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