Re: [PATCH] userfaultfd: hugetlbfs: Fix userfaultfd_huge_must_wait pte access

[Janosch Frank <frankja@linux.ibm.com>]

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On 26.06.2018 19:00, Mike Kravetz wrote:
> On 06/26/2018 06:24 AM, Janosch Frank wrote:
>> Use huge_ptep_get to translate huge ptes to normal ptes so we can
>> check them with the huge_pte_* functions. Otherwise some architectures
>> will check the wrong values and will not wait for userspace to bring
>> in the memory.
>>
>> Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
>> Fixes: 369cd2121be4 ("userfaultfd: hugetlbfs: userfaultfd_huge_must_wait for hugepmd ranges")
> Adding linux-mm and Andrew on Cc:
> 
> Thanks for catching and fixing this.

Sure
I'd be happy if we get less of these problems with time, this one was
rather painful to debug. :)

> I think this needs to be fixed in stable as well.  Correct?  Assuming
> userfaultfd is/can be enabled for impacted architectures.

Correct, it seems I forgot the CC stable...

> 
> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>

Thanks

> -- Mike Kravetz
>> ---
>>  fs/userfaultfd.c | 12 +++++++-----
>>  1 file changed, 7 insertions(+), 5 deletions(-)
>>
>> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
>> index 123bf7d516fc..594d192b2331 100644
>> --- a/fs/userfaultfd.c
>> +++ b/fs/userfaultfd.c
>> @@ -222,24 +222,26 @@ static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
>>  					 unsigned long reason)
>>  {
>>  	struct mm_struct *mm = ctx->mm;
>> -	pte_t *pte;
>> +	pte_t *ptep, pte;
>>  	bool ret = true;
>>  
>>  	VM_BUG_ON(!rwsem_is_locked(&mm->mmap_sem));
>>  
>> -	pte = huge_pte_offset(mm, address, vma_mmu_pagesize(vma));
>> -	if (!pte)
>> +	ptep = huge_pte_offset(mm, address, vma_mmu_pagesize(vma));
>> +
>> +	if (!ptep)
>>  		goto out;
>>  
>>  	ret = false;
>> +	pte = huge_ptep_get(ptep);
>>  
>>  	/*
>>  	 * Lockless access: we're in a wait_event so it's ok if it
>>  	 * changes under us.
>>  	 */
>> -	if (huge_pte_none(*pte))
>> +	if (huge_pte_none(pte))
>>  		ret = true;
>> -	if (!huge_pte_write(*pte) && (reason & VM_UFFD_WP))
>> +	if (!huge_pte_write(pte) && (reason & VM_UFFD_WP))
>>  		ret = true;
>>  out:
>>  	return ret;
>>



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