr4_valid(vcpu, kvm_read_cr4(vcpu))) {
kvm_queue_exception(vcpu, UD_VECTOR);
return 1;
}
- /*
- * CPL=0 and all other checks that are lower priority than VM-Exit must
- * be checked manually.
- */
if (vmx_get_cpl(vcpu)) {
kvm_inject_gp(vcpu, 0);
return 1;
base-commit: 476d5fb78ea6438941559af4814a2795849cb8f0
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* [Bug 216033] KVM VMX nested virtualization: VMXON does not check guest CR0 against IA32_VMX_CR0_FIXED0
2022-05-26 3:54 [Bug 216033] New: KVM VMX nested virtualization: VMXON does not check guest CR0 against IA32_VMX_CR0_FIXED0 bugzilla-daemon
` (2 preceding siblings ...)
2022-09-02 19:00 ` bugzilla-daemon
@ 2022-09-02 19:45 ` bugzilla-daemon
2022-09-02 20:57 ` bugzilla-daemon
4 siblings, 0 replies; 6+ messages in thread
From: bugzilla-daemon @ 2022-09-02 19:45 UTC (permalink / raw)
To: kvm
https://bugzilla.kernel.org/show_bug.cgi?id=216033
--- Comment #4 from Eric Li (ercli@ucdavis.edu) ---
> if (!nested_host_cr0_valid(vcpu, kvm_read_cr0(vcpu)) ||
> !nested_host_cr4_valid(vcpu, kvm_read_cr4(vcpu))) {
> kvm_queue_exception(vcpu, UD_VECTOR);
> return 1;
> }
Thanks for the reply. I think there is still a typo. Do you mean the following?
if (!nested_host_cr0_valid(vcpu, kvm_read_cr0(vcpu)) ||
!nested_host_cr4_valid(vcpu, kvm_read_cr4(vcpu))) {
kvm_queue_exception_e(vcpu, GP_VECTOR, 0);
return 1;
}
Or maybe:
if (!nested_host_cr0_valid(vcpu, kvm_read_cr0(vcpu)) ||
!nested_host_cr4_valid(vcpu, kvm_read_cr4(vcpu))) {
kvm_inject_gp(vcpu, 0);
return 1;
}
I am not familiar with KVM code so not sure which one should be used. Thanks
again!
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* [Bug 216033] KVM VMX nested virtualization: VMXON does not check guest CR0 against IA32_VMX_CR0_FIXED0
2022-05-26 3:54 [Bug 216033] New: KVM VMX nested virtualization: VMXON does not check guest CR0 against IA32_VMX_CR0_FIXED0 bugzilla-daemon
` (3 preceding siblings ...)
2022-09-02 19:45 ` bugzilla-daemon
@ 2022-09-02 20:57 ` bugzilla-daemon
4 siblings, 0 replies; 6+ messages in thread
From: bugzilla-daemon @ 2022-09-02 20:57 UTC (permalink / raw)
To: kvm
https://bugzilla.kernel.org/show_bug.cgi?id=216033
--- Comment #5 from Sean Christopherson (seanjc@google.com) ---
On Fri, Sep 02, 2022, bugzilla-daemon@kernel.org wrote:
> https://bugzilla.kernel.org/show_bug.cgi?id=216033
>
> --- Comment #4 from Eric Li (ercli@ucdavis.edu) ---
> > if (!nested_host_cr0_valid(vcpu, kvm_read_cr0(vcpu)) ||
> > !nested_host_cr4_valid(vcpu, kvm_read_cr4(vcpu))) {
> > kvm_queue_exception(vcpu, UD_VECTOR);
> > return 1;
> > }
>
> Thanks for the reply. I think there is still a typo. Do you mean the
> following?
Yes, yes I did.
> if (!nested_host_cr0_valid(vcpu, kvm_read_cr0(vcpu)) ||
> !nested_host_cr4_valid(vcpu, kvm_read_cr4(vcpu))) {
> kvm_queue_exception_e(vcpu, GP_VECTOR, 0);
> return 1;
> }
>
> Or maybe:
>
> if (!nested_host_cr0_valid(vcpu, kvm_read_cr0(vcpu)) ||
> !nested_host_cr4_valid(vcpu, kvm_read_cr4(vcpu))) {
> kvm_inject_gp(vcpu, 0);
> return 1;
> }
>
> I am not familiar with KVM code
Heh, for all the good that being familiar with KVM is doing me.
> so not sure which one should be used. Thanks again!
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