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

[Fenghua Yu <fenghua.yu@intel.com>]

Hi, Dave,

On Thu, Apr 28, 2022 at 08:09:04AM -0700, Dave Hansen wrote:
> On 4/25/22 21:20, Fenghua Yu wrote:
> >>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().
> 
> Is it really just ARM?

Actually it should happen on X86 as well. I will remove "on ARM" in the
changelog.

> 
> > 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.
> 
> Does this changelog work for everyone?
> 
> ==
> 
> tl;dr: The PASID is being freed too early.  It needs to stay around
> until after device drivers that might be using it have had a chance to
> clear it out of the hardware.
> 

Do you want me to change the changlog to add both this paragraph and the
following paragraph?

> --
> 
> As a reminder:
> 
> mmget() /mmput()  refcount the mm's address space
> mmgrab()/mmdrop() refcount the mm itself
> 
> The PASID is currently tied to the life of the mm's address space and
> freed in __mmput().  This makes logical sense because the PASID can't be
> used once the address space is gone.
> 
> But, this misses an important point: even after the address space is
> gone, the PASID will still be programmed into a device.  Device drivers
> might, for instance, still need to flush operations that are outstanding
> and need to use that PASID.  They do this at ->release() time.
> 
> Device drivers hold a reference on the mm itself and drop it at
> ->release() time.  But, the device driver holds a reference mm itself,
> not the address space.  The address space (and the PASID) is long gone
> by the time the driver tries to clean up.  This is effectively a
> use-after-free bug on the PASID.
> 
> To fix this, move the PASID free operation from __mmput() to __mmdrop().
>  This ensures that the device drivers' existing mmgrab() keeps the PASID
> allocated until they drop their mm reference.
> 

Thank you very much!

-Fenghua