[PATCH v8 12/17] KVM: x86: Fix compute_guest_tsc() to handle negative time deltas

[Sean Christopherson <seanjc@google.com>]

From: David Woodhouse <dwmw@amazon.co.uk>

The compute_guest_tsc() function computes the guest TSC at a given
kernel_ns timestamp. When the master clock reference point
(master_kernel_ns) is earlier than vcpu->arch.this_tsc_nsec, the delta
is negative. Since pvclock_scale_delta() takes a u64, the negative
value wraps to a huge positive number, producing a wildly wrong result.

Handle negative deltas explicitly by scaling the absolute value of the
delta and applying it to this_tsc_write with the appropriate sign.

This is believed to be unreachable in practice; no path has been
identified which invokes compute_guest_tsc() with a timestamp from
before the vCPU's TSC generation was established. Fix it for
robustness, in the spirit of defence in depth.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/x86.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ec39ca82633b..5667cd17672b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1054,11 +1054,15 @@ static int kvm_set_tsc_khz(struct kvm_vcpu *vcpu, u32 user_tsc_khz)
 
 static u64 compute_guest_tsc(struct kvm_vcpu *vcpu, s64 kernel_ns)
 {
-	u64 tsc = pvclock_scale_delta(kernel_ns-vcpu->arch.this_tsc_nsec,
-				      vcpu->arch.virtual_tsc_mult,
-				      vcpu->arch.virtual_tsc_shift);
-	tsc += vcpu->arch.this_tsc_write;
-	return tsc;
+	s64 delta_ns = kernel_ns - vcpu->arch.this_tsc_nsec;
+	u64 tsc;
+
+	/* Handle negative deltas gracefully (master clock ref may be earlier) */
+	tsc = pvclock_scale_delta(abs(delta_ns),
+				  vcpu->arch.virtual_tsc_mult,
+				  vcpu->arch.virtual_tsc_shift);
+
+	return vcpu->arch.this_tsc_write + (delta_ns >= 0 ? tsc : -tsc);
 }
 
 #ifdef CONFIG_X86_64
-- 
2.55.0.571.g244d577d93-goog