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

[Peter Xu <peterx@redhat.com>]

On Wed, Dec 23, 2020 at 12:12:23PM -0700, Yu Zhao wrote:
> On Wed, Dec 23, 2020 at 11:24:16AM -0500, Peter Xu wrote:
> > On Wed, Dec 23, 2020 at 03:06:30AM -0700, Yu Zhao wrote:
> > > On Wed, Dec 23, 2020 at 01:44:42AM -0800, Linus Torvalds wrote:
> > > > On Tue, Dec 22, 2020 at 4:01 PM Linus Torvalds
> > > > <torvalds@linux-foundation.org> wrote:
> > > > >
> > > > > The more I look at the mprotect code, the less I like it. We seem to
> > > > > be much better about the TLB flushes in other places (looking at
> > > > > mremap, for example). The mprotect code seems to be very laissez-faire
> > > > > about the TLB flushing.
> > > > 
> > > > No, this doesn't help.
> > > > 
> > > > > Does adding a TLB flush to before that
> > > > >
> > > > >         pte_unmap_unlock(pte - 1, ptl);
> > > > >
> > > > > fix things for you?
> > > > 
> > > > It really doesn't fix it. Exactly because - as pointed out earlier -
> > > > the actual page *copy* happens outside the pte lock.
> > > 
> > > I appreciate all the pointers. It seems to me it does.
> > > 
> > > > So what can happen is:
> > > > 
> > > >  - CPU 1 holds the page table lock, while doing the write protect. It
> > > > has cleared the writable bit, but hasn't flushed the TLB's yet
> > > > 
> > > >  - CPU 2 did *not* have the TLB entry, sees the new read-only state,
> > > > takes a COW page fault, and reads the PTE from memory (into
> > > > vmf->orig_pte)
> > > 
> > > In handle_pte_fault(), we lock page table and check pte_write(), so
> > > we either see a RW pte before CPU 1 runs or a RO one with no stale tlb
> > > entries after CPU 1 runs, assume CPU 1 flushes tlb while holding the
> > > same page table lock (not mmap_lock).
> > 
> > I think this is not against Linus's example - where cpu2 does not have tlb
> > cached so it sees RO while cpu3 does have tlb cached so cpu3 can still modify
> > it.  So IMHO there's no problem here.
> 
> None of the CPUs has stale entries when CPU 2 sees a RO PTE.

In this example we have - Please see [1] below.

> We are
> assuming that TLB flush will be done on CPU 1 while it's still holding
> page table lock.
> 
> CPU 2 (re)locks page table and (re)checks the PTE under question when
> it decides if copy is necessary. If it sees a RO PTE, it means the
> flush has been done on all CPUs, therefore it fixes the problem.

I guess you had the assumption that pgtable lock released in step 1 already.
But it's not happening in this specific example, not until step5 [2] below.

Indeed if pgtable lock is not released from cpu1, then COW path won't even
triger, afaiu... do_wp_page() needs the pgtable lock.  It seems just even safer.

Irrelevant of the small confusions here and there... I believe we're always on
the same page, at least the conclusion.

Thanks,

> 
> > But I do think in step 2 here we overlooked _PAGE_UFFD_WP bit.  Note that if
> > it's uffd-wp wr-protection it's always applied along with removing of the write
> > bit in change_pte_range():
> > 
> >         if (uffd_wp) {
> >                 ptent = pte_wrprotect(ptent);
> >                 ptent = pte_mkuffd_wp(ptent);
> >         }
> > 
> > So instead of being handled as COW page do_wp_page() will always trap
> > userfaultfd-wp first, hence no chance to race with COW.
> > 
> > COW could only trigger after another uffd-wp-resolve ioctl which could remove
> > the _PAGE_UFFD_WP bit, but with Andrea's theory unprotect will only happen
> > after all wr-protect completes, which guarantees that when reaching the COW
> > path the tlb must has been flushed anyways.  Then no one should be modifying
> > the page anymore even without pgtable lock in COW path.
> > 
> > So IIUC what Linus proposed on "flushing tlb within pgtable lock" seems to
> > work, but it just may cause more tlb flush than Andrea's proposal especially
> > when the protection range is large (it's common to have a huge protection range
> > for e.g. VM live snapshotting, where we'll protect all guest rw ram).
> > 
> > My understanding of current issue is that either we can take Andrea's proposal
> > (although I think the group rwsem may not be extremely better than a per-mm
> > rwsem, which I don't know... at least not worst than that?), or we can also go
> > the other way (also as Andrea mentioned) so that when wr-protect:
> > 
> >   - for <=2M range (pmd or less), we take read rwsem, but flush tlb within
> >     pgtable lock
> > 
> >   - for >2M range, we take write rwsem directly but flush tlb once
> >   
> > Thanks,
> > 
> > > 
> > > >  - CPU 2 correctly decides it needs to be a COW, and copies the page contents
> > > > 
> > > >  - CPU 3 *does* have a stale TLB (because TLB invalidation hasn't
> > > > happened yet), and writes to that page in users apce

[1]

> > > > 
> > > >  - CPU 1 now does the TLB invalidate, and releases the page table lock

[2]

> > > > 
> > > >  - CPU 2 gets the page table lock, sees that its PTE matches
> > > > vmf->orig_pte, and switches it to be that writable copy of the page.
> > > > 
> > > > where the copy happened before CPU 3 had stopped writing to the page.
> > > > 
> > > > So the pte lock doesn't actually matter, unless we actually do the
> > > > page copy inside of it (on CPU2), in addition to doing the TLB flush
> > > > inside of it (on CPU1).
> > > > 
> > > > mprotect() is actually safe for two independent reasons: (a) it does
> > > > the mmap_sem for writing (so mprotect can't race with the COW logic at
> > > > all), and (b) it changes the vma permissions so turning something
> > > > read-only actually disables COW anyway, since it won't be a COW, it
> > > > will be a SIGSEGV.
> > > > 
> > > > So mprotect() is irrelevant, other than the fact that it shares some
> > > > code with that "turn it read-only in the page tables".
> > > > 
> > > > fork() is a much closer operation, in that it actually triggers that
> > > > COW behavior, but fork() takes the mmap_sem for writing, so it avoids
> > > > this too.
> > > > 
> > > > So it's really just userfaultfd and that kind of ilk that is relevant
> > > > here, I think. But that "you need to flush the TLB before releasing
> > > > the page table lock" was not true (well, it's true in other
> > > > circumstances - just not *here*), and is not part of the solution.
> > > > 
> > > > Or rather, if it's part of the solution here, it would have to be
> > > > matched with that "page copy needs to be done under the page table
> > > > lock too".
> > > > 
> > > >               Linus
> > > > 
> > > 
> > 
> > -- 
> > Peter Xu
> > 
> 

-- 
Peter Xu