[PATCH v2 1/2] KVM: x86: relax canonical check for some x86 architectural msrs

Maxim Levitsky
2024-08-02T15:16:25Z

Several architectural msrs (e.g MSR_KERNEL_GS_BASE) must contain
a canonical address, and according to Intel PRM, this is enforced
by a #GP canonical check during MSR write.

However as it turns out, the supported address width
used for this canonical check is determined only
by host cpu model:
if CPU *supports* 5 level paging, the width will be 57
regardless of the state of CR4.LA57.

Experemental tests on a Sapphire Rapids CPU and on a Zen4 CPU
confirm this behavior.

In addition to that, the Intel ISA extension manual mentions that this might
be the architectural behavior:

Architecture Instruction Set Extensions and Future Features Programming Reference [1].
Chapter 6.4:

"CANONICALITY CHECKING FOR DATA ADDRESSES WRITTEN TO CONTROL REGISTERS AND
MSRS"

"In Processors that support LAM continue to require the addresses written to
control registers or MSRs to be 57-bit canonical if the processor _supports_
5-level paging or 48-bit canonical if it supports only 4-level paging"

[1]: https://cdrdv2.intel.com/v1/dl/getContent/671368

Suggested-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
arch/x86/kvm/x86.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a6968eadd418..3582f0bb7644 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1844,7 +1844,16 @@ static int __kvm_set_msr(struct kvm_vcpu *vcpu, u32 index, u64 data,
case MSR_KERNEL_GS_BASE:
case MSR_CSTAR:
case MSR_LSTAR:
- if (is_noncanonical_address(data, vcpu))
+
+ /*
+ * Both AMD and Intel cpus allow values which
+ * are canonical in the 5 level paging mode but are not
+ * canonical in the 4 level paging mode to be written
+ * to the above MSRs, as long as the host CPU supports
+ * 5 level paging, regardless of the state of the CR4.LA57.
+ */
+ if (!__is_canonical_address(data,
+ kvm_cpu_cap_has(X86_FEATURE_LA57) ? 57 : 48))
return 1;
break;
case MSR_IA32_SYSENTER_EIP:
--
2.40.1