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:   Thu, 10 Feb 2022 09:24:50 -0800
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Fenghua Yu <fenghua.yu@...el.com>
Cc:     Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        iommu@...ts.linux-foundation.org, x86 <x86@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID
 allocation and free it on mm exit

On Thu, Feb 10, 2022 at 08:27:50AM -0800, Fenghua Yu wrote:
> Hi, Jacob,
> 
> On Wed, Feb 09, 2022 at 07:16:14PM -0800, Jacob Pan wrote:
> > Hi Fenghua,
> > 
> > On Mon,  7 Feb 2022 15:02:48 -0800, Fenghua Yu <fenghua.yu@...el.com> wrote:
> > 
> > > @@ -1047,8 +1040,6 @@ struct iommu_sva *intel_svm_bind(struct device
> > > *dev, struct mm_struct *mm, void }
> > >  
> > >  	sva = intel_svm_bind_mm(iommu, dev, mm, flags);
> > > -	if (IS_ERR_OR_NULL(sva))
> > > -		intel_svm_free_pasid(mm);
> > If bind fails, the PASID has no IOMMU nor CPU context. It should be safe to
> > free here.
> 
> The PASID can not be freed even if bind fails. The PASID allocated earlier
> (either in this thread or in another thread) might be populated to other
> threads already and being used now.
> 
> Without freeing the PASID on bind failure, the worst case is the PASID might
> not be used in the process (and will be freed on process exit anyway).
> 
> This all matches with the PASID life time described in the commit message.

But what does this mean for the user that failed that intel_svm_bind_mm()?

Here's a scenario:

Process sets up to use PASID capable device #1. Everything works,
so the process has mm->pasid, and the IOMMU has the tables to map
virtual addresses coming from device #1 using that PASID.

Now the same process asks to start using PASID capable device #2,
but there is a failure at intel_svm_bind_mm().

Fenghua is right that we shouldn't free the PASID. It is in use
by at least one thread of the process to access device #1.

But what happens with device #2? Does the caller of intel_svm_bind()
do the right thing with the IS_ERR_OR_NULL return value to let the
user know that device #2 isn't accessible?

-Tony