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KVM: x86: Reject loading KVM if host.PAT[0] != WB

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Reject KVM if entry '0' in the host's IA32_PAT MSR is not programmed to
writeback (WB) memtype.  KVM subtly relies on IA32_PAT entry '0' to be
programmed to WB by leaving the PAT bits in shadow paging and NPT SPTEs
as '0'.  If something other than WB is in PAT[0], at _best_ guests will
suffer very poor performance, and at worst KVM will crash the system by
breaking cache-coherency expecations (e.g. using WC for guest memory).

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20220715230016.3762909-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson
2022-07-15 23:00:13 +00:00
committed by Paolo Bonzini
parent 01e69cef63
commit 94bda2f4cd
1 changed files with 15 additions and 0 deletions
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15
arch/x86/kvm/x86.c
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@@ -9141,6 +9141,7 @@ static struct notifier_block pvclock_gtod_notifier = {
int kvm_arch_init(void *opaque)
{
struct kvm_x86_init_ops *ops = opaque;
u64 host_pat;
int r;
if (kvm_x86_ops.hardware_enable) {
@@ -9179,6 +9180,20 @@ int kvm_arch_init(void *opaque)
goto out;
}
/*
* KVM assumes that PAT entry '0' encodes WB memtype and simply zeroes
* the PAT bits in SPTEs. Bail if PAT[0] is programmed to something
* other than WB. Note, EPT doesn't utilize the PAT, but don't bother
* with an exception. PAT[0] is set to WB on RESET and also by the
* kernel, i.e. failure indicates a kernel bug or broken firmware.
*/
if (rdmsrl_safe(MSR_IA32_CR_PAT, &host_pat) ||
(host_pat & GENMASK(2, 0)) != 6) {
pr_err("kvm: host PAT[0] is not WB\n");
r = -EIO;
goto out;
}
r = -ENOMEM;
x86_emulator_cache = kvm_alloc_emulator_cache();