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

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