Re: [PATCH 05/21] KVM: x86/mmu: Try to avoid crashing KVM if a MMU memory cache is empty
From: Ben Gardon <hidden>
Date: 2020-06-10 22:12:18
Also in:
kvm, kvmarm, linux-mips, lkml
On Fri, Jun 5, 2020 at 2:39 PM Sean Christopherson [off-list ref] wrote:
quoted hunk ↗ jump to hunk
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. Signed-off-by: Sean Christopherson <redacted> --- arch/x86/kvm/mmu/mmu.c | 21 +++++++++++++++------ 1 file changed, 15 insertions(+), 6 deletions(-)diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index ba70de24a5b0..5e773564ab20 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c@@ -1060,6 +1060,15 @@ 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;@@ -1067,10 +1076,7 @@ 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;@@ -1118,8 +1124,11 @@ 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);
Is an atomic allocation really necessary here? In most cases, when topping up the memory cache we are handing a guest page fault. This bug could also be removed by returning null if unable to allocate from the cache, and then re-trying the page fault in that case. I don't know if this is necessary to handle other, non-x86 architectures more easily, but I worry this could cause some unpleasantness if combined with some other bug or the host was in a low memory situation and then this consumed the atomic pool. Perhaps this is a moot point since we log a warning and consider the atomic allocation something of an error.
+ else
+ p = mc->objects[--mc->nobjs];
+ BUG_ON(!p);
return p;
}
--
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