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

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 23/12/2019 22:18, Michael Ellerman wrote:
> Alexey Kardashevskiy <aik@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@suse.cz>
>> Signed-off-by: Alexey Kardashevskiy <aik@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,


> 
> 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