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SubjectRe: [RESEND PATCH] KVM: x86/mmu: make apf token non-zero to fix bug
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On 2/22/22 04:12, 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@huawei.com>
> ---
> 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;
> + 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);

Queued, thanks.

Paolo

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