Re: [RESEND PATCH] KVM: x86/mmu: make apf token non-zero to fix bug

[zhangliang (AG)]

No. Because '(++vcpu->arch.apf.id << 12)' may also produce zero value.

On 2022/3/10 11:34, Xinlong Lin wrote:
> 
> 
> On 2022/2/22 11:12, Liang Zhang wrote:
>> In current async pagefault logic, when a page is ready, KVM relies on
>> kvm_arch_can_dequeue_async_page_present() to determine whether to deliver
>> a READY event to the Guest. This function test token value of struct
>> kvm_vcpu_pv_apf_data, which must be reset to zero by Guest kernel when a
>> READY event is finished by Guest. If value is zero meaning that a READY
>> event is done, so the KVM can deliver another.
>> But the kvm_arch_setup_async_pf() may produce a valid token with zero
>> value, which is confused with previous mention and may lead the loss of
>> this READY event.
>>
>> This bug may cause task blocked forever in Guest:
>>   INFO: task stress:7532 blocked for more than 1254 seconds.
>>         Not tainted 5.10.0 #16
>>   "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>>   task:stress          state:D stack:    0 pid: 7532 ppid:  1409
>>   flags:0x00000080
>>   Call Trace:
>>    __schedule+0x1e7/0x650
>>    schedule+0x46/0xb0
>>    kvm_async_pf_task_wait_schedule+0xad/0xe0
>>    ? exit_to_user_mode_prepare+0x60/0x70
>>    __kvm_handle_async_pf+0x4f/0xb0
>>    ? asm_exc_page_fault+0x8/0x30
>>    exc_page_fault+0x6f/0x110
>>    ? asm_exc_page_fault+0x8/0x30
>>    asm_exc_page_fault+0x1e/0x30
>>   RIP: 0033:0x402d00
>>   RSP: 002b:00007ffd31912500 EFLAGS: 00010206
>>   RAX: 0000000000071000 RBX: ffffffffffffffff RCX: 00000000021a32b0
>>   RDX: 000000000007d011 RSI: 000000000007d000 RDI: 00000000021262b0
>>   RBP: 00000000021262b0 R08: 0000000000000003 R09: 0000000000000086
>>   R10: 00000000000000eb R11: 00007fefbdf2baa0 R12: 0000000000000000
>>   R13: 0000000000000002 R14: 000000000007d000 R15: 0000000000001000
>>
>> Signed-off-by: Liang Zhang <zhangliang5@xxxxxxxxxx>
>> ---
>>   arch/x86/kvm/mmu/mmu.c | 13 ++++++++++++-
>>   1 file changed, 12 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
>> index 593093b52395..8e24f73bf60b 100644
>> --- a/arch/x86/kvm/mmu/mmu.c
>> +++ b/arch/x86/kvm/mmu/mmu.c
>> @@ -3889,12 +3889,23 @@ static void shadow_page_table_clear_flood(struct kvm_vcpu *vcpu, gva_t addr)
>>       walk_shadow_page_lockless_end(vcpu);
>>   }
>>   +static u32 alloc_apf_token(struct kvm_vcpu *vcpu)
>> +{
>> +    /* make sure the token value is not 0 */
>> +    u32 id = vcpu->arch.apf.id;
>> +
>> +    if (id << 12 == 0)
>> +        vcpu->arch.apf.id = 1;
>> +
>> +    return (vcpu->arch.apf.id++ << 12) | vcpu->vcpu_id;
>> +}
>> +
>>   static bool kvm_arch_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>>                       gfn_t gfn)
>>   {
>>       struct kvm_arch_async_pf arch;
>>   -    arch.token = (vcpu->arch.apf.id++ << 12) | vcpu->vcpu_id;
> This patch is completely OK. But I have a question, can we simplify it to
> arch.token = (++vcpu->arch.apf.id << 12) | vcpu->vcpu_id;
>> +    arch.token = alloc_apf_token(vcpu);
>>       arch.gfn = gfn;
>>       arch.direct_map = vcpu->arch.mmu->direct_map;
>>       arch.cr3 = vcpu->arch.mmu->get_guest_pgd(vcpu);
> 
> .

-- 
Best Regards,
Liang Zhang