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

Fenghua Yu
2022-04-28T00:54:28Z

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?

Thank you very much!

-Fenghua