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

Fenghua Yu
2022-04-26T04:20:16Z

Hi, Jean and Zhangfei,

On Mon, Apr 25, 2022 at 05:13:02PM +0100, Jean-Philippe Brucker wrote:
> Could we move mm_pasid_drop() to __mmdrop() instead of __mmput()? For Arm
> we do need to hold the mm_count until unbind(), and mmgrab()/mmdrop() is
> also part of Lu's rework [1].

Is this a right fix for the issue? Could you please test it on ARM?
I don't have an ARM machine.

Thanks.

-Fenghua

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().

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.

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

Reported-by: Zhangfei Gao <zhangfei.gao@foxmail.com>
Suggested-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Suggested-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Fenghua Yu <fenghua.yu@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