Re: [PATCH v2] mm/hugetlb: fix a addressing exception caused by huge_pte_offset()

[Mike Kravetz <mike.kravetz@oracle.com>]

On 3/23/20 11:07 AM, Jason Gunthorpe wrote:
> On Mon, Mar 23, 2020 at 10:27:48AM -0700, Mike Kravetz wrote:
> 
>>>  	pgd = pgd_offset(mm, addr);
>>> -	if (!pgd_present(*pgd))
>>> +	if (!pgd_present(READ_ONCE(*pgd)))
>>>  		return NULL;
>>>  	p4d = p4d_offset(pgd, addr);
>>> -	if (!p4d_present(*p4d))
>>> +	if (!p4d_present(READ_ONCE(*p4d)))
>>>  		return NULL;
>>>  
>>>       pud = pud_offset(p4d, addr);
>>
>> One would argue that pgd and p4d can not change from present to !present
>> during the execution of this code.  To me, that seems like the issue which
>> would cause an issue.  Of course, I could be missing something.
> 
> This I am not sure of, I think it must be true under the read side of
> the mmap_sem, but probably not guarenteed under RCU..
> 
> In any case, it doesn't matter, the fact that *p4d can change at all
> is problematic. Unwinding the above inlines we get:
> 
>   p4d = p4d_offset(pgd, addr)
>   if (!p4d_present(*p4d))
>       return NULL;
>   pud = (pud_t *)p4d_page_vaddr(*p4d) + pud_index(address);
> 
> According to our memory model the compiler/CPU is free to execute this
> as:
> 
>   p4d = p4d_offset(pgd, addr)
>   p4d_for_vaddr = *p4d;
>   if (!p4d_present(*p4d))
>       return NULL;
>   pud = (pud_t *)p4d_page_vaddr(p4d_for_vaddr) + pud_index(address);
> 

Wow!  How do you know this?  You don't need to answer :)

> In the case where p4 goes from !present -> present (ie
> handle_mm_fault()):
> 
> p4d_for_vaddr == p4d_none, and p4d_present(*p4d) == true, meaning the
> p4d_page_vaddr() will crash.
> 
> Basically the problem here is not just missing READ_ONCE, but that the
> p4d is read multiple times at all. It should be written like gup_fast
> does, to guarantee a single CPU read of the unstable data:
> 
>   p4d = READ_ONCE(*p4d_offset(pgdp, addr));
>   if (!p4d_present(p4))
>       return NULL;
>   pud = pud_offset(&p4d, addr);
> 
> At least this is what I've been able to figure out :\

In that case, I believe there are a bunch of similar routines with this issue.

For this patch, I was primarily interested in seeing the obvious multiple
dereferences in C fixed up.  This is above and beyond that! :)

>>> Also, the remark about pmd_offset() seems accurate. The
>>> get_user_fast_pages() pattern seems like the correct one to emulate:
>>>
>>>   pud = READ_ONCE(*pudp);
>>>   if (pud_none(pud)) 
>>>      ..
>>>   if (!pud_'is a pmd pointer')
>>>      ..
>>>   pmdp = pmd_offset(&pud, address);
>>>   pmd = READ_ONCE(*pmd);
>>>   [...]
>>>
>>> Passing &pud in avoids another de-reference of the pudp. Honestly all
>>> these APIs that take in page table pointers and internally
>>> de-reference them seem very hard to use correctly when the page table
>>> access isn't fully locked against write.
> 
> And the same protocol for the PUD, etc.
> 
>>> It looks like at least the p4d read from the pgd is also unlocked here
>>> as handle_mm_fault() writes to it??
>>
>> Yes, there is no locking required to call huge_pte_offset().
> 
> None? Not RCU or read mmap_sem?

Yes, mmap_sem in read mode.
Sorry, I was confusing this with additional locking requirements for hugetlb
specific code.

-- 
Mike Kravetz