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:   Fri, 15 Apr 2022 14:00:02 -0700
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     "zhangfei.gao@...mail.com" <zhangfei.gao@...mail.com>
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Ashok Raj <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>,
        Dave Hansen <dave.hansen@...el.com>,
        iommu <iommu@...ts.linux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <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 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?

Thanks,

Jacob