[PATCH v3] kvm: arm/arm64: Fix locking for kvm_free_stage2_pgd

[Suzuki K Poulose]

In kvm_free_stage2_pgd() we don't hold the kvm->mmu_lock while calling
unmap_stage2_range() on the entire memory range for the guest. This could
cause problems with other callers (e.g, munmap on a memslot) trying to
unmap a range. And since we have to unmap the entire Guest memory range
holding a spinlock, make sure we yield the lock if necessary, after we
unmap each PUD range.

Fixes: commit d5d8184d35c9 ("KVM: ARM: Memory virtualization setup")
Cc: stable at vger.kernel.org # v3.10+
Cc: Paolo Bonzini <pbonzin at redhat.com>
Cc: Marc Zyngier <marc.zyngier at arm.com>
Cc: Christoffer Dall <christoffer.dall at linaro.org>
Cc: Mark Rutland <mark.rutland at arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose at arm.com>
[ Avoid vCPU starvation and lockup detector warnings ]
Signed-off-by: Marc Zyngier <marc.zyngier at arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose at arm.com>

---
Changes since V2:
 - Restrict kvm->mmu_lock relaxation to bigger ranges in unmap_stage2_range(),
   to avoid possible issues like [0]

 [0] http://lists.infradead.org/pipermail/linux-arm-kernel/2017-March/498210.html

Changes since V1:
 - Yield the kvm->mmu_lock if necessary in unmap_stage2_range to prevent
   vCPU starvation and lockup detector warnings.
---
 arch/arm/kvm/mmu.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
index 13b9c1f..db94f3a 100644
--- a/arch/arm/kvm/mmu.c
+++ b/arch/arm/kvm/mmu.c
@@ -292,8 +292,15 @@ static void unmap_stage2_range(struct kvm *kvm, phys_addr_t start, u64 size)
 	phys_addr_t addr = start, end = start + size;
 	phys_addr_t next;
 
+	assert_spin_locked(&kvm->mmu_lock);
 	pgd = kvm->arch.pgd + stage2_pgd_index(addr);
 	do {
+		/*
+		 * If the range is too large, release the kvm->mmu_lock
+		 * to prevent starvation and lockup detector warnings.
+		 */
+		if (size > S2_PUD_SIZE)
+			cond_resched_lock(&kvm->mmu_lock);
 		next = stage2_pgd_addr_end(addr, end);
 		if (!stage2_pgd_none(*pgd))
 			unmap_stage2_puds(kvm, pgd, addr, next);
@@ -831,7 +838,10 @@ void kvm_free_stage2_pgd(struct kvm *kvm)
 	if (kvm->arch.pgd == NULL)
 		return;
 
+	spin_lock(&kvm->mmu_lock);
 	unmap_stage2_range(kvm, 0, KVM_PHYS_SIZE);
+	spin_unlock(&kvm->mmu_lock);
+
 	/* Free the HW pgd, one page at a time */
 	free_pages_exact(kvm->arch.pgd, S2_PGD_SIZE);
 	kvm->arch.pgd = NULL;
-- 
2.7.4