KVM MMU: fix race in invlpg code

PATCH Avi Kivity 2 participants 5 messages Apr 30, 2010 – May 5, 2010 4 messages not in the archive KVM

lore

Share & tools

lore

A message that is not in the archive (sent off-list or not yet received): [email protected]

Avi Kivity

author

2010-04-30 09:52 UTC #1 lore raw

Quoted 38 lines · Xiao Guangrong

It has race in invlpg code, like below sequences:
A: hold mmu_lock and get 'sp'
B: release mmu_lock and do other things
C: hold mmu_lock and continue use 'sp'
if other path freed 'sp' in stage B, then kernel will crash
This patch checks 'sp' whether lived before use 'sp' in stage C
Signed-off-by Xiao Guangrong<[email protected]>
---
  arch/x86/kvm/paging_tmpl.h |   18 +++++++++++++++++-
  1 files changed, 17 insertions(+), 1 deletions(-)
arch/x86/kvm/paging_tmpl.h +5 -1
index 624b38f..641d844 100644
@@ -462,11 +462,15 @@ out_unlock:
 
  static void FNAME(invlpg)(struct kvm_vcpu *vcpu, gva_t gva)
  {
-	struct kvm_mmu_page *sp = NULL;
+	struct kvm_mmu_page *sp = NULL, *s;
  	struct kvm_shadow_walk_iterator iterator;
+	struct hlist_head *bucket;
+	struct hlist_node *node, *tmp;
  	gfn_t gfn = -1;
  	u64 *sptep = NULL, gentry;
  	int invlpg_counter, level, offset = 0, need_flush = 0;
+	unsigned index;
+	bool live = false;
 
  	spin_lock(&vcpu->kvm->mmu_lock);
 

arch/x86/kvm/paging_tmpl.h static void FNAME(invlpg)(struct kvm_vcpu *vcpu, gva_t gva) · line 530

Quoted 8 lines

arch/x86/kvm/paging_tmpl.h +4 -0

@@ -519,10 +523,22 @@ static void FNAME(invlpg)(struct kvm_vcpu *vcpu, gva_t gva)
 
  	mmu_guess_page_from_pte_write(vcpu, gfn_to_gpa(gfn) + offset, gentry);
  	spin_lock(&vcpu->kvm->mmu_lock);
+	index = kvm_page_table_hashfn(gfn);
+	bucket =&vcpu->kvm->arch.mmu_page_hash[index];
+	hlist_for_each_entry_safe(s, node, tmp, bucket, hash_link)
+		if (s == sp) {

At this point, sp might have been freed and re-allocated, now pointing at something completely different. So need to check role etc.

Alternatively, increase root_count. Then sp is guaranteed to be live (though it may have role.invalid set).

Quoted 16 lines

arch/x86/kvm/paging_tmpl.h +8 -0

+			live = true;
+			break;
+		}
+
+	if (!live)
+		goto unlock_exit;
+
  	if (atomic_read(&vcpu->kvm->arch.invlpg_counter) == invlpg_counter) {
  		++vcpu->kvm->stat.mmu_pte_updated;
  		FNAME(update_pte)(vcpu, sp, sptep,&gentry);
  	}
+unlock_exit:
  	spin_unlock(&vcpu->kvm->mmu_lock);
  	mmu_release_page_from_pte_write(vcpu);

Signature · 1 line

Do not meddle in the internals of kernels, for they are subtle and quick to panic.

A message that is not in the archive (sent off-list or not yet received): [email protected]

Avi Kivity

author

2010-04-30 09:54 UTC #4 lore raw

Re: [PATCH 3/4] KVM MMU: allow shadow page become unsync at creating time

Quoted 8 lines · Xiao Guangrong

Allow new shadow page become unsync when is created, then we no need write-protect the 'sp->gfn', this idea is from Avi:
|Another interesting case is to create new shadow pages in the unsync
|state. That can help when the guest starts a short lived process: we
|can avoid write protecting its pagetables completely

Any idea how this improves performance? A kernel build has a lot of short lived processes, might see an improvement there.

Signature · 1 line

Do not meddle in the internals of kernels, for they are subtle and quick to panic.

A message that is not in the archive (sent off-list or not yet received): [email protected]

Avi Kivity

author

2010-04-30 17:21 UTC #6 lore raw

Re: [PATCH 4/4] KVM MMU: do not intercept invlpg if 'oos_shadow' is disabled

Quoted 7 lines · Xiao Guangrong

If 'oos_shadow' == 0, intercepting invlpg command is really unnecessary.
And it's good for us to compare the performance between enable 'oos_shadow' and disable 'oos_shadow'

Review static int dbg = 0; · line 80

Quoted 8 lines

(quoted diff) +2 -1

@@ -74,8 +74,9 @@ static int dbg = 0;
  module_param(dbg, bool, 0644);
  #endif
 
-static int oos_shadow = 1;
+int __read_mostly oos_shadow = 1;
  module_param(oos_shadow, bool, 0644);
+EXPORT_SYMBOL_GPL(oos_shadow);

Please rename to kvm_oos_shadow to reduce potential for conflict with other global names.

But really, this is a debug option, I don't expect people to run with oos_shadow=0, so there's not much motivation to optimize it.

Signature · 1 line

Do not meddle in the internals of kernels, for they are subtle and quick to panic.

Xiao Guangrong

2010-05-05 12:57 UTC #7 lore raw

Re: [PATCH 4/4] KVM MMU: do not intercept invlpg if 'oos_shadow' is disabled

Quoted 21 lines · Avi Kivity

Quoted 8 lines · Xiao Guangrong
If 'oos_shadow' == 0, intercepting invlpg command is really unnecessary.
And it's good for us to compare the performance between enable 'oos_shadow'
and disable 'oos_shadow'
Review static int dbg = 0; · line 80
Quoted 8 lines
(quoted diff) +2 -1
@@ -74,8 +74,9 @@ static int dbg = 0;
  module_param(dbg, bool, 0644);
  #endif
 
-static int oos_shadow = 1;
+int __read_mostly oos_shadow = 1;
  module_param(oos_shadow, bool, 0644);
+EXPORT_SYMBOL_GPL(oos_shadow);
 
Please rename to kvm_oos_shadow to reduce potential for conflict with other global names.

Quoted 2 lines

But really, this is a debug option, I don't expect people to run with oos_shadow=0, so there's not much motivation to optimize it.

Agreed, but, 'oos_shadow' option is document in Documentation/kernel-parameters.txt, if it's just a debug option, i think we do better not document it.

Thanks,
Xiao

Avi Kivity

author

2010-05-05 14:26 UTC #8 lore raw

Re: [PATCH 4/4] KVM MMU: do not intercept invlpg if 'oos_shadow' is disabled

Quoted 25 lines · Xiao Guangrong

Quoted 22 lines · Avi Kivity
Quoted 8 lines · Xiao Guangrong
If 'oos_shadow' == 0, intercepting invlpg command is really unnecessary.
And it's good for us to compare the performance between enable 'oos_shadow'
and disable 'oos_shadow'
Review static int dbg = 0; · line 81
Quoted 9 lines
(quoted diff) +2 -1
@@ -74,8 +74,9 @@ static int dbg = 0;
   module_param(dbg, bool, 0644);
   #endif
 
-static int oos_shadow = 1;
+int __read_mostly oos_shadow = 1;
   module_param(oos_shadow, bool, 0644);
+EXPORT_SYMBOL_GPL(oos_shadow);
 
 
Please rename to kvm_oos_shadow to reduce potential for conflict with other global names.

Quoted 6 lines

Quoted 3 lines
But really, this is a debug option, I don't expect people to run with oos_shadow=0, so there's not much motivation to optimize it.
Agreed, but, 'oos_shadow' option is document in Documentation/kernel-parameters.txt, if it's just a debug option, i think we do better not document it.

It has to be documented, otherwise people complain :)

Anyway the variable name and the option name don't have to be the same (I think).

Signature · 1 line

error compiling committee.c: too many arguments to function