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

[Yu Zhao <yuzhao@google.com>]

On Sun, Dec 20, 2020 at 09:39:06PM -0800, Nadav Amit wrote:
> > On Dec 20, 2020, at 9:25 PM, Nadav Amit <nadav.amit@gmail.com> wrote:
> > 
> >> On Dec 20, 2020, at 9:12 PM, Yu Zhao <yuzhao@google.com> 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@redhat.com> 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@gmail.com> wrote:
> >>>>>> 
> >>>>>> [ cc’ing some more people who have experience with similar problems ]
> >>>>>> 
> >>>>>>> On Dec 19, 2020, at 11:15 AM, Andrea Arcangeli <aarcange@redhat.com> 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?

Yes, it has the same problem.

> Stupid me. You hold mmap_lock for write, so no, it cannot happen when clear
> soft-dirty.

mmap_write_lock is temporarily held to update vm_page_prot for write
notifications. It doesn't help in the context of this problem.