KVM: x86/mmu: Try to avoid crashing KVM if a MMU memory cache is empty

Attempt to allocate a new object instead of crashing KVM (and likely the
kernel) if a memory cache is unexpectedly empty.  Use GFP_ATOMIC for the
allocation as the caches are used while holding mmu_lock.  The immediate
BUG_ON() makes the code unnecessarily explosive and led to confusing
minimums being used in the past, e.g. allocating 4 objects where 1 would
suffice.

Reviewed-by: Ben Gardon <bgardon@google.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200703023545.8771-6-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 7fae1c3..a8eddb8 100644 (file)
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -1061,6 +1061,15 @@ static void walk_shadow_page_lockless_end(struct kvm_vcpu *vcpu)
        local_irq_enable();
 }
 
+static inline void *mmu_memory_cache_alloc_obj(struct kvm_mmu_memory_cache *mc,
+                                              gfp_t gfp_flags)
+{
+       if (mc->kmem_cache)
+               return kmem_cache_zalloc(mc->kmem_cache, gfp_flags);
+       else
+               return (void *)__get_free_page(gfp_flags);
+}
+
 static int mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int min)
 {
        void *obj;
@@ -1068,10 +1077,7 @@ static int mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int min)
        if (mc->nobjs >= min)
                return 0;
        while (mc->nobjs < ARRAY_SIZE(mc->objects)) {
-               if (mc->kmem_cache)
-                       obj = kmem_cache_zalloc(mc->kmem_cache, GFP_KERNEL_ACCOUNT);
-               else
-                       obj = (void *)__get_free_page(GFP_KERNEL_ACCOUNT);
+               obj = mmu_memory_cache_alloc_obj(mc, GFP_KERNEL_ACCOUNT);
                if (!obj)
                        return mc->nobjs >= min ? 0 : -ENOMEM;
                mc->objects[mc->nobjs++] = obj;
@@ -1119,8 +1125,11 @@ static void *mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc)
 {
        void *p;
 
-       BUG_ON(!mc->nobjs);
-       p = mc->objects[--mc->nobjs];
+       if (WARN_ON(!mc->nobjs))
+               p = mmu_memory_cache_alloc_obj(mc, GFP_ATOMIC | __GFP_ACCOUNT);
+       else
+               p = mc->objects[--mc->nobjs];
+       BUG_ON(!p);
        return p;
 }