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

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

[Fenghua Yu]

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>
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>
---

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);
 }
 
-- 
2.32.0

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

[Zhangfei Gao]

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>
> ---
>
> 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);
>   }
>   

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

[Zhangfei Gao]

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);
>>   }
>

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

[Thomas Gleixner]

On Fri, May 06 2022 at 09:49, Zhangfei Gao wrote:
> Will this be taken for 5.18?

It's queued in tip core/urgent and will go to Linus this week.

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