* [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for vcpu->arch.guest_supported_xcr0 @ 2022-01-23 5:50 Like Xu 2022-01-24 7:06 ` Tian, Kevin 0 siblings, 1 reply; 8+ messages in thread From: Like Xu @ 2022-01-23 5:50 UTC (permalink / raw) To: Paolo Bonzini Cc: Jing Liu, Wanpeng Li, Sean Christopherson, Jim Mattson, Vitaly Kuznetsov, Joerg Roedel, kvm, linux-kernel From: Like Xu <likexu@tencent.com> A malicious user space can bypass xstate_get_guest_group_perm() in the KVM_GET_SUPPORTED_CPUID mechanism and obtain unpermitted xfeatures, since the validity check of xcr0 depends only on guest_supported_xcr0. Fixes: 445ecdf79be0 ("kvm: x86: Exclude unpermitted xfeatures at KVM_GET_SUPPORTED_CPUID") Signed-off-by: Like Xu <likexu@tencent.com> --- arch/x86/kvm/cpuid.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c index 3902c28fb6cb..1bd4d560cbdd 100644 --- a/arch/x86/kvm/cpuid.c +++ b/arch/x86/kvm/cpuid.c @@ -266,7 +266,8 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu) vcpu->arch.guest_supported_xcr0 = 0; else vcpu->arch.guest_supported_xcr0 = - (best->eax | ((u64)best->edx << 32)) & supported_xcr0; + (best->eax | ((u64)best->edx << 32)) & + (supported_xcr0 & xstate_get_guest_group_perm()); /* * Bits 127:0 of the allowed SECS.ATTRIBUTES (CPUID.0x12.0x1) enumerate -- 2.33.1 ^ permalink raw reply related [flat|nested] 8+ messages in thread
* RE: [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for vcpu->arch.guest_supported_xcr0 2022-01-23 5:50 [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for vcpu->arch.guest_supported_xcr0 Like Xu @ 2022-01-24 7:06 ` Tian, Kevin 2022-01-24 7:17 ` Like Xu 0 siblings, 1 reply; 8+ messages in thread From: Tian, Kevin @ 2022-01-24 7:06 UTC (permalink / raw) To: Like Xu, Paolo Bonzini Cc: Liu, Jing2, Wanpeng Li, Christopherson,, Sean, Jim Mattson, Vitaly Kuznetsov, Joerg Roedel, kvm, linux-kernel > From: Like Xu <like.xu.linux@gmail.com> > Sent: Sunday, January 23, 2022 1:50 PM > > From: Like Xu <likexu@tencent.com> > > A malicious user space can bypass xstate_get_guest_group_perm() in the > KVM_GET_SUPPORTED_CPUID mechanism and obtain unpermitted xfeatures, > since the validity check of xcr0 depends only on guest_supported_xcr0. Unpermitted xfeatures cannot pass kvm_check_cpuid()... > > Fixes: 445ecdf79be0 ("kvm: x86: Exclude unpermitted xfeatures at > KVM_GET_SUPPORTED_CPUID") > Signed-off-by: Like Xu <likexu@tencent.com> > --- > arch/x86/kvm/cpuid.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c > index 3902c28fb6cb..1bd4d560cbdd 100644 > --- a/arch/x86/kvm/cpuid.c > +++ b/arch/x86/kvm/cpuid.c > @@ -266,7 +266,8 @@ static void kvm_vcpu_after_set_cpuid(struct > kvm_vcpu *vcpu) > vcpu->arch.guest_supported_xcr0 = 0; > else > vcpu->arch.guest_supported_xcr0 = > - (best->eax | ((u64)best->edx << 32)) & > supported_xcr0; > + (best->eax | ((u64)best->edx << 32)) & > + (supported_xcr0 & xstate_get_guest_group_perm()); > > /* > * Bits 127:0 of the allowed SECS.ATTRIBUTES (CPUID.0x12.0x1) > enumerate > -- > 2.33.1 ^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for vcpu->arch.guest_supported_xcr0 2022-01-24 7:06 ` Tian, Kevin @ 2022-01-24 7:17 ` Like Xu 2022-01-24 16:23 ` Sean Christopherson 0 siblings, 1 reply; 8+ messages in thread From: Like Xu @ 2022-01-24 7:17 UTC (permalink / raw) To: Tian, Kevin, Paolo Bonzini Cc: Liu, Jing2, Wanpeng Li, Christopherson,, Sean, Jim Mattson, Vitaly Kuznetsov, Joerg Roedel, kvm, linux-kernel On 24/1/2022 3:06 pm, Tian, Kevin wrote: >> From: Like Xu <like.xu.linux@gmail.com> >> Sent: Sunday, January 23, 2022 1:50 PM >> >> From: Like Xu <likexu@tencent.com> >> >> A malicious user space can bypass xstate_get_guest_group_perm() in the >> KVM_GET_SUPPORTED_CPUID mechanism and obtain unpermitted xfeatures, >> since the validity check of xcr0 depends only on guest_supported_xcr0. > > Unpermitted xfeatures cannot pass kvm_check_cpuid()... Indeed, 5ab2f45bba4894a0db4af8567da3efd6228dd010. This part of logic is pretty fragile and fragmented due to semantic inconsistencies between supported_xcr0 and guest_supported_xcr0 in other three places: - __do_cpuid_func - kvm_mpx_supported - kvm_vcpu_ioctl_x86_set_xsave Have you identified all their areas of use ? > >> >> Fixes: 445ecdf79be0 ("kvm: x86: Exclude unpermitted xfeatures at >> KVM_GET_SUPPORTED_CPUID") >> Signed-off-by: Like Xu <likexu@tencent.com> >> --- >> arch/x86/kvm/cpuid.c | 3 ++- >> 1 file changed, 2 insertions(+), 1 deletion(-) >> >> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c >> index 3902c28fb6cb..1bd4d560cbdd 100644 >> --- a/arch/x86/kvm/cpuid.c >> +++ b/arch/x86/kvm/cpuid.c >> @@ -266,7 +266,8 @@ static void kvm_vcpu_after_set_cpuid(struct >> kvm_vcpu *vcpu) >> vcpu->arch.guest_supported_xcr0 = 0; >> else >> vcpu->arch.guest_supported_xcr0 = >> - (best->eax | ((u64)best->edx << 32)) & >> supported_xcr0; >> + (best->eax | ((u64)best->edx << 32)) & >> + (supported_xcr0 & xstate_get_guest_group_perm()); >> >> /* >> * Bits 127:0 of the allowed SECS.ATTRIBUTES (CPUID.0x12.0x1) >> enumerate >> -- >> 2.33.1 > ^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for vcpu->arch.guest_supported_xcr0 2022-01-24 7:17 ` Like Xu @ 2022-01-24 16:23 ` Sean Christopherson 2022-01-24 16:29 ` Paolo Bonzini 0 siblings, 1 reply; 8+ messages in thread From: Sean Christopherson @ 2022-01-24 16:23 UTC (permalink / raw) To: Like Xu Cc: Tian, Kevin, Paolo Bonzini, Liu, Jing2, Wanpeng Li, Jim Mattson, Vitaly Kuznetsov, Joerg Roedel, kvm, linux-kernel On Mon, Jan 24, 2022, Like Xu wrote: > On 24/1/2022 3:06 pm, Tian, Kevin wrote: > > > From: Like Xu <like.xu.linux@gmail.com> > > > Sent: Sunday, January 23, 2022 1:50 PM > > > > > > From: Like Xu <likexu@tencent.com> > > > > > > A malicious user space can bypass xstate_get_guest_group_perm() in the > > > KVM_GET_SUPPORTED_CPUID mechanism and obtain unpermitted xfeatures, > > > since the validity check of xcr0 depends only on guest_supported_xcr0. > > > > Unpermitted xfeatures cannot pass kvm_check_cpuid()... > > Indeed, 5ab2f45bba4894a0db4af8567da3efd6228dd010. > > This part of logic is pretty fragile and fragmented due to semantic > inconsistencies between supported_xcr0 and guest_supported_xcr0 > in other three places: There are no inconsistencies, at least not in the examples below, as the examples are intended to work in host context. guest_supported_xcr0 is about what the guest is/isn't allowed to access, it has no bearing on what host userspace can/can't do. Or are you talking about a different type of inconsistency? > - __do_cpuid_func Reporting what KVM supports to host userspace. > - kvm_mpx_supported This is a check on host support. > - kvm_vcpu_ioctl_x86_set_xsave "write" from host userspace. > Have you identified all their areas of use ? ^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for vcpu->arch.guest_supported_xcr0 2022-01-24 16:23 ` Sean Christopherson @ 2022-01-24 16:29 ` Paolo Bonzini 2022-01-25 1:54 ` Tian, Kevin 0 siblings, 1 reply; 8+ messages in thread From: Paolo Bonzini @ 2022-01-24 16:29 UTC (permalink / raw) To: Sean Christopherson, Like Xu Cc: Tian, Kevin, Liu, Jing2, Wanpeng Li, Jim Mattson, Vitaly Kuznetsov, Joerg Roedel, kvm, linux-kernel, Zhong, Yang On 1/24/22 17:23, Sean Christopherson wrote: > On Mon, Jan 24, 2022, Like Xu wrote: >> On 24/1/2022 3:06 pm, Tian, Kevin wrote: >>>> From: Like Xu <like.xu.linux@gmail.com> >>>> Sent: Sunday, January 23, 2022 1:50 PM >>>> >>>> From: Like Xu <likexu@tencent.com> >>>> >>>> A malicious user space can bypass xstate_get_guest_group_perm() in the >>>> KVM_GET_SUPPORTED_CPUID mechanism and obtain unpermitted xfeatures, >>>> since the validity check of xcr0 depends only on guest_supported_xcr0. >>> >>> Unpermitted xfeatures cannot pass kvm_check_cpuid()... >> >> Indeed, 5ab2f45bba4894a0db4af8567da3efd6228dd010. >> >> This part of logic is pretty fragile and fragmented due to semantic >> inconsistencies between supported_xcr0 and guest_supported_xcr0 >> in other three places: > > There are no inconsistencies, at least not in the examples below, as the examples > are intended to work in host context. guest_supported_xcr0 is about what the guest > is/isn't allowed to access, it has no bearing on what host userspace can/can't do. > Or are you talking about a different type of inconsistency? The extra complication is that arch_prctl(ARCH_REQ_XCOMP_GUEST_PERM) changes what host userspace can/can't do. It would be easier if we could just say that KVM_GET_SUPPORTED_CPUID returns "the most" that userspace can do, but we already have the contract that userspace can take KVM_GET_SUPPORTED_CPUID and pass it straight to KVM_SET_CPUID2. Therefore, KVM_GET_SUPPORTED_CPUID must limit its returned values to what has already been enabled. While reviewing the QEMU part of AMX support (this morning), I also noticed that there is no equivalent for guest permissions of ARCH_GET_XCOMP_SUPP. This needs to know KVM's supported_xcr0, so it's probably best realized as a new KVM_CHECK_EXTENSION rather than as an arch_prctl. Paolo >> - __do_cpuid_func > > Reporting what KVM supports to host userspace. > >> - kvm_mpx_supported > > This is a check on host support. > >> - kvm_vcpu_ioctl_x86_set_xsave > > "write" from host userspace. > >> Have you identified all their areas of use ? > ^ permalink raw reply [flat|nested] 8+ messages in thread
* RE: [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for vcpu->arch.guest_supported_xcr0 2022-01-24 16:29 ` Paolo Bonzini @ 2022-01-25 1:54 ` Tian, Kevin 2022-01-25 8:27 ` Paolo Bonzini 0 siblings, 1 reply; 8+ messages in thread From: Tian, Kevin @ 2022-01-25 1:54 UTC (permalink / raw) To: Paolo Bonzini, Christopherson,, Sean, Like Xu Cc: Liu, Jing2, Wanpeng Li, Jim Mattson, Vitaly Kuznetsov, Joerg Roedel, kvm, linux-kernel, Zhong, Yang > From: Paolo Bonzini <pbonzini@redhat.com> > Sent: Tuesday, January 25, 2022 12:30 AM > > On 1/24/22 17:23, Sean Christopherson wrote: > > On Mon, Jan 24, 2022, Like Xu wrote: > >> On 24/1/2022 3:06 pm, Tian, Kevin wrote: > >>>> From: Like Xu <like.xu.linux@gmail.com> > >>>> Sent: Sunday, January 23, 2022 1:50 PM > >>>> > >>>> From: Like Xu <likexu@tencent.com> > >>>> > >>>> A malicious user space can bypass xstate_get_guest_group_perm() in > the > >>>> KVM_GET_SUPPORTED_CPUID mechanism and obtain unpermitted > xfeatures, > >>>> since the validity check of xcr0 depends only on guest_supported_xcr0. > >>> > >>> Unpermitted xfeatures cannot pass kvm_check_cpuid()... > >> > >> Indeed, 5ab2f45bba4894a0db4af8567da3efd6228dd010. > >> > >> This part of logic is pretty fragile and fragmented due to semantic > >> inconsistencies between supported_xcr0 and guest_supported_xcr0 > >> in other three places: > > > > There are no inconsistencies, at least not in the examples below, as the > examples > > are intended to work in host context. guest_supported_xcr0 is about what > the guest > > is/isn't allowed to access, it has no bearing on what host userspace > can/can't do. > > Or are you talking about a different type of inconsistency? > > The extra complication is that arch_prctl(ARCH_REQ_XCOMP_GUEST_PERM) > changes what host userspace can/can't do. It would be easier if we > could just say that KVM_GET_SUPPORTED_CPUID returns "the most" that > userspace can do, but we already have the contract that userspace can > take KVM_GET_SUPPORTED_CPUID and pass it straight to KVM_SET_CPUID2. > > Therefore, KVM_GET_SUPPORTED_CPUID must limit its returned values to > what has already been enabled. > > While reviewing the QEMU part of AMX support (this morning), I also > noticed that there is no equivalent for guest permissions of > ARCH_GET_XCOMP_SUPP. This needs to know KVM's supported_xcr0, so it's > probably best realized as a new KVM_CHECK_EXTENSION rather than as an > arch_prctl. > Would that lead to a weird situation where although KVM says no support of guest permissions while the user can still request them via prctl()? I wonder whether it's cleaner to do it still via prctl() if we really want to enhance this part. But as you said then it needs a mechanism to know KVM's supported_xcr0 (and if KVM is not loaded then no guest permission support at all)... Thanks Kevin ^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for vcpu->arch.guest_supported_xcr0 2022-01-25 1:54 ` Tian, Kevin @ 2022-01-25 8:27 ` Paolo Bonzini 2022-01-26 1:23 ` Tian, Kevin 0 siblings, 1 reply; 8+ messages in thread From: Paolo Bonzini @ 2022-01-25 8:27 UTC (permalink / raw) To: Tian, Kevin, Christopherson,, Sean, Like Xu Cc: Liu, Jing2, Wanpeng Li, Jim Mattson, Vitaly Kuznetsov, Joerg Roedel, kvm, linux-kernel, Zhong, Yang On 1/25/22 02:54, Tian, Kevin wrote: >> The extra complication is that arch_prctl(ARCH_REQ_XCOMP_GUEST_PERM) >> changes what host userspace can/can't do. It would be easier if we >> could just say that KVM_GET_SUPPORTED_CPUID returns "the most" that >> userspace can do, but we already have the contract that userspace can >> take KVM_GET_SUPPORTED_CPUID and pass it straight to KVM_SET_CPUID2. >> >> Therefore, KVM_GET_SUPPORTED_CPUID must limit its returned values to >> what has already been enabled. >> >> While reviewing the QEMU part of AMX support (this morning), I also >> noticed that there is no equivalent for guest permissions of >> ARCH_GET_XCOMP_SUPP. This needs to know KVM's supported_xcr0, so it's >> probably best realized as a new KVM_CHECK_EXTENSION rather than as an >> arch_prctl. >> > Would that lead to a weird situation where although KVM says no support > of guest permissions while the user can still request them via prctl()? This is already the case for the current implementation of KVM_GET_SUPPORTED_CPUID. Paolo > I wonder whether it's cleaner to do it still via prctl() if we really want to > enhance this part. But as you said then it needs a mechanism to know > KVM's supported_xcr0 (and if KVM is not loaded then no guest permission > support at all)... ^ permalink raw reply [flat|nested] 8+ messages in thread
* RE: [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for vcpu->arch.guest_supported_xcr0 2022-01-25 8:27 ` Paolo Bonzini @ 2022-01-26 1:23 ` Tian, Kevin 0 siblings, 0 replies; 8+ messages in thread From: Tian, Kevin @ 2022-01-26 1:23 UTC (permalink / raw) To: Paolo Bonzini, Christopherson,, Sean, Like Xu Cc: Liu, Jing2, Wanpeng Li, Jim Mattson, Vitaly Kuznetsov, Joerg Roedel, kvm, linux-kernel, Zhong, Yang > From: Paolo Bonzini <pbonzini@redhat.com> > Sent: Tuesday, January 25, 2022 4:28 PM > > On 1/25/22 02:54, Tian, Kevin wrote: > >> The extra complication is that > arch_prctl(ARCH_REQ_XCOMP_GUEST_PERM) > >> changes what host userspace can/can't do. It would be easier if we > >> could just say that KVM_GET_SUPPORTED_CPUID returns "the most" that > >> userspace can do, but we already have the contract that userspace can > >> take KVM_GET_SUPPORTED_CPUID and pass it straight to > KVM_SET_CPUID2. > >> > >> Therefore, KVM_GET_SUPPORTED_CPUID must limit its returned values > to > >> what has already been enabled. > >> > >> While reviewing the QEMU part of AMX support (this morning), I also > >> noticed that there is no equivalent for guest permissions of > >> ARCH_GET_XCOMP_SUPP. This needs to know KVM's supported_xcr0, so > it's > >> probably best realized as a new KVM_CHECK_EXTENSION rather than as > an > >> arch_prctl. > >> > > Would that lead to a weird situation where although KVM says no support > > of guest permissions while the user can still request them via prctl()? > > This is already the case for the current implementation of > KVM_GET_SUPPORTED_CPUID. > fair enough. ^ permalink raw reply [flat|nested] 8+ messages in thread
end of thread, other threads:[~2022-01-26 1:24 UTC | newest] Thread overview: 8+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2022-01-23 5:50 [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for vcpu->arch.guest_supported_xcr0 Like Xu 2022-01-24 7:06 ` Tian, Kevin 2022-01-24 7:17 ` Like Xu 2022-01-24 16:23 ` Sean Christopherson 2022-01-24 16:29 ` Paolo Bonzini 2022-01-25 1:54 ` Tian, Kevin 2022-01-25 8:27 ` Paolo Bonzini 2022-01-26 1:23 ` Tian, Kevin
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