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

[Nadav Amit]

> 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.