linux-kernel - [PATCH] KVM: x86: Take a u64 when checking for a valid dr7 value

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Date:   Fri, 24 Jan 2020 15:07:22 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Krish Sadhukhan <krish.sadhukhan@...cle.com>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: [PATCH] KVM: x86: Take a u64 when checking for a valid dr7 value

Take a u64 instead of an unsigned long in kvm_dr7_valid() to fix a build
warning on i386 due to right-shifting a 32-bit value by 32 when checking
for bits being set in dr7[63:32].

Alternatively, the warning could be resolved by rewriting the check to
use an i386-friendly method, but taking a u64 fixes another oddity on
32-bit KVM.  Beause KVM implements natural width VMCS fields as u64s to
avoid layout issues between 32-bit and 64-bit, a devious guest can stuff
vmcs12->guest_dr7 with a 64-bit value even when both the guest and host
are 32-bit kernels.  KVM eventually drops vmcs12->guest_dr7[63:32] when
propagating vmcs12->guest_dr7 to vmcs02, but ideally KVM would not rely
on that behavior for correctness.

Cc: Jim Mattson <jmattson@...gle.com>
Cc: Krish Sadhukhan <krish.sadhukhan@...cle.com>
Fixes: ecb697d10f70 ("KVM: nVMX: Check GUEST_DR7 on vmentry of nested guests")
Reported-by: Randy Dunlap <rdunlap@...radead.org>
Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
---
 arch/x86/kvm/x86.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index 2d2ff855773b..3624665acee4 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -357,7 +357,7 @@ static inline bool kvm_pat_valid(u64 data)
 	return (data | ((data & 0x0202020202020202ull) << 1)) == data;
 }

-static inline bool kvm_dr7_valid(unsigned long data)
+static inline bool kvm_dr7_valid(u64 data)
 {
 	/* Bits [63:32] are reserved */
 	return !(data >> 32);
-- 
2.24.1