linux-kernel - [PATCH] KVM: vmx: fix underflow in TSC deadline calculation

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Message-Id: <1467033073-3746-1-git-send-email-pbonzini@redhat.com>
Date:	Mon, 27 Jun 2016 15:11:13 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:	wanpeng.li@...mail.com, Yunhong Jiang <yunhong.jiang@...il.com>
Subject: [PATCH] KVM: vmx: fix underflow in TSC deadline calculation

If the TSC deadline timer is programmed really close to the deadline or
even in the past, the computation in vmx_set_hv_timer can underflow and
cause delta_tsc to be set to a huge value.  This generally results
in vmx_set_hv_timer returning -ERANGE, but we can fix it by limiting
delta_tsc to be positive or zero.

Reported-by: Wanpeng Li <wanpeng.li@...mail.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/vmx.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index c1d655c10fd2..85e2f0a882ca 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -10829,9 +10829,9 @@ static inline int u64_shl_div_u64(u64 a, unsigned int shift,
 static int vmx_set_hv_timer(struct kvm_vcpu *vcpu, u64 guest_deadline_tsc)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
-	u64 tscl = rdtsc(), delta_tsc;
-
-	delta_tsc = guest_deadline_tsc - kvm_read_l1_tsc(vcpu, tscl);
+	u64 tscl = rdtsc();
+	u64 guest_tscl = kvm_read_l1_tsc(vcpu, tscl);
+	u64 delta_tsc = max(guest_deadline_tsc, guest_tscl) - guest_tscl;
 
 	/* Convert to host delta tsc if tsc scaling is enabled */
 	if (vcpu->arch.tsc_scaling_ratio != kvm_default_tsc_scaling_ratio &&
-- 
1.8.3.1