| Message ID | 20211222124052.644626-6-jing2.liu@intel.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | AMX Support in KVM | expand |
On Wed, Dec 22, 2021, Jing Liu wrote: > Guest xstate permissions should be set by userspace VMM before vcpu > creation. Extend KVM_SET_CPUID2 to verify that every feature reported > in CPUID[0xD] has proper permission set. Why? Nothing in the changelog, code, or comments explains why KVM _needs_ to prevent userspace from advertising bogus features to the guest. E.g. the virtual address width check exists because _KVM_ will do the wrong thing given a width other than 48 or 57, and explicity says as much in a comment.
> From: Sean Christopherson <seanjc@google.com> > Sent: Wednesday, December 29, 2021 7:39 AM > > On Wed, Dec 22, 2021, Jing Liu wrote: > > Guest xstate permissions should be set by userspace VMM before vcpu > > creation. Extend KVM_SET_CPUID2 to verify that every feature reported > > in CPUID[0xD] has proper permission set. > > Why? Nothing in the changelog, code, or comments explains why KVM > _needs_ to prevent > userspace from advertising bogus features to the guest. E.g. the virtual > address > width check exists because _KVM_ will do the wrong thing given a width > other than 48 > or 57, and explicity says as much in a comment. Advertising a known bogus feature due to lacking of permission does no good compared to failing it early even before the guest is running. This also avoids tons of complexity at run-time to deal with permission violation (e.g. introducing new kvm exit reason if you tracked the v2 discussion). But yes, we should add a clear comment here. Thanks Kevin
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c index 4855344091b8..a068373a7fbd 100644 --- a/arch/x86/kvm/cpuid.c +++ b/arch/x86/kvm/cpuid.c @@ -81,7 +81,9 @@ static inline struct kvm_cpuid_entry2 *cpuid_entry2_find( return NULL; } -static int kvm_check_cpuid(struct kvm_cpuid_entry2 *entries, int nent) +static int kvm_check_cpuid(struct kvm_vcpu *vcpu, + struct kvm_cpuid_entry2 *entries, + int nent) { struct kvm_cpuid_entry2 *best; @@ -97,6 +99,16 @@ static int kvm_check_cpuid(struct kvm_cpuid_entry2 *entries, int nent) return -EINVAL; } + /* Check guest permissions for dynamically-enabled xfeatures */ + best = cpuid_entry2_find(entries, nent, 0xd, 0); + if (best) { + u64 xfeatures; + + xfeatures = best->eax | ((u64)best->edx << 32); + if (xfeatures & ~vcpu->arch.guest_fpu.perm) + return -ENXIO; + } + return 0; } @@ -277,21 +289,21 @@ u64 kvm_vcpu_reserved_gpa_bits_raw(struct kvm_vcpu *vcpu) static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2, int nent) { - int r; + int r; - r = kvm_check_cpuid(e2, nent); - if (r) - return r; + r = kvm_check_cpuid(vcpu, e2, nent); + if (r) + return r; - kvfree(vcpu->arch.cpuid_entries); - vcpu->arch.cpuid_entries = e2; - vcpu->arch.cpuid_nent = nent; + kvfree(vcpu->arch.cpuid_entries); + vcpu->arch.cpuid_entries = e2; + vcpu->arch.cpuid_nent = nent; - kvm_update_kvm_cpuid_base(vcpu); - kvm_update_cpuid_runtime(vcpu); - kvm_vcpu_after_set_cpuid(vcpu); + kvm_update_kvm_cpuid_base(vcpu); + kvm_update_cpuid_runtime(vcpu); + kvm_vcpu_after_set_cpuid(vcpu); - return 0; + return 0; } /* when an old userspace process fills a new kernel module */
Guest xstate permissions should be set by userspace VMM before vcpu creation. Extend KVM_SET_CPUID2 to verify that every feature reported in CPUID[0xD] has proper permission set. Signed-off-by: Jing Liu <jing2.liu@intel.com> --- arch/x86/kvm/cpuid.c | 36 ++++++++++++++++++++++++------------ 1 file changed, 24 insertions(+), 12 deletions(-)