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@xxxxxxxxx>
Sent: Sunday, January 23, 2022 1:50 PM
From: Like Xu <likexu@xxxxxxxxxxx>
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 ?
