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

[Zhangfei Gao <zhangfei.gao@linaro.org>]

On 2022/4/26 下午12:20, Fenghua Yu wrote:
> 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>
Thanks for the fix.

Tested-by: Zhangfei Gao <zhangfei.gao@linaro.org>


> ---
>   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);
>   }
>