Re: [syzbot] general protection fault in kvm_mmu_uninit_tdp_mmu

Pavel Skripkin
2022-03-25T15:51:55Z

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On 3/25/22 16:10, syzbot wrote:
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: f9006d9269ea Add linux-next specific files for 20220321
> git tree: linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=101191bd700000
> kernel config: https://syzkaller.appspot.com/x/.config?x=c1619ffa2b0259a1
> dashboard link: https://syzkaller.appspot.com/bug?extid=717ed82268812a643b28
> compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=109e8f5d700000
> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1666180b700000
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+717ed82268812a643b28@syzkaller.appspotmail.com
>

The following code is not safe:

arch/x86/kvm/mmu/tdp_mmu.c:

28 kvm->arch.tdp_mmu_zap_wq =
29 alloc_workqueue("kvm", WQ_UNBOUND|WQ_MEM_RECLAIM|WQ_CPU_INTENSIVE, 0);
30
31 return true;

Looks like kvm_mmu_init_tdp_mmu() error value is just ignored, and then
all kvm_*_init_vm() functions are void, so the easiest solution is to
check that tdp_mmu_zap_wq is valid pointer

#syz test:
git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git master

With regards,
Pavel Skripkin

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diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index e7e7876251b3..b3e8ff7ac5b0 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -48,8 +48,10 @@ void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm)
if (!kvm->arch.tdp_mmu_enabled)
return;

- flush_workqueue(kvm->arch.tdp_mmu_zap_wq);
- destroy_workqueue(kvm->arch.tdp_mmu_zap_wq);
+ if (kvm->arch.tdp_mmu_zap_wq) {
+ flush_workqueue(kvm->arch.tdp_mmu_zap_wq);
+ destroy_workqueue(kvm->arch.tdp_mmu_zap_wq);
+ }

WARN_ON(!list_empty(&kvm->arch.tdp_mmu_pages));
WARN_ON(!list_empty(&kvm->arch.tdp_mmu_roots));
@@ -119,9 +121,11 @@ static void tdp_mmu_zap_root_work(struct work_struct *work)

static void tdp_mmu_schedule_zap_root(struct kvm *kvm, struct kvm_mmu_page *root)
{
- root->tdp_mmu_async_data = kvm;
- INIT_WORK(&root->tdp_mmu_async_work, tdp_mmu_zap_root_work);
- queue_work(kvm->arch.tdp_mmu_zap_wq, &root->tdp_mmu_async_work);
+ if (kvm->arch.tdp_mmu_zap_wq) {
+ root->tdp_mmu_async_data = kvm;
+ INIT_WORK(&root->tdp_mmu_async_work, tdp_mmu_zap_root_work);
+ queue_work(kvm->arch.tdp_mmu_zap_wq, &root->tdp_mmu_async_work);
+ }
}

static inline bool kvm_tdp_root_mark_invalid(struct kvm_mmu_page *page)