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Subject: Re: [PATCH v2] iommu/sva: Fix PASID use-after-free issue
To:     Fenghua Yu <fenghua.yu@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@redhat.com>,
        Will Deacon <will@kernel.org>,
        Robin Murphy <robin.murphy@arm.com>,
        Tony Luck <tony.luck@intel.com>,
        Jacob Pan <jacob.jun.pan@linux.intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>, x86 <x86@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        iommu@lists.linux-foundation.org
References: <20220428180041.806809-1-fenghua.yu@intel.com>
 <8f50c673-fe92-3c42-993d-43e65fc7235c@linaro.org>
From:   Zhangfei Gao <zhangfei.gao@linaro.org>
Message-ID: <da5eafcc-1cb0-07fc-aa0d-0dccb8b8875a@linaro.org>
Date:   Fri, 6 May 2022 09:49:50 +0800
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On 2022/4/29 上午9:39, Zhangfei Gao wrote:
>
>
> On 2022/4/29 上午2:00, Fenghua Yu wrote:
>> 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@foxmail.com>
>> Tested-by: Zhangfei Gao <zhangfei.gao@foxmail.com>
>
> Tested-by: Zhangfei Gao <zhangfei.gao@linaro.org>
>
> Use the formal email, thanks
>
>> Suggested-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
>> Suggested-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
>> Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
>> Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>

Hi,

Will this be taken for 5.18?

Thanks

>> ---
>>
>> v2:
>> - Dave Hansen rewrites the change log.
>> - Add Tested-by: Zhangfei Gao <zhangfei.gao@foxmail.com>
>> - Add Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.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);
>>   }
>