linux-kernel - Re: [PATCH v5 08/14] iommufd/viommu: Add iommufd_viommu_report

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

Message-ID: <20250113192144.GT5556@nvidia.com>
Date: Mon, 13 Jan 2025 15:21:44 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: kevin.tian@...el.com, corbet@....net, will@...nel.org, joro@...tes.org,
	suravee.suthikulpanit@....com, robin.murphy@....com,
	dwmw2@...radead.org, baolu.lu@...ux.intel.com, shuah@...nel.org,
	linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org,
	linux-kselftest@...r.kernel.org, linux-doc@...r.kernel.org,
	eric.auger@...hat.com, jean-philippe@...aro.org, mdf@...nel.org,
	mshavit@...gle.com, shameerali.kolothum.thodi@...wei.com,
	smostafa@...gle.com, ddutile@...hat.com, yi.l.liu@...el.com,
	patches@...ts.linux.dev
Subject: Re: [PATCH v5 08/14] iommufd/viommu: Add iommufd_viommu_report_event
 helper

On Sun, Jan 12, 2025 at 09:37:41PM -0800, Nicolin Chen wrote:

> > > > I supposed we will need a way to indicate lost events to userspace on
> > > > top of this?
> > > 
> > > Perhaps another u32 flag in the arm_smmuv3_vevent struct to report
> > > an overflow. That said, what userspace/VMM will need to do with it?
> > 
> > Trigger the above code in the VM somehow?
> 
> I found two ways of forwarding an overflow flag:
> 
> 1. Return -EOVERFLOW to read(). But it cannot return the read bytes
> any more:

You could not return any bytes, it would have to be 0 bytes read, ie
immediately return EOVERFLOW and do nothing else.

Returning EOVERFLOW from read would have to also clear the overflow
indicator.

The other approach would be to add a sequence number to each event and
let userspace detect the non-montonicity. It would require adding a
header to the native ARM evt.

> 2. Return EPOLLERR via pollfd.revents. But it cannot use POLLERR
> for other errors any more:
> --------------------------------------------------
> @@ -420,2 +421,4 @@ static __poll_t iommufd_eventq_fops_poll(struct file *filep,
>         poll_wait(filep, &eventq->wait_queue, wait);
> +       if (test_bit(IOMMUFD_VEVENTQ_ERROR_OVERFLOW, veventq->errors))
> +               return EPOLLERR;
>         mutex_lock(&eventq->mutex);

But then how do you clear the error? I've only seen POLLERR used for
fatal conditions so there is no recovery, it is permanent.

Jason