[PATCH 4/4] KVM: arm64: Avoid mistaken attempts to save SVE state for vcpus

[Dave.Martin@arm.com (Dave Martin)]

Commit e6b673b ("KVM: arm64: Optimise FPSIMD handling to reduce
guest/host thrashing") uses fpsimd_save() to save the FPSIMD state
for a vcpu when scheduling the vcpu out.  However, currently
current's value of TIF_SVE is restored before calling fpsimd_save()
which means that fpsimd_save() may erroneously attempt to save SVE
state from the vcpu.  This enables current's vector state to be
polluted with guest data.  current->thread.sve_state may be
unallocated or not large enough, so this can also trigger a NULL
dereference or buffer overrun.

Instead of this, TIF_SVE should be configured properly for the
guest when calling fpsimd_save() with the vcpu context loaded.

This patch ensures this my delaying restoration of current's
TIF_SVE until after the call to fpsimd_save().

Fixes: e6b673b741ea ("KVM: arm64: Optimise FPSIMD handling to reduce guest/host thrashing")
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
---
 arch/arm64/kvm/fpsimd.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
index 30c7a34..4aaf78e 100644
--- a/arch/arm64/kvm/fpsimd.c
+++ b/arch/arm64/kvm/fpsimd.c
@@ -99,9 +99,6 @@ void kvm_arch_vcpu_ctxsync_fp(struct kvm_vcpu *vcpu)
  */
 static void __kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu)
 {
-	update_thread_flag(TIF_SVE,
-			   vcpu->arch.flags & KVM_ARM64_HOST_SVE_IN_USE);
-
 	if (vcpu->arch.flags & KVM_ARM64_FP_ENABLED) {
 		/* Clean guest FP state to memory and invalidate cpu view */
 		fpsimd_save();
@@ -119,6 +116,9 @@ static void __kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu)
 		else
 			sysreg_clear_set(CPACR_EL1, CPACR_EL1_ZEN_EL0EN, 0);
 	}
+
+	update_thread_flag(TIF_SVE,
+			   vcpu->arch.flags & KVM_ARM64_HOST_SVE_IN_USE);
 }
 
 
-- 
2.1.4