[PATCH 5.15 014/139] KVM: x86/mmu: make apf token non-zero to fix bug

Greg Kroah-Hartman
2022-02-28T17:51:58Z

From: Liang Zhang <zhangliang5@huawei.com>

commit 6f3c1fc53d86d580d8d6d749c4af23705e4f6f79 upstream.

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>
Message-Id: <20220222031239.1076682-1-zhangliang5@huawei.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/kvm/mmu/mmu.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)

--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3889,12 +3889,23 @@ static void shadow_page_table_clear_floo
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);