Messages in this thread | | | Subject | Re: [PATCH v2] s390/vfio-ap: fix memory leak in mdev remove callback | From | Tony Krowiak <> | Date | Tue, 18 May 2021 14:40:50 -0400 |
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On 5/18/21 2:22 PM, Christian Borntraeger wrote: > > > On 18.05.21 20:14, Tony Krowiak wrote: >> >> >> On 5/18/21 9:59 AM, Christian Borntraeger wrote: >>> >>> >>> On 18.05.21 15:42, Tony Krowiak wrote: >>>> >>>> >>>> On 5/18/21 5:30 AM, Christian Borntraeger wrote: >>>>> >>>>> >>>>> On 17.05.21 21:10, Halil Pasic wrote: >>>>>> On Mon, 17 May 2021 09:37:42 -0400 >>>>>> Tony Krowiak <akrowiak@linux.ibm.com> wrote: >>>>>> >>>>>>>> >>>>>>>> Because of this, I don't think the rest of your argument is valid. >>>>>>> >>>>>>> Okay, so your concern is that between the point in time the >>>>>>> vcpu->kvm->arch.crypto.pqap_hook pointer is checked in >>>>>>> priv.c and the point in time the handle_pqap() function >>>>>>> in vfio_ap_ops.c is called, the memory allocated for the >>>>>>> matrix_mdev containing the struct kvm_s390_module_hook >>>>>>> may get freed, thus rendering the function pointer invalid. >>>>>>> While not impossible, that seems extremely unlikely to >>>>>>> happen. Can you articulate a scenario where that could >>>>>>> even occur? >>>>>> >>>>>> Malicious userspace. We tend to do the pqap aqic just once >>>>>> in the guest right after the queue is detected. I do agree >>>>>> it ain't very likely to happen during normal operation. But why are >>>>>> you asking? >>>>> >>>>> Would it help, if the code in priv.c would read the hook once >>>>> and then only work on the copy? We could protect that with rcu >>>>> and do a synchronize rcu in vfio_ap_mdev_unset_kvm after >>>>> unsetting the pointer? >>>> >>>> I'll look into this. >>> >>> I think it could work. in priv.c use rcu_readlock, save the >>> pointer, do the check and call, call rcu_read_unlock. >>> In vfio_ap use rcu_assign_pointer to set the pointer and >>> after setting it to zero call sychronize_rcu. >>> >>> Halil, I think we can do this as an addon patch as it makes >>> sense to have this callback pointer protected independent of >>> this patch. Agree? >> >> I agree that this is a viable option; however, this does not >> guarantee that the matrix_mdev is not freed thus rendering >> the function pointer to the interception handler invalid unless >> that is also included within the rcu_readlock/rcu_read_unlock. > > The trick should be the sychronize_rcu. This will put the deleting > code (vfio_ap_mdev_unset_kvm) to sleep until the rcu read section > has finished. So if you first set the pointer to zero, then call > synchronize_rcu the code will only progress until all users of > the old poiner have finished.
Yes, that is my understanding too.
> >> That is not possible given the matrix_mdev is freed within >> the remove callback and the pointer to the structure that >> contains the interception handler function pointer is cleared >> in the vfio_ap_mdev_unset_kvm() function. I am working on >> a patch and should be able to post it before EOD or first thing >> tomorrow. >>
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