Messages in this thread | | | Date | Thu, 28 Apr 2022 08:09:04 -0700 | Subject | Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID allocation and free it on mm exit | From | Dave Hansen <> |
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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?
> 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.
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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); > } >
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