[PATCH v2 3/3] fork: lock VMAs of the parent process when forking

[Suren Baghdasaryan]

On Sat, Jul 8, 2023 at 2:18 PM Linus Torvalds
<torvalds at linux-foundation.org> wrote:
>
> On Sat, 8 Jul 2023 at 12:12, Suren Baghdasaryan <surenb at google.com> wrote:
> >
> >  kernel/fork.c | 1 +
> >  1 file changed, 1 insertion(+)
>
> I ended up editing your explanation a lot.
>
> I'm not convinced that the bug has much to do with the delayed tlb flushing.
>
> I think it's more fundamental than some tlb coherence issue: our VM
> copying simply expects to not have any unrelated concurrent page fault
> activity, and various random internal data structures simply rely on
> that.
>
> I made up an example that I'm not sure is relevant to any of the
> particular failures, but that I think is a non-TLB case: the parent
> 'vma->anon_vma' chain is copied by dup_mmap() in anon_vma_fork(), and
> it's possible that the parent vma didn't have any anon_vma associated
> with it at that point.
>
> But a concurrent page fault to the same vma - even *before* the page
> tables have been copied, and when the TLB is still entirely coherent -
> could then cause a anon_vma_prepare() on that parent vma, and
> associate one of the pages with that anon-vma.
>
> Then the page table copy happens, and that page gets marked read-only
> again, and is added to both the parent and the child vma's, but the
> child vma never got associated with the parents new anon_vma, because
> it didn't exist when anon_vma_fork() happened.
>
> Does this ever happen? I have no idea. But it would seem to be an
> example that really has nothing to do with any TLB state, and is just
> simply "we cannot handle concurrent page faults while we're busy
> copying the mm".
>
> Again - maybe I messed up, but it really feels like the missing
> vma_start_write() was more fundamental, and not some "TLB coherency"
> issue.

Sounds plausible. I'll try to use the reproducer to verify if that's
indeed happening here. It's likely there are multiple problematic
scenarios due to this missing lock though.
Thanks,
Suren.

>
>             Linus