[PATCH] LoongArch: KVM: Fix TOCTOU race on pv_features

[Tao Cui <cui.tao@linux.dev>]

From: Tao Cui <cuitao@kylinos.cn>

kvm_loongarch_cpucfg_set_attr() validates and writes the VM-wide
pv_features with a lockless check-then-set, so two vCPUs racing it can
both pass the "all-vCPUs-must-match" check and install divergent values.
Make the check-then-set atomic with a cmpxchg loop; the UPDATED bit
already packs the configured state into the same word.

Signed-off-by: Tao Cui <cuitao@kylinos.cn>
---
 arch/loongarch/kvm/vcpu.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/arch/loongarch/kvm/vcpu.c b/arch/loongarch/kvm/vcpu.c
index 20c207d80e31..55030c37cf06 100644
--- a/arch/loongarch/kvm/vcpu.c
+++ b/arch/loongarch/kvm/vcpu.c
@@ -1164,12 +1164,18 @@ static int kvm_loongarch_cpucfg_set_attr(struct kvm_vcpu *vcpu,
 		if (val & ~valid)
 			return -EINVAL;
 
-		/* All vCPUs need set the same PV features */
-		if ((kvm->arch.pv_features & LOONGARCH_PV_FEAT_UPDATED)
-				&& ((kvm->arch.pv_features & valid) != val))
-			return -EINVAL;
-		kvm->arch.pv_features = val | LOONGARCH_PV_FEAT_UPDATED;
-		return 0;
+		/* Atomically install val; the cmpxchg serializes concurrent setters. */
+		for (;;) {
+			unsigned long old, new;
+
+			old = READ_ONCE(kvm->arch.pv_features);
+			if ((old & LOONGARCH_PV_FEAT_UPDATED) &&
+			    ((old & valid) != val))
+				return -EINVAL;
+			new = val | LOONGARCH_PV_FEAT_UPDATED;
+			if (cmpxchg(&kvm->arch.pv_features, old, new) == old)
+				return 0;
+		}
 	default:
 		return -ENXIO;
 	}
-- 
2.43.0