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

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Message-ID: <YmdzFFx7fN586jcf@fyu1.sc.intel.com>
Date:   Mon, 25 Apr 2022 21:20:36 -0700
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Jean-Philippe Brucker <jean-philippe@...aro.org>
Cc:     Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Peter Zijlstra <peterz@...radead.org>, robin.murphy@....com,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        x86 <x86@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        iommu <iommu@...ts.linux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>, zhangfei.gao@...aro.org,
        Thomas Gleixner <tglx@...utronix.de>, will@...nel.org
Subject: Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID
 allocation and free it on mm exit

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