Re: [PATCH 1/3] mm: Move arch_do_swap_page() call to before swap_free()

Peter Collingbourne
2023-05-16T02:35:41Z

On Mon, May 15, 2023 at 05:16:09PM -0700, Peter Collingbourne wrote:
> On Sat, May 13, 2023 at 05:29:53AM +0200, David Hildenbrand wrote:
> > On 13.05.23 01:57, Peter Collingbourne wrote:
> > > Commit c145e0b47c77 ("mm: streamline COW logic in do_swap_page()") moved
> > > the call to swap_free() before the call to set_pte_at(), which meant that
> > > the MTE tags could end up being freed before set_pte_at() had a chance
> > > to restore them. One other possibility was to hook arch_do_swap_page(),
> > > but this had a number of problems:
> > >
> > > - The call to the hook was also after swap_free().
> > >
> > > - The call to the hook was after the call to set_pte_at(), so there was a
> > > racy window where uninitialized metadata may be exposed to userspace.
> > > This likely also affects SPARC ADI, which implements this hook to
> > > restore tags.
> > >
> > > - As a result of commit 1eba86c096e3 ("mm: change page type prior to
> > > adding page table entry"), we were also passing the new PTE as the
> > > oldpte argument, preventing the hook from knowing the swap index.
> > >
> > > Fix all of these problems by moving the arch_do_swap_page() call before
> > > the call to free_page(), and ensuring that we do not set orig_pte until
> > > after the call.
> > >
> > > Signed-off-by: Peter Collingbourne <pcc@google.com>
> > > Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
> > > Link: https://linux-review.googlesource.com/id/I6470efa669e8bd2f841049b8c61020c510678965
> > > Cc: <stable@vger.kernel.org> # 6.1
> > > Fixes: ca827d55ebaa ("mm, swap: Add infrastructure for saving page metadata on swap")
> > > Fixes: 1eba86c096e3 ("mm: change page type prior to adding page table entry")
> >
> > I'm confused. You say c145e0b47c77 changed something (which was after above
> > commits), indicate that it fixes two other commits, and indicate "6.1" as
> > stable which does not apply to any of these commits.
>
> Sorry, the situation is indeed a bit confusing.
>
> - In order to make the arch_do_swap_page() hook suitable for fixing the
> bug introduced by c145e0b47c77, patch 1 addresses a number of issues,
> including fixing bugs introduced by ca827d55ebaa and 1eba86c096e3,
> but we haven't fixed the c145e0b47c77 bug yet, so there's no Fixes:
> tag for it yet.
>
> - Patch 2, relying on the fixes in patch 1, makes MTE install an
> arch_do_swap_page() hook (indirectly, by making arch_swap_restore()
> also hook arch_do_swap_page()), thereby fixing the c145e0b47c77 bug.
>
> - 6.1 is the first stable version in which all 3 commits in my Fixes: tags
> are present, so that is the version that I've indicated in my stable
> tag for this series. In theory patch 1 could be applied to older kernel
> versions, but it wouldn't fix any problems that we are facing with MTE
> (because it only fixes problems relating to the arch_do_swap_page()
> hook, which older kernel versions don't hook with MTE), and there are
> some merge conflicts if we go back further anyway. If the SPARC folks
> (the previous only user of this hook) want to fix these issues with ADI,
> they can propose their own backport.
>
> > > @@ -3959,7 +3960,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > > VM_BUG_ON(!folio_test_anon(folio) ||
> > > (pte_write(pte) && !PageAnonExclusive(page)));
> > > set_pte_at(vma->vm_mm, vmf->address, vmf->pte, pte);
> > > - arch_do_swap_page(vma->vm_mm, vma, vmf->address, pte, vmf->orig_pte);
> > > folio_unlock(folio);
> > > if (folio != swapcache && swapcache) {
> >
> >
> > You are moving the folio_free_swap() call after the folio_ref_count(folio)
> > == 1 check, which means that such (previously) swapped pages that are
> > exclusive cannot be detected as exclusive.
>
> Ack. I will fix this in v2.

I gave this some thought and concluded that the added complexity needed
to make this hook suitable for arm64 without breaking sparc probably
isn't worth it in the end, and as I explained in patch 2, sparc ought
to be moving away from this hook anyway. So in v2 I replaced patches 1
and 2 with a patch that adds a direct call to the arch_swap_restore()
hook before the call to swap_free().

Peter