linux-kernel - Handling active DMA during a VFIO application crash

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Date:   Thu, 15 Feb 2018 16:34:06 +0530
From:   Linu Cherian <linuc.decode@...il.com>
To:     alex.williamson@...hat.com
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linu.cherian@...ium.com, Sunil.Goutham@...ium.com
Subject: Handling active DMA during a VFIO application crash

Hi,

Was exploring the implications of an application crash while DMA 
is active from a vfio PCI device; the DMA being configured and 
started by the application using vfio APIs.               

The expectation is that, DMA is stopped/reset before we tear down the IOMMU mappings 
and finally free the mmapped pages(on which DMA is happening).                                            

>From the below stack trace(with dump_stack in vfio_pci_release),             
[  201.564273] [<ffffff8008798b50>] vfio_pci_release+0x80/0x458
[  201.564276] [<ffffff8008792b74>] vfio_device_fops_release+0x2c/0x50
[  201.564279] [<ffffff8008269ef4>] __fput+0x9c/0x218
[  201.564283] [<ffffff800826a0e8>] ____fput+0x20/0x30
[  201.564286] [<ffffff80080e7fe0>] task_work_run+0xa0/0xc8
[  201.564289] [<ffffff80080cbc7c>] do_exit+0x2bc/0x9c8
[  201.564293] [<ffffff80080cd0ec>] do_group_exit+0x3c/0xa8
[  201.564296] [<ffffff80080d94c4>] get_signal+0x3e4/0x538
[  201.564299] [<ffffff80080892f0>] do_signal+0x70/0x660
[  201.564302] [<ffffff8008089ce8>] do_notify_resume+0xe0/0x120

PCI device is disabled/reset from vfio_pci_release invoked as part of 
device fd release. The fd releases are in turn invoked from exit_files
and exit_task_work.

But exit_mm, gets called before exit_files/exit_task_work in do_exit.

Assuming all pages allocated/mmaped to a process gets freed in exit_mm,                                                                                               
is there is a possibility that user pages configured for DMA can get freed 
to kernel before the vfio device is stopped/reset ? 

Thanks.

-- 
Linu cherian