[PATCH kernel v3] powerpc/book3s64: Fix error handling in mm_iommu_do_alloc()

[Alexey Kardashevskiy]

On 24/12/2019 10:32, Alexey Kardashevskiy wrote:
> 
> 
> On 23/12/2019 22:18, Michael Ellerman wrote:
>> Alexey Kardashevskiy <aik at ozlabs.ru> writes:
>>
>>> The last jump to free_exit in mm_iommu_do_alloc() happens after page
>>> pointers in struct mm_iommu_table_group_mem_t were already converted to
>>> physical addresses. Thus calling put_page() on these physical addresses
>>> will likely crash.
>>>
>>> This moves the loop which calculates the pageshift and converts page
>>> struct pointers to physical addresses later after the point when
>>> we cannot fail; thus eliminating the need to convert pointers back.
>>>
>>> Fixes: eb9d7a62c386 ("powerpc/mm_iommu: Fix potential deadlock")
>>> Reported-by: Jan Kara <jack at suse.cz>
>>> Signed-off-by: Alexey Kardashevskiy <aik at ozlabs.ru>
>>> ---
>>> Changes:
>>> v3:
>>> * move pointers conversion after the last possible failure point
>>> ---
>>>  arch/powerpc/mm/book3s64/iommu_api.c | 39 +++++++++++++++-------------
>>>  1 file changed, 21 insertions(+), 18 deletions(-)
>>>
>>> diff --git a/arch/powerpc/mm/book3s64/iommu_api.c b/arch/powerpc/mm/book3s64/iommu_api.c
>>> index 56cc84520577..ef164851738b 100644
>>> --- a/arch/powerpc/mm/book3s64/iommu_api.c
>>> +++ b/arch/powerpc/mm/book3s64/iommu_api.c
>>> @@ -121,24 +121,6 @@ static long mm_iommu_do_alloc(struct mm_struct *mm, unsigned long ua,
>>>  		goto free_exit;
>>>  	}
>>>  
>>> -	pageshift = PAGE_SHIFT;
>>> -	for (i = 0; i < entries; ++i) {
>>> -		struct page *page = mem->hpages[i];
>>> -
>>> -		/*
>>> -		 * Allow to use larger than 64k IOMMU pages. Only do that
>>> -		 * if we are backed by hugetlb.
>>> -		 */
>>> -		if ((mem->pageshift > PAGE_SHIFT) && PageHuge(page))
>>> -			pageshift = page_shift(compound_head(page));
>>> -		mem->pageshift = min(mem->pageshift, pageshift);
>>> -		/*
>>> -		 * We don't need struct page reference any more, switch
>>> -		 * to physical address.
>>> -		 */
>>> -		mem->hpas[i] = page_to_pfn(page) << PAGE_SHIFT;
>>> -	}
>>> -
>>>  good_exit:
>>>  	atomic64_set(&mem->mapped, 1);
>>>  	mem->used = 1;
>>> @@ -158,6 +140,27 @@ static long mm_iommu_do_alloc(struct mm_struct *mm, unsigned long ua,
>>>  		}
>>>  	}
>>>  
>>> +	if (mem->dev_hpa == MM_IOMMU_TABLE_INVALID_HPA) {
>>
>> Couldn't you avoid testing this again ...
>>
>>> +		/*
>>> +		 * Allow to use larger than 64k IOMMU pages. Only do that
>>> +		 * if we are backed by hugetlb. Skip device memory as it is not
>>> +		 * backed with page structs.
>>> +		 */
>>> +		pageshift = PAGE_SHIFT;
>>> +		for (i = 0; i < entries; ++i) {
>>
>> ... by making this loop up to `pinned`.
>>
>> `pinned` is only incremented in the loop that does the GUP, and there's
>> a check that pinned == entries after that loop.
>>
>> So when we get here we know pinned == entries, and if pinned is zero
>> it's because we took the (dev_hpa != MM_IOMMU_TABLE_INVALID_HPA) case at
>> the start of the function to get here.
>>
>> Or do you think that's too subtle to rely on?
> 
> 
> I had 4 choices:
> 
> 1. for (;i < pinned;)
> 
> 2. if (dev_hpa == MM_IOMMU_TABLE_INVALID_HPA) (dev_hpa is a function
> parameter)
> 
> 3. if (mem->dev_hpa == MM_IOMMU_TABLE_INVALID_HPA)
> 
> 4. if (mem->hpages)
> 
> 
> The function is already ugly. 3) seemed as the most obvious way of
> telling what is going on here: "we have just initialized @mem and it is
> not for a device memory, lets finish the initialization".
> 
> I could rearrange the code even more but since there is no NVLink3
> coming ever, I'd avoid changing it more than necessary. Thanks,



Repost? Rework?



> 
> 
>>
>> cheers
>>
>>> +			struct page *page = mem->hpages[i];
>>> +
>>> +			if ((mem->pageshift > PAGE_SHIFT) && PageHuge(page))
>>> +				pageshift = page_shift(compound_head(page));
>>> +			mem->pageshift = min(mem->pageshift, pageshift);
>>> +			/*
>>> +			 * We don't need struct page reference any more, switch
>>> +			 * to physical address.
>>> +			 */
>>> +			mem->hpas[i] = page_to_pfn(page) << PAGE_SHIFT;
>>> +		}
>>> +	}
>>> +
>>>  	list_add_rcu(&mem->next, &mm->context.iommu_group_mem_list);
>>>  
>>>  	mutex_unlock(&mem_list_mutex);
>>> -- 
>>> 2.17.1
> 

-- 
Alexey