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

[Yu Zhao <yuzhao@google.com>]

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).

>  - 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
>