Re: [5.4 PATCH] mm/gup: Do not force a COW break on file-backed memory

[Jann Horn]

On Thu, Dec 2, 2021 at 5:11 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> On Thu, Dec 02, 2021 at 04:51:47AM +0100, Jann Horn wrote:
> > On Thu, Dec 2, 2021 at 12:18 AM Matthew Wilcox (Oracle)
> > <willy@xxxxxxxxxxxxx> wrote:
> > > Commit 17839856fd58 ("gup: document and work around "COW can break either
> > > way" issue") forces a COW break, even for read-only GUP.  This interacts
> > > badly with CONFIG_READ_ONLY_THP_FOR_FS as it tries to write to a read-only
> > > PMD and follow_trans_huge_pmd() returns NULL which induces an endless
> > > loop as __get_user_pages() interprets that as page-not-present, tries
> > > to fault it in and retries the follow_page_mask().
> > >
> > > The issues fixed by 17839856fd58 don't apply to files.  We know which way
> > > the COW breaks; the page cache keeps the original and any modifications
> > > are private to that process.  There's no optimisation that allows a
> > > process to reuse a file-backed MAP_PRIVATE page.  So we can skip the
> > > breaking of the COW for file-backed mappings.
> > >
> > > This problem only exists in v5.4.y; other stable kernels either predate
> > > CONFIG_READ_ONLY_THP_FOR_FS or they include commit a308c71bf1e6 ("mm/gup:
> > > Remove enfornced COW mechanism").
> > >
> > > Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> > > ---
> > >  mm/gup.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/mm/gup.c b/mm/gup.c
> > > index 3ef769529548..d55e02411010 100644
> > > --- a/mm/gup.c
> > > +++ b/mm/gup.c
> > > @@ -176,7 +176,8 @@ static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
> > >   */
> > >  static inline bool should_force_cow_break(struct vm_area_struct *vma, unsigned int flags)
> > >  {
> > > -       return is_cow_mapping(vma->vm_flags) && (flags & FOLL_GET);
> > > +       return is_cow_mapping(vma->vm_flags) && vma_is_anonymous(vma) &&
> > > +               (flags & FOLL_GET);
> > >  }
> >
> > To be fully correct, the check would have to check for PageAnon(), not
> > whether the mapping is anonymous, right? Since a private file mapping
> > can still contain anonymous pages from a prior CoW?
>
> Oh, right.  So parent process maps a file with MAP_PRIVATE, writes to
> it, gets an anon page, forks.  Child stuffs the page into a pipe,
> unmaps page.  Parent writes to page again, now child can read() the
> modification?

Yeah - in theory that could happen e.g. with an ELF's .data section?
Those end up as writable private file mappings.

(I don't know whether that actually has real-world relevance though,
I'm just saying it's semantically off in theory.)

> The problem is that we don't even get to seeing the struct page with
> the current code paths.  And we're looking for a fix for RO THP that's
> less intrusive for v5.4 than backporting
>
> 09854ba94c6a ("mm: do_wp_page() simplification")
> 1a0cf26323c8 ("mm/ksm: Remove reuse_ksm_page()")
> a308c71bf1e6 ("mm/gup: Remove enfornced COW mechanism")
>
> The other patch we've been kicking around (and works) is:
>
>  static inline bool should_force_cow_break(struct vm_area_struct *vma, unsigned
> int flags)
>  {
> -       return is_cow_mapping(vma->vm_flags) && (flags & FOLL_GET);
> +       return is_cow_mapping(vma->vm_flags) &&
> +               (!(vma->vm_flags & VM_DENYWRITE)) && (flags & FOLL_GET);
>  }
>
> That limits the change to be only text pages.  Generally programs do
> not write to their text pages, and they certainly don't write *secrets*
> to their text pages; if somebody else can read it, that's probably not
> a problem in the same way as writing to a page of heap.

Hm, yeah. It's not exactly beautiful, but I guess it should do the job
for fixing stable...

It's a good thing that VM_DENYWRITE still exists in the 5.4 branch. ^^