linux-kernel - Re: [PATCH] VFIO: PCI: eliminate unnecessary overhead in vfio_pci_reflck

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [day] [month] [year] [list]

Date:   Wed, 6 Nov 2019 02:02:59 +0000
From:   linmiaohe <linmiaohe@...wei.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     "cohuck@...hat.com" <cohuck@...hat.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "aik@...abs.ru" <aik@...abs.ru>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "hexin.op@...il.com" <hexin.op@...il.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] VFIO: PCI: eliminate unnecessary overhead in
 vfio_pci_reflck_find


On Wed, 6 Nov 2019 03:25
Alex Williamson wrote
>> From: Miaohe Lin <linmiaohe@...wei.com>
>> 
>> The driver of the pci device may not equal to vfio_pci_driver, but we 
>> fetch vfio_device from pci_dev unconditionally in func 
>> vfio_pci_reflck_find. This overhead, such as the competition of 
>> vfio.group_lock, can be eliminated by check pci_dev_driver with 
>> vfio_pci_driver first.
>> 
>
>I believe this introduces a race.  When we hold a reference to the vfio device, an unbind from a vfio bus driver will be blocked in vfio_del_group_dev().  Therefore if we test the driver is vfio-pci while holding this reference, we know that it cannot be released and the device_data is a valid vfio_pci_device. Testing the driver prior to acquiring a vfio device reference is meaningless as we have no guarantee that the driver has not changed by the time we acquire a reference.  Are you actually seeing contention here or was this a code inspection optimization?  Thanks,
>
>Alex

Thanks for your reply and patient explanation.  It was a code inspection optimization, and obviously, I missed something. I'm really sorry about that.

Best wishes.
Have a nice day.