linux-kernel - [PATCH 3.16 113/148] KVM: x86: do not modify masked bits of shared MSRs

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Message-ID: <lsq.1581185940.373083172@decadent.org.uk>
Date:   Sat, 08 Feb 2020 18:20:52 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Jim Mattson" <jmattson@...gle.com>,
        "Paolo Bonzini" <pbonzini@...hat.com>
Subject: [PATCH 3.16 113/148] KVM: x86: do not modify masked bits of
 shared MSRs

3.16.82-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Paolo Bonzini <pbonzini@...hat.com>

commit de1fca5d6e0105c9d33924e1247e2f386efc3ece upstream.

"Shared MSRs" are guest MSRs that are written to the host MSRs but
keep their value until the next return to userspace.  They support
a mask, so that some bits keep the host value, but this mask is
only used to skip an unnecessary MSR write and the value written
to the MSR is always the guest MSR.

Fix this and, while at it, do not update smsr->values[slot].curr if
for whatever reason the wrmsr fails.  This should only happen due to
reserved bits, so the value written to smsr->values[slot].curr
will not match when the user-return notifier and the host value will
always be restored.  However, it is untidy and in rare cases this
can actually avoid spurious WRMSRs on return to userspace.

Reviewed-by: Jim Mattson <jmattson@...gle.com>
Tested-by: Jim Mattson <jmattson@...gle.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/x86/kvm/x86.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -242,13 +242,14 @@ int kvm_set_shared_msr(unsigned slot, u6
 	struct kvm_shared_msrs *smsr = per_cpu_ptr(shared_msrs, cpu);
 	int err;

-	if (((value ^ smsr->values[slot].curr) & mask) == 0)
+	value = (value & mask) | (smsr->values[slot].host & ~mask);
+	if (value == smsr->values[slot].curr)
 		return 0;
-	smsr->values[slot].curr = value;
 	err = wrmsrl_safe(shared_msrs_global.msrs[slot], value);
 	if (err)
 		return 1;

+	smsr->values[slot].curr = value;
 	if (!smsr->registered) {
 		smsr->urn.on_user_return = kvm_on_user_return;
 		user_return_notifier_register(&smsr->urn);