Re: [PATCH v2 05/21] KVM: x86/mmu: Try to avoid crashing KVM if a MMU memory cache is empty

[Ben Gardon]

On Mon, Jun 22, 2020 at 1:09 PM Sean Christopherson
 wrote:
>
> 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 
> Signed-off-by: Sean Christopherson 
> ---
>  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);
> +   else
> +   p = mc->objects[--mc->nobjs];
> +   BUG_ON(!p);
> return p;
>  }
>
> --
> 2.26.0
>

[PATCH v2 05/21] KVM: x86/mmu: Try to avoid crashing KVM if a MMU memory cache is empty

[Sean Christopherson]

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 
---
 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);
+   else
+   p = mc->objects[--mc->nobjs];
+   BUG_ON(!p);
return p;
 }
 
-- 
2.26.0