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Date:   Thu, 28 Apr 2022 09:43:31 +0100
From:   Jean-Philippe Brucker <jean-philippe@linaro.org>
To:     Fenghua Yu <fenghua.yu@intel.com>
Cc:     Zhangfei Gao <zhangfei.gao@linaro.org>,
        Jacob Pan <jacob.jun.pan@linux.intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        Peter Zijlstra <peterz@infradead.org>, robin.murphy@arm.com,
        Dave Hansen <dave.hansen@linux.intel.com>,
        x86 <x86@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        iommu <iommu@lists.linux-foundation.org>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>, will@kernel.org
Subject: 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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On Wed, Apr 27, 2022 at 05:54:57PM -0700, Fenghua Yu wrote:
> Hi, Dave and Jean,
> 
> On Tue, Apr 26, 2022 at 01:04:45PM +0800, Zhangfei Gao wrote:
> > 
> > 
> > On 2022/4/26 下午12:20, Fenghua Yu wrote:
> > > Hi, Jean and Zhangfei,
> > > 
> > > On Mon, Apr 25, 2022 at 05:13:02PM +0100, Jean-Philippe Brucker wrote:
> > > > Could we move mm_pasid_drop() to __mmdrop() instead of __mmput()?  For Arm
> > > > we do need to hold the mm_count until unbind(), and mmgrab()/mmdrop() is
> > > > also part of Lu's rework [1].
> > > Is this a right fix for the issue? Could you please test it on ARM?
> > > I don't have an ARM machine.
> > > 
> > > Thanks.
> > > 
> > > -Fenghua
> > > 
> > >  From 84aa68f6174439d863c40cdc2db0e1b89d620dd0 Mon Sep 17 00:00:00 2001
> > > From: Fenghua Yu <fenghua.yu@intel.com>
> > > Date: Fri, 15 Apr 2022 00:51:33 -0700
> > > Subject: [PATCH] iommu/sva: Fix PASID use-after-free issue
> > > 
> > > A PASID might be still used on ARM after it is freed in __mmput().
> > > 
> > > process:
> > > 	open()->sva_bind()->ioasid_alloc() = N; // Get PASID N for the mm
> > > 	exit();
> > > 	exit_mm()->__mmput()->mm_pasid_drop()->mm->pasid = -1; // PASID -1
> > > 	exit_files()->release(dev)->sva_unbind()->use mm->pasid; // Failure
> > > 
> > > To avoid the use-after-free issue, free the PASID after no device uses it,
> > > i.e. after all devices are unbound from the mm.
> > > 
> > > sva_bind()/sva_unbind() call mmgrab()/mmdrop() to track mm->mm_count.
> > > __mmdrop() is called only after mm->mm_count is zero. So freeing the PASID
> > > in __mmdrop() guarantees the PASID is safely freed only after no device
> > > is bound to the mm.
> > > 
> > > Fixes: 701fac40384f ("iommu/sva: Assign a PASID to mm on PASID allocation and free it on mm exit")
> > > 
> > > Reported-by: Zhangfei Gao <zhangfei.gao@foxmail.com>
> > > Suggested-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
> > > Suggested-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
> > > Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
> > Thanks for the fix.
> > 
> > Tested-by: Zhangfei Gao <zhangfei.gao@linaro.org>
> > 
> > 
> > > ---
> > >   kernel/fork.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/fork.c b/kernel/fork.c
> > > index 9796897560ab..35a3beff140b 100644
> > > --- a/kernel/fork.c
> > > +++ b/kernel/fork.c
> > > @@ -792,6 +792,7 @@ void __mmdrop(struct mm_struct *mm)
> > >   	mmu_notifier_subscriptions_destroy(mm);
> > >   	check_mm(mm);
> > >   	put_user_ns(mm->user_ns);
> > > +	mm_pasid_drop(mm);
> > >   	free_mm(mm);
> > >   }
> > >   EXPORT_SYMBOL_GPL(__mmdrop);
> > > @@ -1190,7 +1191,6 @@ static inline void __mmput(struct mm_struct *mm)
> > >   	}
> > >   	if (mm->binfmt)
> > >   		module_put(mm->binfmt->module);
> > > -	mm_pasid_drop(mm);
> > >   	mmdrop(mm);
> > >   }
> > 
> 
> Is this patch a good fix? Will you help push the fix into upstream?

Yes, I think it's the right thing to do for now. Could you resend it
separately so it gets visibility from the maintainers?

Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.org>