linux-kernel - [PATCH v2] iommu/sva: Fix PASID use-after-free issue

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Message-Id: <20220428180041.806809-1-fenghua.yu@intel.com>
Date:   Thu, 28 Apr 2022 11:00:41 -0700
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     "Dave Hansen" <dave.hansen@...ux.intel.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Jean-Philippe Brucker" <jean-philippe@...aro.org>,
        "Borislav Petkov" <bp@...en8.de>, "Ingo Molnar" <mingo@...hat.com>,
        "Zhangfei Gao" <zhangfei.gao@...aro.org>,
        "Will Deacon" <will@...nel.org>,
        "Robin Murphy" <robin.murphy@....com>,
        "Tony Luck" <tony.luck@...el.com>,
        "Jacob Pan" <jacob.jun.pan@...ux.intel.com>,
        "Ravi V Shankar" <ravi.v.shankar@...el.com>,
        "Peter Zijlstra" <peterz@...radead.org>,
        "Andy Lutomirski" <luto@...nel.org>, "x86" <x86@...nel.org>,
        "linux-kernel" <linux-kernel@...r.kernel.org>,
        iommu@...ts.linux-foundation.org
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Zhangfei Gao <zhangfei.gao@...mail.com>
Subject: [PATCH v2] iommu/sva: Fix PASID use-after-free issue

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 file->release() time.

Device drivers call the IOMMU driver to hold a reference on the mm itself
and drop it at file->release() time.  But, the IOMMU driver holds a
reference on the 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 IOMMU driver's existing mmgrab() keeps the PASID
allocated until it drops its mm reference.

Fixes: 701fac40384f ("iommu/sva: Assign a PASID to mm on PASID allocation and free it on mm exit")

Reported-by: Zhangfei Gao <zhangfei.gao@...mail.com>
Tested-by: Zhangfei Gao <zhangfei.gao@...mail.com>
Suggested-by: Jean-Philippe Brucker <jean-philippe@...aro.org>
Suggested-by: Jacob Pan <jacob.jun.pan@...ux.intel.com>
Reviewed-by: Jean-Philippe Brucker <jean-philippe@...aro.org>
Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
---

v2:
- Dave Hansen rewrites the change log.
- Add Tested-by: Zhangfei Gao <zhangfei.gao@...mail.com>
- Add Reviewed-by: Jean-Philippe Brucker <jean-philippe@...aro.org>

The original patch was posted and discussed in:
https://lore.kernel.org/lkml/YmdzFFx7fN586jcf@fyu1.sc.intel.com/

 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);
 }

-- 
2.32.0