linux-kernel - Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID allocation and free it on mm exit

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Date:   Mon, 18 Apr 2022 11:11:35 -0700
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     "zhangfei.gao@...mail.com" <zhangfei.gao@...mail.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        jean-philippe <jean-philippe@...aro.org>,
        "Peter Zijlstra" <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        x86 <x86@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        iommu <iommu@...ts.linux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "Poimboe, Josh" <jpoimboe@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID
 allocation and free it on mm exit

Hi Kevin,

On Mon, 18 Apr 2022 06:34:19 +0000, "Tian, Kevin" <kevin.tian@...el.com>
wrote:

> > From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> > Sent: Saturday, April 16, 2022 5:00 AM
> > 
> > Hi zhangfei.gao@...mail.com,
> > 
> > On Fri, 15 Apr 2022 19:52:03 +0800, "zhangfei.gao@...mail.com"
> > <zhangfei.gao@...mail.com> wrote:
> >   
> > > >>> A PASID might be still used even though it is freed on mm exit.
> > > >>>
> > > >>> process A:
> > > >>> 	sva_bind();
> > > >>> 	ioasid_alloc() = N; // Get PASID N for the mm
> > > >>> 	fork(): // spawn process B
> > > >>> 	exit();
> > > >>> 	ioasid_free(N);
> > > >>>
> > > >>> process B:
> > > >>> 	device uses PASID N -> failure
> > > >>> 	sva_unbind();
> > > >>>
> > > >>> Dave Hansen suggests to take a refcount on the mm whenever
> > > >>> binding  
> > the  
> > > >>> PASID to a device and drop the refcount on unbinding. The mm
> > > >>> won't  
> > be  
> > > >>> dropped if the PASID is still bound to it.
> > > >>>
> > > >>> Fixes: 701fac40384f ("iommu/sva: Assign a PASID to mm on PASID
> > > >>> allocation and free it on mm exit")
> > > >>>  
> > Is process A's mm intended to be used by process B? Or you really should
> > use PASID N on process B's mm? If the latter, it may work for a while
> > until B changes mapping.
> > 
> > It seems you are just extending the life of a defunct mm?
> >   
> 
> IMHO the intention is not to allow B to access A's mm.
> 
> The problem is that PASID N is released on exit() of A and then
> reallocated to B before iommu driver gets the chance to quiesce
> the device and clear the PASID entry. According to the discussion
> the quiesce operation must be done when driver calls unbind()
> instead of in mm exit. In this case a failure is reported when
> B tries to call bind() on PASID N due to an already-present entry.
> 
> Dave's patch extending the life of A's mm until unbind() is called.
> With it B either gets a different PASID before A's unbind() is 
> completed or same PASID N pointing to B's mm after A's unbind().
> 
As long as B gets a different PASID, that is fine. It seems PASID N has no
use then.

> Thanks
> Kevin


Thanks,

Jacob