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

From: Dave Hansen
Date: Thu Apr 28 2022 - 11:08:54 EST

On 4/25/22 21:20, Fenghua Yu wrote:
>>From 84aa68f6174439d863c40cdc2db0e1b89d620dd0 Mon Sep 17 00:00:00 2001
> From: Fenghua Yu <fenghua.yu@xxxxxxxxx>
> 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?

> 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.


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.

> 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);
> }