Re: [PATCH v1 5/9] KVM: x86: Add new hypercall to lock control registers
From: Wei Liu <wei.liu@kernel.org>
Date: 2023-05-08 21:11:58
Also in:
kvm, linux-hardening, linux-hyperv, lkml, qemu-devel, xen-devel
On Fri, May 05, 2023 at 05:20:42PM +0200, Mickaël Salaün wrote:
This enables guests to lock their CR0 and CR4 registers with a subset of X86_CR0_WP, X86_CR4_SMEP, X86_CR4_SMAP, X86_CR4_UMIP, X86_CR4_FSGSBASE and X86_CR4_CET flags. The new KVM_HC_LOCK_CR_UPDATE hypercall takes two arguments. The first is to identify the control register, and the second is a bit mask to pin (i.e. mark as read-only). These register flags should already be pinned by Linux guests, but once compromised, this self-protection mechanism could be disabled, which is not the case with this dedicated hypercall. Cc: Borislav Petkov <bp@alien8.de> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Ingo Molnar <mingo@redhat.com> Cc: Kees Cook <redacted> Cc: Madhavan T. Venkataraman <redacted> Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Sean Christopherson <seanjc@google.com> Cc: Thomas Gleixner <redacted> Cc: Vitaly Kuznetsov <vkuznets@redhat.com> Cc: Wanpeng Li <redacted> Signed-off-by: Mickaël Salaün <mic@digikod.net> Link: https://lore.kernel.org/r/20230505152046.6575-6-mic@digikod.net (local)
[...]
quoted hunk ↗ jump to hunk
hw_cr4 = (cr4_read_shadow() & X86_CR4_MCE) | (cr4 & ~X86_CR4_MCE); if (is_unrestricted_guest(vcpu))diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index ffab64d08de3..a529455359ac 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c@@ -7927,11 +7927,77 @@ static unsigned long emulator_get_cr(struct x86_emulate_ctxt *ctxt, int cr) return value; } +#ifdef CONFIG_HEKI + +extern unsigned long cr4_pinned_mask; +
Can this be moved to a header file?
+static int heki_lock_cr(struct kvm *const kvm, const unsigned long cr,
+ unsigned long pin)
+{
+ if (!pin)
+ return -KVM_EINVAL;
+
+ switch (cr) {
+ case 0:
+ /* Cf. arch/x86/kernel/cpu/common.c */
+ if (!(pin & X86_CR0_WP))
+ return -KVM_EINVAL;
+
+ if ((read_cr0() & pin) != pin)
+ return -KVM_EINVAL;
+
+ atomic_long_or(pin, &kvm->heki_pinned_cr0);
+ return 0;
+ case 4:
+ /* Checks for irrelevant bits. */
+ if ((pin & cr4_pinned_mask) != pin)
+ return -KVM_EINVAL;
+It is enforcing the host mask on the guest, right? If the guest's set is a super set of the host's then it will get rejected.
+ /* Ignores bits not present in host. */
+ pin &= __read_cr4();
+ atomic_long_or(pin, &kvm->heki_pinned_cr4);
+ return 0;
+ }
+ return -KVM_EINVAL;
+}
+
+int heki_check_cr(const struct kvm *const kvm, const unsigned long cr,
+ const unsigned long val)
+{
+ unsigned long pinned;
+
+ switch (cr) {
+ case 0:
+ pinned = atomic_long_read(&kvm->heki_pinned_cr0);
+ if ((val & pinned) != pinned) {
+ pr_warn_ratelimited(
+ "heki-kvm: Blocked CR0 update: 0x%lx\n", val);I think if the message contains the VM and VCPU identifier it will become more useful. Thanks, Wei.