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

[Nadav Amit]

> On Dec 20, 2020, at 9:12 PM, Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
> 
> On Sun, Dec 20, 2020 at 08:36:15PM -0800, Nadav Amit wrote:
>>> On Dec 19, 2020, at 6:20 PM, Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote:
>>> 
>>> On Sat, Dec 19, 2020 at 02:06:02PM -0800, Nadav Amit wrote:
>>>>> On Dec 19, 2020, at 1:34 PM, Nadav Amit <nadav.amit@xxxxxxxxx> 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 ]
>>> 
>>> Is the uffd selftest failing with upstream or after your kernel
>>> modification that removes the tlb flush from unprotect?
>> 
>> Please see my reply to Yu. I was wrong in this analysis, and I sent a
>> correction to my analysis. The problem actually happens when
>> userfaultfd_writeprotect() unprotects the memory.
>> 
>>> } else if (uffd_wp_resolve) {
>>> 				/*
>>> 				 * Leave the write bit to be handled
>>> 				 * by PF interrupt handler, then
>>> 				 * things like COW could be properly
>>> 				 * handled.
>>> 				 */
>>> 				ptent = pte_clear_uffd_wp(ptent);
>>> 			}
>>> 
>>> Upstraem this will still do pages++, there's a tlb flush before
>>> change_protection can return here, so I'm confused.
>> 
>> You are correct. The problem I encountered with userfaultfd_writeprotect()
>> is during unprotecting path.
>> 
>> Having said that, I think that there are additional scenarios that are
>> problematic. Consider for instance madvise_dontneed_free() that is racing
>> with userfaultfd_writeprotect(). If madvise_dontneed_free() completed
>> removing the PTEs, but still did not flush, change_pte_range() will see
>> non-present PTEs, say a flush is not needed, and then
>> change_protection_range() will not do a flush, and return while
>> the memory is still not protected.
>> 
>>> I don't share your concern. What matters is the PT lock, so it
>>> wouldn't be one per pte, but a least an order 9 higher, but let's
>>> assume one flush per pte.
>>> 
>>> It's either huge mapping and then it's likely running without other
>>> tlb flushing in background (postcopy snapshotting), or it's a granular
>>> protect with distributed shared memory in which case the number of
>>> changd ptes or huge_pmds tends to be always 1 anyway. So it doesn't
>>> matter if it's deferred.
>>> 
>>> I agree it may require a larger tlb flush review not just mprotect
>>> though, but it didn't sound particularly complex. Note the
>>> UFFDIO_WRITEPROTECT is still relatively recent so backports won't
>>> risk to reject so heavy as to require a band-aid.
>>> 
>>> My second thought is, I don't see exactly the bug and it's not clear
>>> if it's upstream reproducing this, but assuming this happens on
>>> upstream, even ignoring everything else happening in the tlb flush
>>> code, this sounds like purely introduced by userfaultfd_writeprotect()
>>> vs userfaultfd_writeprotect() (since it's the only place changing
>>> protection with mmap_sem for reading and note we already unmap and
>>> flush tlb with mmap_sem for reading in MADV_DONTNEED/MADV_FREE clears
>>> the dirty bit etc..). Flushing tlbs with mmap_sem for reading is
>>> nothing new, the only new thing is the flush after wrprotect.
>>> 
>>> So instead of altering any tlb flush code, would it be possible to
>>> just stick to mmap_lock for reading and then serialize
>>> userfaultfd_writeprotect() against itself with an additional
>>> mm->mmap_wprotect_lock mutex? That'd be a very local change to
>>> userfaultfd too.
>>> 
>>> Can you look if the rule mmap_sem for reading plus a new
>>> mm->mmap_wprotect_lock mutex or the mmap_sem for writing, whenever
>>> wrprotecting ptes, is enough to comply with the current tlb flushing
>>> code, so not to require any change non local to uffd (modulo the
>>> additional mutex).
>> 
>> So I did not fully understand your solution, but I took your point and
>> looked again on similar cases. To be fair, despite my experience with these
>> deferred TLB flushes as well as Peter Zijlstra’s great documentation, I keep
>> getting confused (e.g., can’t we somehow combine tlb_flush_batched and
>> tlb_flush_pending ?)
>> 
>> As I said before, my initial scenario was wrong, and the problem is not
>> userfaultfd_writeprotect() racing against itself. This one seems actually
>> benign to me.
>> 
>> Nevertheless, I do think there is a problem in change_protection_range().
>> Specifically, see the aforementioned scenario of a race between
>> madvise_dontneed_free() and userfaultfd_writeprotect().
>> 
>> So an immediate solution for such a case can be resolve without holding
>> mmap_lock for write, by just adding a test for mm_tlb_flush_nested() in
>> change_protection_range():
>> 
>>        /*
>> 	 * Only flush the TLB if we actually modified any entries
>> 	 * or if there are pending TLB flushes.
>> 	 */
>>        if (pages || mm_tlb_flush_nested(mm))
>>                flush_tlb_range(vma, start, end);
>> 
>> To be fair, I am not confident I did not miss other problematic cases.
>> 
>> But for now, this change, with the preserve_write change should address the
>> immediate issues. Let me know if you agree.
>> 
>> Let me know whether you agree.
> 
> The problem starts in UFD, and is related to tlb flush. But its focal
> point is in do_wp_page(). I'd suggest you look at function and see
> what it does before and after the commits I listed, with the following
> conditions
> 
> PageAnon(), !PageKsm(), !PageSwapCache(), !pte_write(),
> page_mapcount() = 1, page_count() > 1 or PageLocked()
> 
> when it runs against the two UFD examples you listed.

Thanks for your quick response. I wanted to write a lengthy response, but I
do want to sleep on it. I presume page_count() > 1, since I have multiple
concurrent page-faults on the same address in my test, but I will check.

Anyhow, before I give a further response, I was just wondering - since you
recently dealt with soft-dirty issue as I remember - isn't this problematic
COW for non-COW page scenario, in which the copy races with writes to a page
which is protected in the PTE but not in all TLB, also problematic for
soft-dirty clearing?