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Subject[PATCH 5.4 220/309] KVM: x86: Dont let userspace set host-reserved cr4 bits
Date
From: Sean Christopherson <sean.j.christopherson@intel.com>

commit b11306b53b2540c6ba068c4deddb6a17d9f8d95b upstream.

Calculate the host-reserved cr4 bits at runtime based on the system's
capabilities (using logic similar to __do_cpuid_func()), and use the
dynamically generated mask for the reserved bit check in kvm_set_cr4()
instead using of the static CR4_RESERVED_BITS define. This prevents
userspace from "enabling" features in cr4 that are not supported by the
system, e.g. by ignoring KVM_GET_SUPPORTED_CPUID and specifying a bogus
CPUID for the vCPU.

Allowing userspace to set unsupported bits in cr4 can lead to a variety
of undesirable behavior, e.g. failed VM-Enter, and in general increases
KVM's attack surface. A crafty userspace can even abuse CR4.LA57 to
induce an unchecked #GP on a WRMSR.

On a platform without LA57 support:

KVM_SET_CPUID2 // CPUID_7_0_ECX.LA57 = 1
KVM_SET_SREGS // CR4.LA57 = 1
KVM_SET_MSRS // KERNEL_GS_BASE = 0x0004000000000000
KVM_RUN

leads to a #GP when writing KERNEL_GS_BASE into hardware:

unchecked MSR access error: WRMSR to 0xc0000102 (tried to write 0x0004000000000000)
at rIP: 0xffffffffa00f239a (vmx_prepare_switch_to_guest+0x10a/0x1d0 [kvm_intel])
Call Trace:
kvm_arch_vcpu_ioctl_run+0x671/0x1c70 [kvm]
kvm_vcpu_ioctl+0x36b/0x5d0 [kvm]
do_vfs_ioctl+0xa1/0x620
ksys_ioctl+0x66/0x70
__x64_sys_ioctl+0x16/0x20
do_syscall_64+0x4c/0x170
entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x7fc08133bf47

Note, the above sequence fails VM-Enter due to invalid guest state.
Userspace can allow VM-Enter to succeed (after the WRMSR #GP) by adding
a KVM_SET_SREGS w/ CR4.LA57=0 after KVM_SET_MSRS, in which case KVM will
technically leak the host's KERNEL_GS_BASE into the guest. But, as
KERNEL_GS_BASE is a userspace-defined value/address, the leak is largely
benign as a malicious userspace would simply be exposing its own data to
the guest, and attacking a benevolent userspace would require multiple
bugs in the userspace VMM.

Cc: stable@vger.kernel.org
Cc: Jun Nakajima <jun.nakajima@intel.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/x86/kvm/x86.c | 35 ++++++++++++++++++++++++++++++++++-
1 file changed, 34 insertions(+), 1 deletion(-)

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -92,6 +92,8 @@ u64 __read_mostly efer_reserved_bits = ~
static u64 __read_mostly efer_reserved_bits = ~((u64)EFER_SCE);
#endif

+static u64 __read_mostly cr4_reserved_bits = CR4_RESERVED_BITS;
+
#define VM_STAT(x, ...) offsetof(struct kvm, stat.x), KVM_STAT_VM, ## __VA_ARGS__
#define VCPU_STAT(x, ...) offsetof(struct kvm_vcpu, stat.x), KVM_STAT_VCPU, ## __VA_ARGS__

@@ -886,9 +888,38 @@ int kvm_set_xcr(struct kvm_vcpu *vcpu, u
}
EXPORT_SYMBOL_GPL(kvm_set_xcr);

+static u64 kvm_host_cr4_reserved_bits(struct cpuinfo_x86 *c)
+{
+ u64 reserved_bits = CR4_RESERVED_BITS;
+
+ if (!cpu_has(c, X86_FEATURE_XSAVE))
+ reserved_bits |= X86_CR4_OSXSAVE;
+
+ if (!cpu_has(c, X86_FEATURE_SMEP))
+ reserved_bits |= X86_CR4_SMEP;
+
+ if (!cpu_has(c, X86_FEATURE_SMAP))
+ reserved_bits |= X86_CR4_SMAP;
+
+ if (!cpu_has(c, X86_FEATURE_FSGSBASE))
+ reserved_bits |= X86_CR4_FSGSBASE;
+
+ if (!cpu_has(c, X86_FEATURE_PKU))
+ reserved_bits |= X86_CR4_PKE;
+
+ if (!cpu_has(c, X86_FEATURE_LA57) &&
+ !(cpuid_ecx(0x7) & bit(X86_FEATURE_LA57)))
+ reserved_bits |= X86_CR4_LA57;
+
+ if (!cpu_has(c, X86_FEATURE_UMIP) && !kvm_x86_ops->umip_emulated())
+ reserved_bits |= X86_CR4_UMIP;
+
+ return reserved_bits;
+}
+
static int kvm_valid_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
{
- if (cr4 & CR4_RESERVED_BITS)
+ if (cr4 & cr4_reserved_bits)
return -EINVAL;

if (!guest_cpuid_has(vcpu, X86_FEATURE_XSAVE) && (cr4 & X86_CR4_OSXSAVE))
@@ -9341,6 +9372,8 @@ int kvm_arch_hardware_setup(void)
if (r != 0)
return r;

+ cr4_reserved_bits = kvm_host_cr4_reserved_bits(&boot_cpu_data);
+
if (kvm_has_tsc_control) {
/*
* Make sure the user can only configure tsc_khz values that

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