| Message ID | 20240408231500.1388122-1-seanjc@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: x86: Allow, don't ignore, same-value writes to immutable MSRs | expand |
On Mon, 08 Apr 2024 16:15:00 -0700, Sean Christopherson wrote: > When handling userspace writes to immutable feature MSRs for a vCPU that > has already run, fall through into the normal code to set the MSR instead > of immediately returning '0'. I.e. allow such writes, instead of ignoring > such writes. This fixes a bug where KVM incorrectly allows writes to the > VMX MSRs that enumerate which CR{0,4} can be set, but only if the vCPU has > already run. > > [...] Applied to kvm-x86 misc. [1/1] KVM: x86: Allow, don't ignore, same-value writes to immutable MSRs https://github.com/kvm-x86/linux/commit/1d294dfaba8c -- https://github.com/kvm-x86/linux/tree/next
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 47d9f03b7778..a3358c80a88e 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -2230,16 +2230,13 @@ static int do_set_msr(struct kvm_vcpu *vcpu, unsigned index, u64 *data) /* * Disallow writes to immutable feature MSRs after KVM_RUN. KVM does * not support modifying the guest vCPU model on the fly, e.g. changing - * the nVMX capabilities while L2 is running is nonsensical. Ignore + * the nVMX capabilities while L2 is running is nonsensical. Allow * writes of the same value, e.g. to allow userspace to blindly stuff * all MSRs when emulating RESET. */ - if (kvm_vcpu_has_run(vcpu) && kvm_is_immutable_feature_msr(index)) { - if (do_get_msr(vcpu, index, &val) || *data != val) - return -EINVAL; - - return 0; - } + if (kvm_vcpu_has_run(vcpu) && kvm_is_immutable_feature_msr(index) && + (do_get_msr(vcpu, index, &val) || *data != val)) + return -EINVAL; return kvm_set_msr_ignored_check(vcpu, index, *data, true); }
When handling userspace writes to immutable feature MSRs for a vCPU that has already run, fall through into the normal code to set the MSR instead of immediately returning '0'. I.e. allow such writes, instead of ignoring such writes. This fixes a bug where KVM incorrectly allows writes to the VMX MSRs that enumerate which CR{0,4} can be set, but only if the vCPU has already run. The intent of returning '0' and thus ignoring the write, was to avoid any side effects, e.g. refreshing the PMU and thus doing weird things with perf events while the vCPU is running. That approach sounds nice in theory, but in practice it makes it all but impossible to maintain a sane ABI, e.g. all VMX MSRs return -EBUSY if the CPU is post-VMXON, and the VMX MSRs for fixed-1 CR bits are never writable, etc. As for refreshing the PMU, kvm_set_msr_common() explicitly skips the PMU refresh if MSR_IA32_PERF_CAPABILITIES is being written with the current value, specifically to avoid unwanted side effects. And if necessary, adding similar logic for other MSRs is not difficult. Fixes: 0094f62c7eaa ("KVM: x86: Disallow writes to immutable feature MSRs after KVM_RUN") Reported-by: Jim Mattson <jmattson@google.com> Cc: Raghavendra Rao Ananta <rananta@google.com> Signed-off-by: Sean Christopherson <seanjc@google.com> --- arch/x86/kvm/x86.c | 11 ++++------- 1 file changed, 4 insertions(+), 7 deletions(-) base-commit: 8cb4a9a82b21623dbb4b3051dd30d98356cf95bc