Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect

[Nadav Amit]

> On Dec 20, 2020, at 1:54 AM, Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
> 
> On Sun, Dec 20, 2020 at 12:06:38AM -0800, Nadav Amit wrote:
>>> On Dec 19, 2020, at 10:05 PM, Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
>>> 
>>> On Sat, Dec 19, 2020 at 01:34:29PM -0800, Nadav Amit wrote:
>>>> [ cc’ing some more people who have experience with similar problems ]
>>>> 
>>>>> On Dec 19, 2020, at 11:15 AM, Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote:
>>>>> 
>>>>> Hello,
>>>>> 
>>>>> On Fri, Dec 18, 2020 at 08:30:06PM -0800, Nadav Amit wrote:
>>>>>> Analyzing this problem indicates that there is a real bug since
>>>>>> mmap_lock is only taken for read in mwriteprotect_range(). This might
>>>>> 
>>>>> Never having to take the mmap_sem for writing, and in turn never
>>>>> blocking, in order to modify the pagetables is quite an important
>>>>> feature in uffd that justifies uffd instead of mprotect. It's not the
>>>>> most important reason to use uffd, but it'd be nice if that guarantee
>>>>> would remain also for the UFFDIO_WRITEPROTECT API, not only for the
>>>>> other pgtable manipulations.
>>>>> 
>>>>>> Consider the following scenario with 3 CPUs (cpu2 is not shown):
>>>>>> 
>>>>>> cpu0				cpu1
>>>>>> ----				----
>>>>>> userfaultfd_writeprotect()
>>>>>> [ write-protecting ]
>>>>>> mwriteprotect_range()
>>>>>> mmap_read_lock()
>>>>>> change_protection()
>>>>>> change_protection_range()
>>>>>> ...
>>>>>> change_pte_range()
>>>>>> [ defer TLB flushes]
>>>>>> 				userfaultfd_writeprotect()
>>>>>> 				 mmap_read_lock()
>>>>>> 				 change_protection()
>>>>>> 				 [ write-unprotect ]
>>>>>> 				 ...
>>>>>> 				  [ unprotect PTE logically ]
>>>>>> 				...
>>>>>> 				[ page-fault]
>>>>>> 				...
>>>>>> 				wp_page_copy()
>>>>>> 				[ set new writable page in PTE]
>>> 
>>> I don't see any problem in this example -- wp_page_copy() calls
>>> ptep_clear_flush_notify(), which should take care of the stale entry
>>> left by cpu0.
>>> 
>>> That being said, I suspect the memory corruption you observed is
>>> related this example, with cpu1 running something else that flushes
>>> conditionally depending on pte_write().
>>> 
>>> Do you know which type of pages were corrupted? file, anon, etc.
>> 
>> First, Yu, you are correct. My analysis is incorrect, but let me have
>> another try (below). To answer your (and Andrea’s) question - this happens
>> with upstream without any changes, excluding a small fix to the selftest,
>> since it failed (got stuck) due to missing wake events. [1]
>> 
>> We are talking about anon memory.
>> 
>> So to correct myself, I think that what I really encountered was actually
>> during MM_CP_UFFD_WP_RESOLVE (i.e., when the protection is removed). The
>> problem was that in this case the “write”-bit was removed during unprotect.
> 
> Thanks. You are right about when the problem happens: UFD write-
> UNprotecting. But it's not UFD write-UNprotecting that removes the
> writable bit -- the bit can only be removed during COW or UFD
> write-protecting. So your original example was almost correct, except
> the last line describing cpu1.

The scenario is a bit confusing, so stay with me. The idea behind uffd
unprotect is indeed only to mark the PTE logically as uffd-unprotected, and
not to *set* the writable bit, allowing the #PF handler to do COW or
whatever correctly upon #PF.

However, the problem that we have is that if a page is already writable,
write-unprotect *clears* the writable bit, making it write-protected (at
least for anonymous pages). This is not good from performance point-of-view,
but also a correctness issue, as I pointed out.

In some more detail: mwriteprotect_range() uses vm_get_page_prot() to
compute the new protection. For anonymous private memory, at least on x86,
this means the write-bit in the protection is clear. So later,
change_pte_range() *clears* the write-bit during *unprotection*.

That’s the reason the second part of my patch - the change to preserve_write
- fixes the problem.

> The problem is how do_wp_page() handles non-COW pages. (For COW pages,
> do_wp_page() works correctly by either reusing an existing page or
> make a new copy out of it.) In UFD case, the existing page may not
> have been properly write-protected. As you pointed out, the tlb flush
> may not be done yet. Making a copy can potentially race with the
> writer on cpu2.

Just to clarify the difference - You regard a scenario of UFFD
write-protect, while I am pretty sure the problem I encountered is during
write-unprotect.

I am not sure we are on the same page (but we may be). The problem I have is
with cow_user_page() that is called by do_wp_page() before any TLB flush
took place (either by change_protection_range() or by do_wp_page() which
does flush, but after the copy).

Let me know if you regard a different scenario.

> Should we fix the problem by ensuring integrity of the copy? IMO, no,
> because do_wp_page() shouldn't copy at all in this case. It seems it
> was recently broken by
> 
>  be068f29034f mm: fix misplaced unlock_page in do_wp_page()
>  09854ba94c6a mm: do_wp_page() simplification
> 
> I haven't study them carefully. But if you could just revert them and
> run the test again, we'd know where exactly to look at next.

These patches regard the wp_page_reuse() case, which makes me think we
are not on the same page. I do not see a problem with wp_page_reuse()
since it does not make a copy of the page. If you can explain what I
am missing, it would be great.