kvm_clear_exception_queue() should clear pending exception.
This also includes exceptions which were only marked pending but not
yet injected. This is because exception.pending is used for both L1
and L2 to determine if an exception should be raised to guest.
Note that an exception which is pending but not yet injected will
be raised again once the guest will be resumed.

Consider the following scenario:
1) L0 KVM with ignore_msrs=false.
2) L1 prepare vmcs12 with the following:
    a) No intercepts on MSR (MSR_BITMAP exist and is filled with 0).
    b) No intercept for #GP.
    c) vmx-preemption-timer is configured.
3) L1 enters into L2.
4) L2 reads an unhandled MSR that exists in MSR_BITMAP
(such as 0x1fff).

L2 RDMSR could be handled as described below:
1) L2 exits to L0 on RDMSR and calls handle_rdmsr().
2) handle_rdmsr() calls kvm_inject_gp() which sets
KVM_REQ_EVENT, exception.pending=true and exception.injected=false.
3) vcpu_enter_guest() consumes KVM_REQ_EVENT and calls
inject_pending_event() which calls vmx_check_nested_events()
which sees that exception.pending=true but
nested_vmx_check_exception() returns 0 and therefore does nothing at
this point. However let's assume it later sees vmx-preemption-timer
expired and therefore exits from L2 to L1 by calling
nested_vmx_vmexit().
4) nested_vmx_vmexit() calls prepare_vmcs12()
which calls vmcs12_save_pending_event() but it does nothing as
exception.injected is false. Also prepare_vmcs12() calls
kvm_clear_exception_queue() which does nothing as
exception.injected is already false.
5) We now return from vmx_check_nested_events() with 0 while still
having exception.pending=true!
6) Therefore inject_pending_event() continues
and we inject L2 exception to L1!...

This commit will fix above issue by changing step (4) to
clear exception.pending in kvm_clear_exception_queue().

Fixes: 664f8e26 ("KVM: X86: Fix loss of exception which has not yet been injected")
Signed-off-by: Liran Alon <liran.alon@oracle.com>
Reviewed-by: Nikita Leshenko <nikita.leshchenko@oracle.com>
Reviewed-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
Signed-off-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>